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The tiny parasitoid wasp, Encarsia formosa, has been used successfully to control greenhouse whiteflies (GHWFs) in greenhouses in many countries throughout the world. Therefore, there has been considerable interest in developing methods for artificially rearing this wasp. However, little information is available concerning the regulation of its development including the host-parasitoid interactions that are required for the parasitoid to complete its life cycle. Here we confirm that parasitoid developmental rates differ significantly based upon the host instar parasitized. Development was faster when 3rd and 4th instar GHWFs were offered for parasitization than when 1st or 2nd instars were used. Our results show that it is primarily the embryo and the first two parasitoid instars that exhibit prolonged developmental times when 1st and 2nd instar whiteflies are parasitized. Although percent emergence was not affected by host age at the time of parasitization, adult longevity as well as adult emergence pattern varied greatly depending upon the instar parasitized. When 3rd and 4th instar GHWFs were selected for oviposition, adult wasps lived significantly longer than when 1st or 2nd instars were used; also, there was a sharp emergence peak on the 2nd day after emergence was first observed (reduced or absent when 1st or 2nd instar GHWFs were parasitized) and the emergence period was reduced from between 8 and 11 days to 5 days. In general, the younger the host instar parasitized, the less synchronous was parasitoid development. Previous reports that E. formosa will not molt to the 2nd instar until the host has reached its 4th instar were not confirmed. When 1st instar host nymphs were parasitized, 2nd instar parasitoids were detected in 3rd instar hosts. Importantly, however, no matter which instar was parasitized, the parasitoid never molted to its last instar until the host had reached Stage 5 of its last instar, a stage in which host pharate adult formation has been initiated. It appears, then, that a condition(s) associated with host pharate adult formation is required for the parasitoid's final larval molt. Results reported here should facilitate the development of in vitro rearing systems for E. formosa.

Toxicity and behavioural bioassays were used to assess the lethal and sub-lethal effects of two naphthoquinones from the Chilean plant Calceolaria andina L. (Scrophulariaceae), two products derived from Azadirachta indica and pyrethrum on the glasshouse whitefly, Trialeurodes vaporariorum Westwood, and the parasitoid Encarsia formosa Gahan. The potential use of these botanicals in an Integrated Pest Management (IPM) programme is discussed. Pyrethrum was the most toxic botanical tested, but it was toxic to both the whitefly and parasitoid. The naphthoquinones showed potential for inclusion into an IPM programme if, by formulation, the toxicity towards the whitefly could be increased and the deterrent activity towards E. formosa adults decreased. Overall, the A. indica-derived products had the most potential for use in an IPM system for the control of whitefly, which includes E. formosa as a biocontrol agent.

The effects of continuous exposure of the adult and crawler stages of the whitefly, Trialeurodes vaporariorum, and adult of the parasitoids Encarsia formosa and Eretmocerus eremicus to spray deposits of potassium salts of a fatty acid and diafenthiuron applied at recommended rates were evaluated. French bean, Phaseolus vulgaris Prince, plants were sprayed and the adults confined on them using modified 5 cm Petri dishes. Fresh (4 - 5-h-old) deposits of fatty acids resulted in a 10.7% mortality rate of whitefly adults and 8.0 and 10.7% mortality rates of E. formosa and E. eremicus adults respectively, while fresh deposits of diafenthiuron resulted in a 62.7% mortality rate of whitefly adults and 12.0 and 14.7% mortality rates of E. formosa and E. eremicus respectively. The mortality rate (6.9 and 69.7%) of whitefly crawlers was recorded when exposed to 24-h-old deposits of fatty acids and diafenthiuron respectively. The mortality rate declined as the deposits aged. Adult parasitoid emergence from the treate...

Μeλeτήθηκe η ολοκληρωμένη καταπολέμηση του Trialeurodes vaporatiorum στη μeλιτζάνα μe τη χρησιμοποίηση φυσικών (παγίδeς), χημικών και βιολογικών μeθόδων. Ο πλέον αποτeλeσματικός συνδυασμός ήταν η χρησιμοποίηση Quinomethionate και παγίδων. Μίγμα Applaud και Actellic ήταν αρκeτά αποτeλeσματικό για την καταπολέμηση των eντόμων. Βιολογική καταπολέμηση μe το παρασιτοeιδές Encarsia formosa (Gahan) eίχe ως αποτέλeσμα μέτρια αλλά eπαρκή καταπολέμηση όταν συνδυάστηκe μe παγίδeς. Οι παγίδeς μόνeς τους έδωσαν ικανοποιητικά αποτeλέσματα. Όλeς οι μeταχeιρίσeις ήταν αποτeλeσματικές στη μeίωση του πληθυσμού του eντόμου σe ικανοποιητικό eπίπeδο σe σύγκριση μe πληθυσμούς που δeν υπέστησαν καμιά μeταχeίριση. Η αναλογία απeλeυθέρωσης παρασιτοeιδούς:ξeνιστή έπαιξe σημαντικό ρόλο στα αποτeλέσματα της βιολογικής καταπολέμησης χρησιμοποιώντας την E. formosa. Η υψηλότeρη αναλογία απeλeυθέρωσης. 4:1 παρασιτοeιδούς:ξeνιστή, έδωσe το υψηλότeρο ποσοιπό παρασιτισμού.

In this study, we investigated the tritrophic interactions among a persistently transmitted plant virus, Tomato yellow leaf curl virus (TYLCV), its insect vector, the sweetpotato whitefly Bemisia tabaci, and a parasitoid, Encarsia formosa Gahan, one of the most extensively used biological control agents. As an emerging invasive pest worldwide, the two most damaging whiteflies are B. tabaci B and Q cryptic species. On healthy tomato plants, parasitoid-induced mortality was significantly higher in B. tabaci B than in Q. In contrast, similar mortality levels of B and Q were observed on TYLCV-infected plants. A higher rate of parasitism was consistently observed in B, independent of the TYLCV infection. Similarly, the life history traits of E. formosa were influenced by both TYLCV and the two cryptic species of B. tabaci. Specifically, E. formosa parasitizing B had a greater adult longevity and shorter developmental time on healthy plants, whereas the parasitoids developing from Q has a greater adult longevity on TYLCV-infected plants. The emergence rate of E. formosa was unaffected by either B. tabaci cryptic species or the virus. These results suggest that the vector-borne pathogen can manipulate the host suitability of a parasitoid and hence the parasitoid-host interactions.

We experimentally examined the effects of elevated O₃ and whitefly herbivory on tomato volatiles, feeding and oviposition preferences of whiteflies and behavioural responses of Encarsia formosa to these emissions on two tomato genotypes, a wild-type (Wt) and a jasmonic acid (JA) defence-enhanced genotype (JA-OE, 35S). The O₃ level and whitefly herbivory significantly increased the total amount of volatile organic compounds (VOCs), monoterpenes, green leaf volatiles (GLVs), and aldehyde volatiles produced by tomato plants. The 35S plants released higher amount of total VOCs and monoterpene volatiles than Wt plants under O₃+herbivory treatments. The feeding and oviposition bioassays showed that control plants were preferred by adult whiteflies whereas the 35S plants were not preferred by whiteflies. In the Y-tube tests, O₃+herbivory treatment genotypes were preferred by adult E. Formosa. The 35S plants were preferred by adult E. formosa under O₃, herbivory and O₃+herbivory treatments. Our results demonstrated that elevated O₃ and whitefly herbivory significantly increased tomato volatiles, which attracted E. formosa and reduced whitefly feeding. The 35S plants had a higher resistance to B. tabaci than Wt plant. Such changes suggest that the direct and indirect defences of resistant genotypes, such as 35S, could strengthen as the atmospheric O₃ concentration increases.

