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With climate change, the occurrence of extreme weather events such as heatwaves is expected to increase. Extreme temperature events are short periods of high temperature (i.e., >95th percentile of temperature distribution), and their occurrences are difficult to predict. These extreme temperature events pose great challenges to living organisms, especially ectotherms such as insects whose body temperature is greatly influenced by environmental temperature. Temperature is known to play an essential role in driving insect performance as well as inducing behavioral changes. Exposure to extreme temperatures causes physiological damage leading to potential long-lasting effects on insect performance. We aimed at assessing the impact of direct (i.e., current) and indirect (previously experienced) exposure to extreme temperature on the oviposition success of Diadegma semiclausum Hellén (Hymenoptera: Ichneumonidae). Diadegma semiclausum is a larval endoparasitoid of Plutella xylostella L. (Lepidoptera: Plutellidae), a worldwide pest of brassicaceous crops. Oviposition success, measured as the percentage of parasitized P. xylostella, was significantly lower at 33 °C than at 25 °C. However, previous exposure to 33 °C did not significantly impact the egg-laying performance of D. semiclausum, indicating that, after a heatwave, the egg-laying performance can recover. Neither the direct nor indirect exposure to extreme temperature altered D. semiclausum host-instar preference. These results indicate that temperature directly impacts D. semiclausum parasitism performance, although more detailed research is needed to disentangle direct physiological mechanisms from behavioral responses. Overall, these study results confirm others, suggesting that control of P. xylostella by D. semiclausum is likely to be impaired by more frequent heatwaves predicted under climate change scenarios.

Ecologists study how populations are regulated, while scientists studying biological pest control apply population regulation processes to reduce numbers of harmful organisms: an organism (a natural enemy) is used to reduce the population density of another organism (a pest). Finding an effective biological control agent among the tens to hundreds of natural enemies of a pest is a daunting task. Evaluation criteria help in a first selection to remove clearly ineffective or risky species from the list of candidates. Next, we propose to use an aggregate evaluation criterion, the pest kill rate, to compare the pest population reduction capacity of species not eliminated during the first selection. The pest kill rate is the average daily lifetime killing of the pest by the natural enemy under consideration. Pest kill rates of six species of predators and seven species of parasitoids of Tuta absoluta were calculated and compared. Several natural enemies had pest kill rates that were too low to be able to theoretically reduce the pest population below crop damaging densities. Other species showed a high pest reduction capacity and their potential for practical application can now be tested under commercial crop production conditions., Centro de Estudios Parasitológicos y de Vectores

Eggplant shoot and fruit borer, Leucinodes orbonalis, is a major pest in eggplant production in South and South-East Asia. Farmers frequently spray insecticides to control it. Integrated pest management (IPM) based on mass trapping or pheromone trapping and sanitation (removal of infested shoots and fruits) has been suggested but poorly adopted. This study tested, together with farmers, combinations of IPM components that fit their farming practices, increase income, and preserve natural enemy populations. A 2-year participatory study was negotiated with these farmers, comparing (i) an untreated control, (ii) farmers’ conventional weekly spraying, with pheromone trapping either (iii) alone, or combined with (iv) trap-based biorational spraying, or (v) bi-weekly conventional insecticide spraying. Farmers rejected testing sanitation as too labor-intensive. In both years, pheromone trapping alone or combined with biorational spraying reduced fruit infestation, increased yield and income, and preserved natural enemies, showing technical efficacy at costs comparable with farmers’ practice. Replacing biorational spraying by conventional insecticides did not provide any control beyond pheromone trapping alone but reduced natural enemies. In contrast, farmers’ practice neither reduced infestation nor increased yield but reduced populations of natural enemies. Aphid and jassid populations were reduced only by biorational and conventional spraying. As farmers were reluctant to use only pheromone trapping, the addition of biorational spraying might be suitable. Discussion with farmers allowed us to understand how practical applicability of the tested IPM depends on farmers’ knowledge levels on insect biology, farmers’ desire to still use some spraying, and labor constraints to sanitation. Although technically and economically viable, the tested IPM may prove difficult to scale out, as farmers had difficulty understanding the lifecycle of Leucinodes orbonalis and the pheromone trapping mechanisms. This study is the first to disentangle the technical efficacy of pheromone-trapping-based IPM from its practical applicability for the targeted smallholder eggplant growers.

Augmentative biological control by predaceous ladybird beetles can be improved by using flightless morphs, which have longer residence times on the host plants. The two-spot ladybird beetle, Adalia bipunctata (L.) (Coleoptera: Coccinellidae), is used for the biological control of aphids in greenhouses and on urban trees. Flightlessness due to truncated wings occurs at very low frequency in some natural populations of A. bipunctata. Pure-breeding strains of this 'wingless' genotype of A. bipunctata can easily be obtained in the laboratory. Such strains have not been commercialized yet due to concerns about their reduced fitness compared to wild-type strains, which renders mass production more expensive. Wingless strains exhibit, however, wide intra-population phenotypic variation in the extent of wing truncation which is related to fitness traits. We here use classical quantitative genetic techniques to study the heritability and genetic architecture of variation in wing truncation in a wingless strain of A. bipunctata. Split-families reared at one of two temperatures revealed strong family-by-temperature interaction: heritability was estimated as 0.64 ± 0.09 at 19 °C and 0.29 ± 0.06 at 29 °C. Artificial selection in opposite directions at 21 °C demonstrated that the degree of wing truncation can be altered within a few generations resulting in wingless phenotypes without any wing tissue (realized h2 = 0.72), as well as those with minimal truncations (realized h2 = 0.61) in two replicates. The latter lines produced more than twice as many individuals. This indicates that selective breeding of wing truncation may be exploited to improve mass rearing of flightless strains of A. bipunctata for commercial biological control. Our work illustrates that cryptic variation can also be a source for the selective breeding of natural enemies.