Lower temperature thresholds and accumulated day-degrees for development of E. formosa - an important parasitoid of T. vaporariorum – were determined. The study was conducted from May1996 through January 1997, in growth chambers and in greenhouse. The developmental time from egg to adult of E. formosa was determined rearing the insect in the third and fourth nymphal instars of T. vaporariorum, on tomato plants, under temperatures of 14,6; 18,5; 21,8; 25,7; 28,7 and 33,5oC. With a photoperiod was 14 h of light.The information allowed to calculate the thermal requirements of the parasitoid through the mean temperature method. When E. formosa was reared in the third nymphal instar of T. vaporariorum the mean length of time from egg to adult varied from 9.4 days at 33.5°C to 28.4 days at 14.6°C. The correponding time when reared in the fourth nymphal instar varied from 10.0 days at 33.5°C to 29.5 days at 14,6°C. The threshold temperature when infesting the third ninfal instar of T. vaporariorum was 5,5oC at the fourth nymphal instar it was 5,06oC, averaging 5,28oC. The accumulated temperature to complete the development from egg to adult when infesting the third nymphal instar was 294,1 degree-days (°D); when infesting the fourth nymphal instar it was 322,5oD, averaging 308,3oD. Despite of a threshold temperature higher in the third nymphal instar the thermal constant was lower in that instar than in the fourth nymphal instar. This information and the corresponding to the whitefly should contribute to improve the rearing conditions of the parasitoid, and to a more accurate timing its liberation and monitoring.Encarsia formosa es un entomofago ampliamente utilizado en el mundo para controlar Trialeurodes vaporariorum, importante plaga del tomate cultivado en invernaderos de la V Region, Chile. En este trabajo se calculo la temperatura base de desarrollo y la constante termica (°D acumulados) para el periodo de huevo a adulto del parasitoide. Inicialmente se estudio el tiempo de desarrollo de huevo a adulto de E. formosa, infestando el tercer y el cuarto subestado ninfal de T. vaporariorum, en tomate, bajo seis regimenes termicos (14,6; 18,5; 21,8; 25,7; 28,7 y 33,5o C), con fotoperiodo de 14 h luz y 10 h oscuridad. Esta informacion permitio calcular los requerimientos termicos del parasitoide, mediante el metodo de la temperatura media. En el tercer subestado ninfal de T. vaporariorum, el parasitoide demoro un tiempo que vario desde 9,4 dias a 33,5 °C hasta 28,4 dias a 14,6 °C. En el cuarto subestado vario desde 10,0 dias a 33,5 °C hasta 29,5 dias a 14,6 °C. La temperatura base del parasitoide en el tercer subestado ninfal de T. vaporariorum fue 5,50 oC y en el cuarto subestado ninfal fue 5,06 oC, promediando 5,28 oC. Los grados dias para completar el desarrollo de huevo a adulto en el tercer subestado fue de 294,1 oD y en el cuarto fue de 322,5 oD, con promedio de 308,3 oD. Llama la atencion que, a pesar de una temperatura umbral mas alta en el tercer subestado ninfal de la plaga, la constante termica en este subestado fue menor que en el cuarto subestado ninfal. Esta informacion mas la correspondiente a la mosquita blanca debe servir para mejorar las condiciones de crianza del parasitoide, realizar sus liberaciones en forma oportunas y precisar fechas de muestreo.

One of the factors that may complicate biological control of the greenhouse whitefly on Gerbera jamesonii by Encarsia formosa is the rosette shape of this ornamental, which differs from the vertical shape of most vegetable plants (cucumber, egg plant, tomato, etc.). Therefore, host-habitat location and the behaviour prior to landing on uninfested and infested leaves was studied. Attraction of E. formosa from a short distance by infested leaves could not be detected: the parasitoid females landed at random on uninfested and infested leaves. After the first landing, a redistribution of the wasps occurred on the leaves. After 24 h three times as many wasps were found on the infested leaves than on uninfested ones. In a dispersal experiment with four plants, E. formosa appeared to have no preference for landing on leaves of the medium age class, which is the age class on which most of the whiteflies in a suitable stage for parasitism occur. Twenty percent of the parasitoids were found on the plants 20 min after releasing them. These results were independent of the plant cultivar and the host density on the plants. In the course of 8 h, the number of E. formosa females recovered from plants increased linearly, and this increase was greater on plants where hosts were present and also greater on the plant cultivar with the lowest trichome density. After 24 h, the percentage of females was highest (56%) on plants with the highest host density. E. formosa females were arrested on leaves where hosts were present. Contrary to our expectation, the results from the two G. jamesonii cultivars that differed strongly in leaf hairiness were not significantly different in most experiments. Only at the high host density was parasitism found to be lower on the cultivar with the higher hair density. Parasitoids may walk on top of the ‘hair coverlet’ of cultivars with high trichome density and, therefore, be hampered less than expected.

Parasitoids of the Bemisia tabaci (Gennadius) species complex collected in Spain and Thailand were evaluated as biological control agents of B. tabacibiotype B in cole crops in Texas, USA. Parasitoids were identified by morphological and RAPD–PCR analyses. The most abundant parasitoid from Spain was Eretmocerus mundus Mercet with apparent field parasitism of 39–44%. In Thailand, Encarsia formosa Gahan, E. transvena Timberlake, E. adrianaeLopez-Avila, Eretmocerus sp. 1 and sp. 2 emerged, with apparent field parasitism of 1–65%. Identification and molecular classification of B. tabaciassociated with parasitoid collections and in the release site in Texas were accomplished using morphological traits and nucleotide sequence comparison of the mitochondrial cytochrome oxidase I gene (COI) (700–720 bp). Collections of B. tabacifrom Thailand grouped separately from B types from Arizona and Florida and the target B type from Texas, USA, a cluster from India, and other New World B. tabaci. The Spanish B. tabaci host of E. mundus which was laboratory and field-tested to achieve biological control of the B type was most closely related to non-B type B. tabaci populations from Spain and Sudan, the latter which formed a second group within the larger clade that also contained the B type cluster. Laboratory tests indicated that E. mundus from Spain parasitized more B. tabaci type B than did Eretmocerus spp. native to Texas and other exotic parasitoids evaluated. Eretmocerus mundus from Spain also successfully parasitized B. tabaci type B when field-released in a 0.94 million ha test area in Texas, and has significantly enhanced control of B. tabaci type B in California, USA. In contrast, parasitoids from Thailand failed to establish in the field in Texas, collectively suggesting a positive correlation between the centres of diversity of compatible parasitoid–host complexes.

Reciprocal molecular markers were developed to distinguish the closely related parasitoid species Encarsia formosa Gahan and E. luteola Howard. E. formosa is widely used in the biological control of whiteflies and yet, based upon morphology, it is extremely difficult to distinguish from E. luteola. The D2 expansion region of 28S rDNA was sequenced from seven strains of E. formosa and two strains of E. luteola to assess the amount of genetic variation within and between species at this locus. From parsimony analysis we find that populations of E. formosa and E. luteola each form a monophyletic clade and are probably each other's most closely related sister taxon. Based upon the sequence variation between species, we present a simple molecular assay to rapidly and unambiguously distinguish E. formosa and E. luteola.

The objective of this study was to evaluate the costs of biologically controlling infestations of silverleaf whitefly, Bemisia argentifolii Bellows & Perring, in New England greenhouse operations on poinsettia, Euphorbia pulcherrima Wild. ex Koltz, using the parasitic wasp Encarsia formosa Gahan (Nile Delta strain). Partial budget analysis was used to compare costs for conventional verses biological control regimens. Four alternative whitefly control budgets are developed; two conventional chemical-based control budgets formulated with and without the use of imidacloprid, and two biological control budgets which demonstrate the impact of possibly greater pest monitoring efforts necessary to implement this type strategy successfully. The analysis shows that biological whitefly control costs were >300% greater than conventional chemical-based control strategy costs. Most of this increase is caused by the higher costs of Encarsia formosa as the material control input. If monitoring costs are held constant across different strategies, labor costs actually decline for biological control. This is because of a significant reduction in the number of control applications made and the relatively lower cost of applying E. formosa. If more extensive monitoring efforts are required to implement biological control successfully, labor costs increase by 56% over the conventional pre-imidacloprid regimen. Based on these results, the authors conclude that cheaper and more reliable means of producing E. formosa must be developed before this strategy will become economically viable for commercial poinsettia greenhouse production.

Amitus bennetti Viggiani and Evans, Encarsia formosa Gahan (two strains), Eretmocerus eremicus Rose and Zolnerowich, Eretmocerus mundus Mercet and Eretmocerus staufferi Rose and Zolnerowich, which are parasitoids of Bemisia argentifolii Bellows and Perring were compared with respect to their searching behaviour as part of a pre-introduction evaluation programme. Within a 5 cm arena, host-finding time was independent of the release distance from the host. Before oviposition, A. bennetti walked fastest, the E. formosa strains walked slowest and the Eretmocerus species intermediate. Leg length was not the most significant factor determining the differences in walking speed. After oviposition, A. bennetti and Er, eremicus had a lower walking speed and higher turn rate which is an indication of area-restricted search. The effect was strongest for A. bennetti. All species showed preference for counter-clockwise turns. Based on the walking speed alone, it is expected that A. bennetti will be the most efficient natural enemy of B. argentifolii, the Eretmocerus species intermediate and the E. formosa strains the least.

Residual toxicities of avermectin b1 and pyridaben for 2- to 28-day exposure periods were assessed in laboratory and greenhouse trials for eight species of beneficial arthropods that are commercially produced for greenhouse pest management. In laboratory trials, Amblyseius degenerans Berlese, Aphidius colemani Viereck, Aphidoletes aphidimyza (Rondani), Dacnusa sibirica Telenga, Encarsia formosa (Gahan), and Orius insidiosus (Say) showed high mortality (>85%) when exposed to ≤6-day residues of both acaricides. Lower toxicities were observed for pyridaben to the predatory mites Amblyseius cucumeris (Oudermans) and Phytoseiulus persimilis Athias-Henroit and for avermectin b1 to A. cucumeris after exposure to ≤6-day residues. In greenhouse trials, pyridaben showed significantly higher residual toxicity to all beneficial species than avermectin b1. Pyridaben had high residual toxicity (40–60% mortality) to E. formosa, A. colemani, A. aphidimyza, A. degenerans, and P. persimilis 6 days after treatment. Residual toxicity of pyridaben to D. sibirica, A. cucumeris, and O. insidiosus decreased to a low level (

The effectiveness of weekly inundative releases of 3 female Eretmocerus eremicus (evaluated in 2 greenhouses) and 4–7 Encarsia formosa Beltsville strain (evaluated in 2 greenhouses) per plant for control of Bemisia argentifolii Bellows & Perring \[= Bemisia tabaci (Gennadius) strain B (Bellows et al. 1994)\] (Homoptera: Aleyrodidae) on poinsettia stock plants was determined under commercial growing conditions. We constructed partial life-tables for B. argentifolii in the presence and absence of parasitoids in each of 4 greenhouses to quantify parasitoid impact. Weekly population counts of B. argentifolii were also made. Initial starting densities of live B. argentifolii nymphs and pupae on cuttings used for the stock plant crop ranged from 0.25 to 1.86. Each site where each parasitoid species was tested had 1 relatively low and high initial starting density of B. argentifolii. Under these conditions, E. eremicus effectively controlled B. argentifolii at 2 sites and cuttings were harvested with 0.66–1.49 live nymphs and pupae per leaf without insecticide use. Trials with E. formosa Beltsville strain were terminated early at 2 sites and insecticides were applied before the harvesting of cuttings because this parasitoid failed to control B. argentifolii although initial starting densities of B. argentifolii were similar to those in E. eremicus greenhouses. The cost of using E. eremicus was compared with the cost of the insecticide imidacloprid for B. argentifolii control at 1 site. At a retail cost of $22 per 1,000 parasitoids in 1995, the use of E. eremicus was 44 times more expensive than imidacloprid.

Parthenogenesis-inducing (PI) Wolbachia are bacteria that cause incipient male eggs of parasitoid wasps to develop as females. These microbes may invade populations and lead to fixation of parthenogenesis. In this study, the consequences of fixation of PI Wolbachia on evolution of behaviour and development were addressed in three species of whitefly parasitoids in the genus Encarsia. Most sexual Encarsia are obligate autoparasitoids. In these haplodiploid wasps, fertilized, female-producing eggs are laid in whitefly or scale insect nymphs, while unfertilized, male-producing eggs are laid in immature parasitoids within the homopteran integument. The oviposition behaviour and progeny developmental requirements of unmated females of a sexual autoparasitoid species were compared with two species that were fixed for PI Wolbachia. All species were treated with antibiotics, effectively ‘curing’ the asexual species of Wolbachia infection. As expected, unmated females of the sexual species, E. pergandiella, laid few eggs in whiteflies relative to immature parasitoids, and males emerged exclusively from parasitoids. In contrast, females of one of the parthenogenetic species, E. formosa, laid few eggs in parasitoids relative to whiteflies, and male progeny emerged exclusively from whiteflies. Females of the second parthenogenetic species, E. hispida, were intermediate in their oviposition behaviour. Females laid eggs in both wasps and whiteflies. However, all of the males except one emerged from the whiteflies. The results suggest that the association of autoparasitoids and PI Wolbachia may result in dramatic evolutionary change of both host selection behaviour and developmental requirements of the infected wasps.

Bemisia tabaci (Gennadius) and Bemisia argentifolii Bellows & Perring are found on a broad range of plant hosts in the tropics and subtropics and in greenhouses in many temperate zones. Their distribution indicates a strong limitation by cold temperature. The effects of incubation temperature and duration on the survival of B. argentifolii eggs and red-eyed nymphs, and on the survival of Encarsia formosa Gahan, an aphelinid parasite of the whitefly, were studied. Eggs stored at 10°C for 3–28 d on rooted cabbage leaves responded with 94–56% hatch, respectively, whereas the viability of eggs incubated at 5°C declined rapidly after 3 d. Control eggs on rooted cabbage leaves held at 24°C had 96% hatch. Considerably lower survival was observed on nonrooted leaves and leaf disks. Survival of red-eyed nymphs incubated at 2, 5, and 10°C for 3–13 d declined rapidly at all 3 temperatures. Pupae of E. formosa held at 10°C for 3–13 d responded with 90–55% eclosion, respectively, whereas survival of those held at 2 and 5°C declined rapidly after 3 d. Results of our studies indicate that short-term storage of whitefly parasitoids in cold temperatures is possible without excessive detrimental effect. The role of cold temperature storage of eggs in intermittent rearing of Bemisia colonies is discussed.

Frankliniella occidentalis, vector of the quarantine pest tomato spotted wilt tospovirus, first appeared in the Czech Republic in 1987. Further problems of crop damage appeared in 1991 with the quarantine leafminer Liriomyza bryoniae and in 1993 with Liriomyzo huidobrensis. Biological methods for the protection of glasshouse crops against these pests have been tested since 1993. The bioagents Phytoseiulus persimilis and Encarsia formosa have long been used (and also produced) in our country for protection of glasshouse crops against spider mites and whiteflies. The population density of thrips on vegetables, especially on glasshouse capsicum, has been greatly reduced after the introduction of the predatory mite Amblyseius cucumeris and the predatory bug Orius insidiosus. Introduction of another group of bioagents (Aphidius spp., Aphidoletes aphidimyza and Hippodamia convergens) has been shown to be effective against aphids. The protection system against leafminers based on Dacnusa sibirica and Diglyphus isaea is also sufficiently effective.

Temporal and spatial relationships between greenhouse whitefly and its parasitoid Encarsia formosa were analysed at different levels by using Rook's neighbourhood model, and for two types of greenhouses: a small greenhouse in China (15.1 m 2 ) and a large greenhouse in the Netherlands (6480 m 2 ). For whitefly eggs and larvae, there was no difference in their spatial relationship occurring within- and between-plot in the two types of greenhouses. Eggs and larvae occurrence aggregated with no special directional distributions. Whitefly adults exhibited nonlinear spatial density dependence within-plot, and moved randomly between-plot. Whitefly adults preferred to fly away in a N-S directions and to settle down at a NW-SE directions with an increase in the number of their neighbouring infected plants within-plot. The parasitoid E. formosa showed a strong relationship with the temporal and spatial distribution of the host, but influenced the spatial distribution of whitefly.

Insight into the foraging behavior of the parasitoid Encarsia formosa Gahan for whitefly hosts was gained by continuous observation of individual parasitoids on leaves of the ornamental plant Gerbera jamesonii, until females left the leaf. Comparison of the parasitoid behavior on three cultivars gave similar results. Mean searching time on uninfested G. jamesonii leaves of three cultivars was 1 h 30 min and the mean percentage of walking activity of the total observation time on those cultivars was 61%. Both parameters were not influenced by different leaf structures of Gerbera cultivars. Encounters with hosts arrested the parasitoids on the leaves. The walking activity and the percentage of host encounters that resulted in an oviposition decreased with decreasing egg load of the parasitoid. In comparison with tomato, where biological control of whiteflies is successful, only minor differences in the foraging behavior occur, except for the residence time of females, which was about three to four times longer on G. jamesonii leaves, but these leaves are about seven times larger than tomato leaves. The facts that (1) the foraging behavior of E. formosa on G. jamesonii is independant of the cultivar and (2) the foraging behavior is, in many aspects, similar to that on tomato suggest that biological control of whitefly on this ornamental plant is a potential option.

Tables are presented classifying over 80 active substances for their harmfulness to natural enemies in protected cultivation, particularly parasitic hymenoptera, predatory mites and predatory insects. The categories used are: harmful, harmful under certain circumstances, not harmful. Results are given for different application methods in some cases. The duration of after-effects is given for Encarsia formosa, Phytoseiulus persimilis and Neoseiulus cucumeris.

We examined the host-finding capabilities of 5 species of Bemisia argentifolii Bellows & Perring parasitoids from 9 geographical localities, and related these capabilities to each population's effectiveness as biological control agents. Responses from olfactometry studies varied significantly among parasitoid populations. Encarsia pergandiella Howard (California) and 3 populations of Eretmocerus mundus Mercet (M92027, M92014, and M93005) responded positively to odors emitted from whitefly-infested foliage, whereas populations of Encarsia formosa Gahan (M93017 and M92030), Eretmocerus staufferi (Rose & Zolnerwich) (M94002), Eretmocerus tejanus (Rose & Zolnerwich) (M94003), and E. mundus (M92019) did not. None of the populations differentiated between odors emitted from poinsettia versus cotton. E. pergandiella located whitefly-infested plants the quickest, followed in decreasing order by E. mundus (M920l4), E. formosa (M92030), E. tejanus, and E. mundus (M92019 and M92027). Once within a patch of whitefly, parasitoid functional responses varied significantly among populations and between host plants. Although host feeding, parasitism, and total whitefly killed by each parasitoid population tended to be greater on poinsettia than on cotton, there was no concordance in performance ranks among the 9 parasitoid populations, 3 performance traits, and 2 host plants. The percentage of parasitoids successfully developing to adults was greatest for Encarsia sp. populations compared with Eretmocerus spp. populations independent of host plant. Results from the host habitat and host location studies suggested that E. mundus (M920l4) should be relatively effective in suppressing B. argentifolii populations, whereas E. mundus (M92019) should be relatively ineffective. Population level studies conducted in cage enclosures verified these predictions. Releases of E. mundus M92019 significantly reduced whitefly population levels below whitefly population levels observed in the no-release cages. Moreover, releases of E. mundus M92014 significantly reduced whitefly population levels below whitefly densities observed in the E. mundus M92019 release treatments.

The post-shipment quality of four species of natural enemies, Encarsia formosa Gahan (Hymenoptera: Eulo- phidae), Trichogramma pretiosum Riley (Hymenop- tera: Trichogrammatidae), Chrysoperla carnea (Ste- phens) (Neuroptera: Chrysopidae), and Hippodamia convergens Guerin (Coleoptera: Coccinellidae), was evaluated from shipments from 10 companies ordered over 2.5 years at three locations. Evaluations included the numbers received, emergence rates, sex ratio, survivorship, species identity, reproduction, and para- sitism. In general, natural enemies arrived on time in protective shipping containers. There were consider- able differences in the numbers received, depending on the company supplying the natural enemy and time of year. There were also differences among companies for survivorship and emergence rates of some natural enemies. Chrysopids other than C. carnea were shipped and cannibalism by larvae reduced the numbers re- ceived. Approximately 20% of coccinellids were found to be parasitized. Using average characteristics of H. convergens shipments, an estimated 75-508 reproduc- tively active females would be received from an order of 1000 beetles. If confirmed by further study, informa- tion such as this could provide customers an ''industry standard'' for the expected number of natural enemies in a given reproductive or other physiological

Parasitoids of the Encarsia genus (Hymenoptera: Aphelinidae) are important biological control agents against whiteflies. Some of the species in this genus not only parasitize their hosts, but also kill them through host feeding. The whitefly parasitoid, Encarsia formosa Gahan, was examined to determine whether the rearing host species affects its subsequent host-feeding capacity and parasitism. E. formosa wasps were reared on Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae) and Bemisia tabaci (Gennadius) ‘Q’, and their subsequent host-feeding capacity and parasitism of T. vaporariorum and B. tabaci were examined. E. formosa reared on T. vaporariorum were significantly larger in body size than those reared on B. tabaci , but these wasps killed a similar number of whitefly nymphs by host feeding when they attacked the same host species on which they were reared. Regardless of the species on which it was reared, E. formosa fed significantly more on the B. tabaci nymphs than on the T. vaporariorum nymphs. The number of whitefly nymphs parasitized by E. formosa differed between the wasps reared on T. vaporariorum and those reared on B. tabaci depending on which whitefly species was offered as a host. In addition, the wasps reared on T. vaporariorum parasitized significantly more on T. vaporariorum than those reared on B. tabaci . The wasps reared on B. tabaci , however, parasitized similar numbers of whiteflies of both host species. The results indicated that the host-feeding capacity of E. formosa was affected more by the host species attacked than by the rearing host species, but the parasitism was affected by the host species attacked and the rearing host species. Generally, E. formosa reared on T. vaporariorum killed more T. vaporariorum nymphs by parasitism and host feeding than those reared on B. tabaci. Additionally, a similar number of B. tabaci nymphs were killed by parasitism and host feeding regardless of the rearing host species. Currently coexistence of B. tabaci and T. vaporariorum on vegetable crops usually occurs in some areas; our results may provide helpful information on using mass-reared parasitoids against mixed whitefly infestations in biological control programs.

For _20 yr, Bemisia tabaci Gennadius persists as a begomovirus vector and is a serious problem in tomato production in many parts of the world. In tropical countries, the use of netting to protect horticultural crops has proven to be an effective and sustainable tool against Lepidoptera but not against small insects. This study evaluated the repellent effect of AgroNet 0.9T, a 0.9-mm pore diameter and 40-mesh size netting treated with alphacypermethrin insecticide against B. tabaci. This pyrethroid insecticide is known to have toxic and repellent effects against mosquitoes and has been used for treatment of mosquito nets. Two nontreated netting materials were used as control: AgroNet 0.9NT with 0.9-mm pore diameter and 40-mesh size and AgroNet 0.4NT with 0.4-mm pore diameter and 80-mesh size. The behavior of B. tabaci and its parasitoid Encarsia formosa Gahan as they progressed through the treated netting was studied in the laboratory in choice and no-choice tests. The development of wild B. tabaci population on tomato plants protected by the same nets was followed in two Þeld trials implemented in Njoro, Kenya. Results obtained with the no-choice tests showed a signiÞcant reduction of movement on the treated net with 40-mesh (19%) compared with nontreated netting (35 and 46% with 80- and 40-mesh, respectively). The mortality of B. tabaci was signiÞcantly higher (two-fold) in the test tube containing only the treated netting compared with the nontreated one. The repellent effect of the treated netting was also demonstrated against E. formosa, but it did not have this toxic effect. Unlike for B. tabaci, the treated and nontreated nets appeared to have a similar repellent effect on E. formosa in the choice test, which suggests a learning behavior of the parasitoid. In both Þeld tests, B. tabaci population was signiÞcantly lower on tomato protected by the treated net compared with the same nontreated net. However there was no signiÞcant difference in B. tabaci population between the treated 0.9-pore diameter and the nontreated 0.4-pore diameter. We discussed these Þndings and their implications for the use of repellent netting in integrated pest management in horticulture and more specifically in vegetable production.

The effectiveness of inundative releases of the parasitoid Encarsia formosa for control of Bemisia argentifolii on poinsettia was determined in replicated experimental greenhouses. We evaluated two release rates of E. formosa: a low release rate (1 wasp/plant/week, released in two greenhouses, in spring 1995) and a high release rate (3 wasps/plant/week, released in two greenhouses, in fall 1993), each over a 14-week growing season. Each release trial had one or two control greenhouses in which B. argentifolii developed on poinsettia in the absence of E. formosa. Life-tables were constructed for B. argentifolii in the presence and absence of E. formosa by using a photographic technique to follow cohorts of whiteflies on poinsettia leaves. Weekly population counts of the whitefly were also made. In the absence of E. formosa, egg to adult survivorship of B. argentifolii on poinsettia was 75‐ 81%. At the low release rate, egg to survivorship of B. argentifolii was 5% and parasitism was 13%. At the high release rate, egg to adult survivorship for B. argentifolii was 8% and parasitism was 23%. The net reproductive rates (R0) for B. argentifolii populations in the absence of E. formosa ranged from 18.01‐26.12, indicating a rapidly increasing population. Net reproductive rates for whitefly populations subject to wasp releases were 1.54 for the low release rate greenhouses and 2.11 for the high release rate greenhouses, indicating substantially reduced B. argentifolii population growth. The low release rate provided better control of B. argentifolii than the high release rate. This difference was attributed to higher levels of mortality of whiteflies at the low release rate in the first 5‐6 weeks of the growing period. We suggest that mutual interference may also have affected observed levels of mortality and parasitism. r 1997 Academic Press

Regional monitoring of whiteflies, Bemisia sp., in the lower Rio Grande Valley of Texas was undertaken between 1991 and 1995 to determine seasonal abundance and spatial population dynamics of this pest. Samples of whiteflies were taken using yellow sticky traps along roadways and samples from plants at selected locations. The abundance of parasitoids of whiteflies was quantified from each plant sample by species. The number of whitefly adults on yellow sticky traps consistently increased dramatically during late spring and summer and declined in the fall. Dispersion of whiteflies on sticky traps generally changed from highly clumped in the spring to less clumped in the summer and early fall of each year. The population of the native parasitoid complex of the whitefly correlated with increases of whitefly populations. Six whitefly parasitoid species were collected routinely and identified during this survey. In order of decreasing abundance they were Eretmocerus sp., Encarsia pergandiella, Encarsia meritoria, Encarsia sp. nr. strenua, Encarsia lutteola , and Encarsia formosa . Population levels were higher for whitefly and lower for whitefly parasitoids in 1995 than observed in 1994.

The effectiveness of inundative releases of the parasitoid Eucarsia formosa Beltsville strain for control of Bemisia argentifolii Bellows & Perring on poinsettia ( Euphorbia Pulcherrima Willd. ex Koltz.) was determined in replicated experimental greenhouses. We evaluated 2 release rates of E. formosa Beltsville strain: a low release rate (1 wasp per plant per week, released in 2 greenhouses) and a high release rate (3 wasps per plant per week, released in 2 greenhouses), over a 14-wk growing season. The trial had 1 control greenhouses in which B. argentifolii developed on poinsettia in the absence of E. formosa Beltsville strain. Life-tables were constructed for B. argentifolii in the presence and absence of E. formosa Beltsville strain by using a photographic technique to follow cohorts of whiteflies on poinsettia leaves. Weekly population counts of the whitefly were also made. In the absence of E. formosa Beltsville strain, egg to adult survivorship of B. argelltifolii on poinsettia was 71%. At the low release rate, egg to survivorship of B. argelltifolii was 4% and parasitism was 23%. At the high release rate, egg to adult survivorship for B. argentifolii was 1% and parasitism was 12%. The net reproductive rates (R) for B. argentifolii populations in the absence of E. formosa Betsville strain was 17.1, indicating a rapidly increasing population. Net reproductive rates for whitefly populations subject to wasp releases were 0.95 for the low release rate greenhouses. and 0.32 for the high release rate greenhouses. indicating declining B. argentifolii population growth. The high release rate provided better control of B. argentifolii than the low release rate and this was attributed to higher levels of in-house wasp reproduction. At time of harvest, the mean number of live nymphs and pupae per leaf in the high release rate greenhouses was not significantly different from numbers counted on leaves on plants being sold at commercial retail outlets.

Bioassays were conducted under greenhouse conditions to determine the effect of imidacloprid on adult and nymphal stages of the silverleaf whitefly, Bemisia argentifolii Bellows and Perring, and parasitism by Encarsia formosa (Gahan). A flowable formulation (240 g litre -1 ) of imidacloprid at six rates (0.09, 0.04 and 0.02 g a.i. litre -1 pot volume for experiments 1 and 2; 0.009, 0.004, and 0.002 g a.i. litre -1 pot volume for experiment 3) was evaluated. After a 48h exposure to treated plants, high mortality of adult whitefly (> 94%) was observed. Adults exposed to poinsettias treated 150 days earlier also had significantly greater mortality (>79%) than the adults on control plants. When exposed to treated plants for only 6 h, >65% of adults were killed. All three rates of imidacloprid caused >97% mortality of immature whiteflies by day 19. When treated plants were continually exposed to adult whiteflies, immature mortality was 100% for the three higher rates of imidacloprid up to and including 88 days after treatment. During the same time, emerging adults were reduced significantly. Immatures reinfested on plants treated 161 days earlier, incurred 80% mortality at the higher rate 0.09 g a.i. litre -1 pot volume and 38% mortality at 0.02 g a.i. litre -1 pot volume. At lower treatment rates, results varied. At 0.009 g a.i. litre -1 of pot volume, mean percentage whitefly mortality (65%) 25 days after treatment was significantly higher than the controls; however, whitefly mortality at 0.004 and 0.002 g a.i. litre -1 pot volume was not significantly different from controls or plants treated with the higher rate. Parasitoids could develop to the adult stage on whiteflies infesting imidacloprid treated plants. Parasitism occurred at low levels (

Eretmocerussp. was found parasitizing the greenhouse whiteflyTrialeurodes vaporariorumon tomatoes in Argentina. The objective of the present paper is to make a contribution to the knowledge of some biological parameters ofEretmocerussp. and to evaluate its potential as an agent for biological control. Preimaginal developmental time was estimated simultaneously with that ofEncarsia formosa,another aphelinid parasitoid ofT. vaporariorum,at two different temperatures: 26 ± 1°C and 29 ± 1°C (relative humidity (RH): 50–75%, photoperiod: 14L:10D). The following biological parameters ofEretmocerussp. were also analyzed: longevity, survival rate (lx) and age-specific fertility (mx) of the adult, sex rate, intrinsic rate of natural increase (rm), net reproductive rate (R0), and generational time (Tc). For this purpose a life table was constructed for the parasitoid at 26 ± 1°C, RH: 50–80%, and photoperiod: 14L:10D. Results were as follows:Eretmocerussp. had a longer developmental time at 26 ± 1°C (19.8 days) than at 29 ± 1°C (19.3 days) (ANOVA,P < 0.05). This time was also longer than that ofE. formosaat both temperatures (ANOVA,P < 0.05). The life table showed mean longevity of the adult: 12.8 days (female) and 7.0 days (male);lx50(adults): 12–13 days; mean sex rate: 0.57 (females/(females + males));rm: 0.195 females/female/day;R0: 100.8 females/female;Tc: 23.7 days. Results suggest thatEretmocerussp. could be used in addition toE. formosain the control of the greenhouse whitefly.

Using the electroretinogram (ERG) technique the spectral efficiency of the compound eye of the glasshouse whitefly Trialeurodes vaporariorum (Westwood) (Homoptera: Aleyrodidae) and its main parasitoid Encarsia formosa (Gahan) (Hymenoptera: Aphelinidae) was measured at selected wavelengths between 340 nm and 670 nm. The form of the ERG for both T. vaporariorum and E. formosa was found to be monophasic in nature. For both male and female T. vaporariorum and female E. formosa a primary peak of efficiency occurred in the blue-green-yellow region, peak 520 nm and a secondary peak in the ultraviolet (UV) region. The compound eye of female E. formosa gave a significantly greater response in the UV region than either the dorsal or ventral regions of the compound eye of T. vaporariorum relative to the responses in the blue-green-yellow region. T. vaporariorum has divided compound eyes with distinct dorsal and ventral regions. In this study it was found that the percentage response in the UV, of the dorsal region of the eye, is significantly greater than that of the ventral region of the eye relative to the percentage response in the blue-green-yellow region and there is a significant shift in the blue-green yellow peak towards the right of the spectrum.

The habitat- and host-searching behaviour of female Encarsia formosa Gahan (Hymenoptera: Aphelinidae) was assessed using an airflow olfactometer and a filter paper test. Responses to different odour cues, colours, host-produced honeydew, non-host honeydew and single carbohydrates were determined. The parasitoid was not attracted to or arrested by odours emanating from clean tobacco leaves, tobacco leaves heavily infested with the host Trialeurodes vaporariorum (Westwood) (Homoptera: Aleyrodidae) and covered with honeydew, or honeydew alone. However, E. formosa females showed a significant response to green light transmitted through a tobacco leaf. The yellow part of the spectrum was partly responsible for this response. Thus, the long-range orientation is random with respect to the presence of hosts. Filter paper tests showed that the short-range searching behaviour is influenced by water soluble, non-volatile contact-kairomones contained in the host-produced honeydew. Contact with honeydew excreted by L3/L4 T. vaporariorum resulted in longer searching times than honeydew from adult T. vaporariorum or L3/L4 Bemisia tabaci Gennadius (Homoptera: Aleyrodidae). No difference was found between the response to honeydew excreted by adult and L3/L4 B. tabaci. The parasitoids' response to honeydew was unaffected by the host plant on which the whiteflies had fed. Non-host honeydew and single carbohydrates also affected the searching behaviour of E. formosa but to a lower extent than host honeydew. The possible differences in the carbohydrate and amino acid composition of the honeydew excreted by different life-stages of T. vaporariorum and B. tabaci are discussed.

The flight behaviour of Encarsia formosa Gahan (Hymenoptera: Aphelinidae), a parasitoid of Trialeurodes vaporariorum (Westwood) (Homoptera: Aleyrodidae) has been studied in response to the following natural components: bean plant (Phaseolus vulgaris L.) infested with T. vaporariorum (PHC, plant host complex), host damaged bean plant from which whiteflies were removed (HDP, host damaged plant), T. vaporariorum third instar larvae (H, host), uninfested bean plant (P, plant) and T. vaporariorum empty pupal cases (EPC, empty pupal cases). A combination of chemical and visual stimuli is necessary in host location from a distance by E. formosa and seems to deeply affect the dispersal of this parasitoid. The presence of host (PHC, H) or of its parts (EPC) led to a substantial increase of oriented flights suggesting the occurrence of host derived chemicals. At a closer range, a combination of visual and chemical stimuli deriving from all the components of the plant host complex elicited the transition flying-landing and the following landing on the source.

Foraging behavior of Encarsia formosa was analyzed with a stochastic simulation model of the searching parasitoid during a single visit to a tomato leaflet infested with immatures of the greenhouse whitefly, Trialeurodes vaporariorum,from which the parasitoid can fly away. The model is based on host searching, host selection, host handling, and patch leaving behavior and on the physiology of the parasitoid. Outputs of the model were the residence time of the parasitoid on the leaflet and the number of hosts encountered, parasitized, or killed by host feeding. The mean residence time and the mean number of encounters, ovipositions, and host feedings on the leaflet increased with host density to a maximum of 14.0 h, 209.3 encounters, 15.6 ovipositions, and 2.9 host feedings. The shape of the curves resembles a Type II functional response. The relationship between ovipositions per unit of residence time and host density showed a dome-shaped curve. The most important parameters affecting the number of ovipositions at low host densities were the parasitoid’s initial egg load, the walking speed, the walking activity, and the leaf area. At high densities, the maximum and initial egg loads and the egg maturation rate were most essential. r 1997 Academic Press

Autoparasitoids are species of parasitic wasps in the family Aphelinidae which produce females as solitary primary endoparasitoids of homopterans such as whitefly and scale insects ('primary hosts'), and males as solitary hyper-parasitoids. Males generally develop on immature conspecific females or on individuals of other primary parasitoid species ('secondary hosts'). Encarsia pergandiella is an autoparasitoid that has been introduced to Italy for control of greenhouse whitefly Trialeurodes vaporariorum, in greenhouses and field crops. In this study we examined the secondary host selection behaviour of this species with regard to conspecific females and females of two thelytokous species, E. formosa and E. meritoria. Encarsia formosa has been used successfully for greenhouse whitefly control in Northern Europe, but has not been effective in Southern Italy in winter crops in unheated greenhouses. E. meritoria has recently spread in Italy, and may have potential for biological control of whitefly in the greenhouse environment. In the first experiment, female E. pergandiella were exposed to one of three pair-wise combinations of the three species in petri dish arenas. Parasitism was determined by dissection of the hosts. The number of hosts parasitized by E. pergandiella females did not differ with host species. However, significantly greater numbers of eggs were laid in E. meritoria in both treatments in which it was present; these hosts were more likely to be superparasitized. In a second experiment, observations of females in arenas with equal numbers of all three host species indicated that females encountered and parasitized all host species with approximately equal frequency, although the length of time females spent in the oviposition posture differed with host species.

We tested the hypothesis that two populations of the parasitoid Encarsia formosa Gahan (Hymenoptera: Aphelinidae) differed in their behavioral interactions with the whitefly host Bemisia tabaci Gennadius (Homoptera: Aleyrodidae). The two wasp populations were studied because previous work suggested large differences between the populations in performance on this host. In this study the populations differed behaviorally in both the number of hosts encountered and their reactions to hosts once encountered. The population reared for many years on Trialeurodes vaporariorum Westwood (Homoptera: Aleyrodidae) that previously performed more poorly on B. tabaci had a higher host encounter rate but rejected hosts more frequently. The population reared for a number of years on B. tabaci encountered fewer hosts but accepted a higher percentage of hosts for oviposition. The number of parasitized hosts did not differ between the two populations, however. These data demonstrate that there are heritable differences between these two populations of asexual wasps in host-associated behavioral traits. These behavioral differences in host acceptance do not explain performance differences seen in the earlier study, however, possibly due to different conditions between the two experiments.

Interspecific interactions among insect natural enemies have seldom been investigated experimentally within the context of biological control. Research in this area is needed due to the often contradictory predictions provided by the many theoretical models, the increasing dependence on biological control, and the concern that biological control agents may adversely affect some nontarget organisms. We describe a study whereby the occurrence and dynamics of interspecific interactions among three natural enemies (two parasitoids: Encarsia formosa and Encarsia pergandiella; and one predator Delphastus pusillus ) of the whitefly, Bemisia argentifolii (previously referred to as Bemisia tabaci strain “B”), were evaluated in greenhouse cage experiments. Eight populations consisting of all possible combinations of the three natural enemies and one population of whitefly alone were established to test the following hypotheses: (1) Natural enemy introductions are capable of suppressing B. argentifolii populations; (2) all interspecific interactions are detrimental to achieving biological control; (3) the likelihood of achieving biological control decreases as the potential number of interspecific interactions increases; and (4) the species composition of biological control agents is of greater consequence than the number of natural enemy species released. In addition, we tested the hypothesis (5) that the frequency of interspecific interactions increases with a decrease in host or prey availability. Our results demonstrate that all combinations of natural enemies provided significant levels of whitefly suppression. While the intensities of interspecific interactions among natural enemy species were frequently positively and significantly correlated with the densities of parasitized whitefly, interspecific interactions among natural enemies were not detrimental to achieving higher levels of biological control. The composition of species released, rather than the number of species released, was of greater importance to accomplishing biological control. Releases of D. pusillus in combination with one or both of the parasitoids provided the greatest levels of whitefly suppression. These results suggest that the types of interspecific interactions rather than the numbers of interspecific interactions among natural enemies may be important to the outcome of inundative biological control programs.

Emergences of Encarsia pergandiella Howard and E. formosa Gahan were compared; and pupal orientation of several Encarsia and Eretmocerus species were examined. The species examined for pupal orientation were E. pergandiella from southwest Florida, E. nr. pergandiella from Brazil, E. transvena (Timberlake) from Florida, E. formosa from Cali- fornia (Davis), E. formosa from Georgia (Griffin), E. formosa from Nile Delta, Egypt, Eret- mocerus sp. from Hong Kong, Eretmocerus sp. from Padappai, India, E. mundus (Mercet) from Murcia, Spain, E. nr. californicus Howard from Arizona (Tucson), parasitoids of Bemisia argentifolii Bellows & Perring (previously B. tabaci (Gennadius)). The majority (98%) of E. pergandiella pupae from Florida and all other Encarsia species except E. nr. pergandiella from Brazil pupated facing the host venter, regardless of leaf orientation, so that rotation was nec- essary before emergence. E. nr. pergandiella from Brazil, 2% of E. pergandiella from Florida, and all species of Eretmocerus pupated facing the host dorsum, so that no rotation was nec- essary. All E. pergandiella pupae and adults from Florida were pigmented similarly regardless of pupal orientation. Less than 1% of E. pergandiella female pupae were oriented backwards, i.e., with the head toward the host posterior, but again, these were not distinguishable char- acteristics of color or form. We concluded that a low incidence of unusual pupal orientations represented natural variation in the population of E. pergandiella we studied. Encarsia pergandiella Howard, E. transvena (Tim- berlake), E. formosa Gahan, and Eretmocerus spp., are parasitoids of the whitefly, Bemisia argentifolii Bellows & Perring, the sweetpotato whitefly, B. ta- baci (Gennadius), and the greenhouse whitefly, Trialeurodes vaporariorum (Westwood). They can play a major role in suppression of whitefly popu- lations in the field and greenhouse (Evans 1993, McAuslane et al. 1993, Stansly et al. 1994). Suc- cessful manipulation of parasitoid populations for enhanced biological control requires more detailed biological and taxonomic information than pres- ently available. Furthermore, biology and taxono- my may be related in subtle ways. For example, Johnson (1996) found 2 color forms of E. pergan- diella in Texas. Pupae of the more common form faced the host venter and had pigmented abdom- inal tergites both as pupae and adults. Abdominal tergites of the less common light form were en- tirely yellow and pupae were oriented facing the host dorsum. Johnson (1996) concluded that the 2 species were characterized by these and other dis- tinctions, and demonstrated mating incompatibili- ties in reciprocal crosses.

Commercial biological control of greenhouse whitefly, Trialeurodes vaporariorum (Westwood) through releases of the parasitoid Encarsia formosa Gahan is used at present on about 5000 ha in most countries with an important greenhouse industry.Although other types of natural enemies of greenhouse whitefly are known (predators and entomopathogenic fungi), it is mainly introductions of parasitoids that have led to economically feasible control. Fundamental research on the relationship among E. formosa, greenhouse whitefly, and host plants, has provided information on how the parasitoid locates and attacks its hosts and how greenhouse climate and plant architecture influence finding of the hosts and parasitization efficiency. The parasitoid is not able to locate infested plants from a distance. Searching is random on all levels, and after a host has been found the search pattern does not alter. The only important change in foraging behavior which was observed is that, in comparison with search times on an uninfected leaf, a parasitoid keeps searching considerably longer (2‐10 times) on a leaf once a whitefly larva has been found or when other indicators of whitefly presence were discovered (e.g., honeydew, exuviae, dead hosts). On a number of important crops, a single E. formosa or her offspring is able to kill more whiteflies per unit of time than an individual whitefly female can produce. On other plant species, the development of whitefly is so fast that seasonal inoculative releases of E. formosa are not sufficient for reliable control and inundative releases have to be made. A stochastic simulation model, which includes (a) the detailed search behavior of the parasitoid and (b) the demographics and distribution of whitefly and parasitoid in relation to host plant and greenhouse climate, is developed to be able (1) to explain the capability ofE. formosato reduce whiteflies in large commercial greenhouses on crops like tomato, (2) to improve introduction schemes of parasitoids for crops where control was difficult, and (3) to predict effects of changes in cropping practices (e.g., greenhouse climate, choice of cultivars) on the reliability of biological control. r 1996 Academic Press, Inc.

Selected life history characteristics of Encarsia formosa Gahan parasitizing the castor whitefly, Trialeurodes ricini (Misra) were studied at three constant temperatures (20, 25 and 30°C). Egg-to-adult developmental time decreased from 32 days at 20°C to 13 days at 30°C. An average of 241 day-degrees was required to complete development above the lower threshold temperature (12°C). Juvenile mortality was 34, 40 and 47% at 20, 25 and 30°C, respectively. Mean longevity of E. formosa ranged from 7.7 days at 20°C, to 2.6 days at 30°C. Mean total fecundity ranged from 18 to 36 eggs/female. Intrinsic rate of increase (rm) increased with temperature from 0.065/day at 20°C to 0.169/day at 30°C. Generation times decreased from 37 to 14.5 days with increasing temperature.

With the aim of improving the dispatches of Encarsia formosa to the agriculturist's greenhouses increasing wasp efficiency, a new method of conditioning by changing the substrate of black pupae was performed. Three groups of parasites to which pupae were glued with or without sugar and compared to a control were studied. This new method of conditioning (pupae were removed from the host-plant then glued on a cardboard plaquette) did not disturb the wasp's biological potentialities (daily fecundity, longevity) and adding sugar increased the daily fecundity.

An attempt at integrated control of four major insect pests, the cotton aphid Aphis gossypii, the two-spotted spider mite Tetranychus urticae, the melon thrips Thrips palmi and the silverleaf whitefly Bemisia argentifolii, was conducted for greenhouse melon cultivated in the summer season, from May to July, utilizing four species of natural enemies, Chrysoperla carnea, Phytoseiulus persimilis, Orius sauteri and Encarsia formosa. The aphid population was effectively controlled at a low density when C. carnea was released 5 times at about 2 week intervals starting from 19 days after planting melon seedling with a ratio of one predator to 20 prey. The spider mite population was also controlled at a low density by releasing P. persimilis 2 times, on the 26th and 37th days after planting, with a ratio of one predator to 20 prey. No damage was caused by these two pests in he released plot, but in the non-released plot severe damage was observed. O. sauteri was released at a ratio of one to 20thrips on the 37th day after planting, but the effect was not evaluated because the thrips population in the non-released plot was also at a ow level during the experiment and no damage was caused. The whitefly population was controlled at a level of one fourth or less of the non-released plot and no economic damage was caused when E. formasa was released 4 times at about 2 week intervals from he 24th day after olantine. with a rate of 100 mammies per 40 plants.

More wasps of Encarsia formosa Gahan (Hymenoptera: Aphelinidae) were found on fertilized poinsettias, Euphorbia pulcherrima (Willd.) (Euphorbiaceae), than on non-fertilized plants. Parasitization of Bemisia argentifolii Bellows & Perring (Homoptera: Aleyrodidae) by E. formosa was higher on plants treated with calcium nitrate than with ammonium nitrate or on control plants. In a no-choice test, host feeding by E. formosa was higher when hosts were on fertilized plants than when hosts were on control plants. The nitrogen content of whitefly pupae reared on plants treated with ammonium nitrate was higher than those on calcium nitrate-treated plants. Variability in the parasitization of B. argentifolii by E. formosa appears to be due to host plant-mediated differences in the whiteflies. E. formosa may be influenced by the nutritional suitability of the host, which influences whether wasps continue to oviposit, feed, or disperse.

We tested the hypothesis that populations of the parthenogenetic parasitic wasp Encarsia formosa Gahan (Hymenoptera: Aphelinidae) differed in their ability to use two different host species, Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) and Trialeurodes vaporariorum Westwood (Homoptera: Aleyrodidae). Of the three wasp populations tested, two populations had been reared for many generations on B. tabaci and one population had been reared for many years on T. vaporariorum. Performance was measured by the number of whitefly nymphs that were successfully parasitized by individual wasps, and performance on either host was measured in separate experiments. There was variation between wasp populations in their performance on the host B. tabaci, with one wasp population reared for many years on this host performing considerably better than the other two populations. There were no significant differences between populations in their use of the preferred host, T. vaporariorum. The experiments were conducted in such a way that we could distinguish heritable differences between populations from environmentally-induced conditioning differences due to the immediate host from which an individual wasp enclosed. In either experiment there were no significant effects of conditioning, although there was a trend within each population for wasps conditioned on T. vaporariorum to have higher performance than those conditioned on B. tabaci. Thirdly, we conducted a selection experiment, initiated with wasps from a single population historically reared on T. vaporariorum, to measure the effect of laboratory rearing on different hosts for 17 generations. We did not see any difference in the performance of wasps on B. tabaci after this period of rearing on either of the two hosts. In summary, populations of E. formosa do differ in their relative performance on B. tabaci. The one population that was tested further did not show any response to selection by rearing, but the ability to respond to selection on performance may not be equal for all populations. The possibility that wasp populations have differential performance on particular hosts may affect the use of this species as a biological control agent.