Eggplant seedling production in homestead nurseries of farmers in Jamalpur (Bangladesh) is greatly compromised by damping-off. Therefore, farmers often do not have enough seedlings to transplant. Effective treatments of soil and seed to reduce disease pressure in the nurseries are available but little is known on the relative contributions of soil-borne and seed-borne pathogens to damping-off and on how nursery management can integrate management options under farmers' conditions. A 2-year nursery study was conducted in consecutive seasons, jointly with farmers and using farmers' preserved seed and farmers' nurseries subject to damping-off problems. Year 1 involved a single nursery, Year 2 nine nurseries. The following treatments were tested: Trichoderma harzianum as soil amendment combined with seed treatment using either hot water or Carbendazim and farmers' conventional practice: curative spraying after appearance of damping-off. In Year 1, a control (no intervention) treatment was also included. Emergence of seedlings, incidence of damping-off, seedling performance variables and farmers' appreciation of seedling vigour were recorded. Soil treatment with T. harzianum combined with seed treatment with hot water increased seedling emergence and produced 25–64 percentage points more healthy seedlings than farmers' conventional practice. This combined treatment also improved seedling performance (height, root length, lateral root development), and reduced stem girdling, a symptom associated with disease infection of transplants. To determine the contributions of nursery soil and farmers’ seed to damping-off, blotter and in vitro studies in the laboratory and tray studies in a screen house were performed. Soil proved to be the major pathogen source and treating nursery soil with T. harzianum had the largest positive effect on seedling performance, both reducing damping-off and enhancing seedling growth.

Host plant resistance has received considerable attention for the management of insect herbivores on crop plants. However, resistance is threatened by the rapid adaptation of target herbivores towards virulence (the ability to survive, develop and damage a host with major resistance genes). This study examines the potential costs and benefits of adaptation for virulence in herbivores. We continuously reared planthoppers, Nilaparvata lugens, on two susceptible and three resistant rice, Oryza sativa, varieties for 20 + generations. We then assessed the performance of selected planthoppers across a range of rice lines with distinct resistance genes. We found that planthoppers with long-term exposure to resistant hosts (particularly IR62 with the Bph3(t) and BPH32 gene loci, and PTB33 with the Bph3(t), BPH32 and BPH26 gene loci) gained virulence against related varieties with the same and different resistance genes, but planthoppers adapted to the resistant host IR65482-4-136-2-2 (BPH10 locus) had reduced performance on phylogenetically distant plants with distinct resistant genes. In choice bioassays, avirulent planthoppers showed marked preferences for susceptible lines, whereas virulent planthoppers were less selective of varieties. We also examined whether virulence was associated with insecticide susceptibility. We tested susceptibility to three insecticides using a topical application method. Populations selectively reared on IR65482-4-136-2-2 had increased susceptibility to imidacloprid and fipronil, representing a possible trade-off with virulence. In contrast, a population with virulence to the highly resistant variety PTB33 was 4.88 × more resistant to imidacloprid and 3.18 × more resistant to BPMC compared to planthoppers of the same origin but reared on the susceptible variety IR22. Our results suggest complex relations between insecticide resistance and virulence that vary according to insecticidal toxins and resistance genes, and include potentially increased insecticide-susceptibility (a trade-off) as well as common detoxification mechanisms (a benefit)., Graphical abstract Image 1, Highlights • Costs and benefits of Nilaparvata lugens adaptation to resistant rice are assessed. • N. lugens selected on IR65482-4-136-2-2 had reduced performance on other hosts. • N. lugens selected on PTB33 and IR62 gained virulence against other resistant hosts. • Virulence against IR65482-4-136-2-2 was associated with insecticide susceptibility. • Virulence against PTB33 was associated with resistance to two insecticides.

Bacterial wilt (causal agent: Ralstonia solanacearum) and Phomopsis fruit rot (causal agent: Phomopsis vexans) are two major diseases constraining eggplant production in Bangladesh. No integrated management options to manage both diseases together are currently available; farmers spray pesticides. A 2-year participatory study was conducted to compare yield and disease severity of plants that originated from preceding nursery studies. Both soil and seed were found to be source of these pathogens. Seedlings raised according to farmers’ practice or in nursery soil treated with Trichoderma harzianum from seed treated either with hot water or Carbendazim were transplanted on fields either with or without soil application of T. harzianum. The seed treatments had comparable effects. Improved nursery management alone reduced wilt and fruit rot by 25–40 and 14–20 percentage points respectively, increased marketable yield by 8–19 t ha−1 and income by €1800-3700 ha−1 compared to farmers’ conventional practice of spraying. In Year 1, a comparable result was obtained treating only the field but not the nursery; farmers, however, did not want to repeat this treatment in Year 2 as costs were higher. Further disease reduction (10–11 and 0.6–6 percentage points respectively for wilt and rot) occurred when improved seedlings were transplanted into T. harzianum amended fields and consequently marketable yield increased by 5–7 t ha−1 and income by €500-900 ha−1. Both labour and material costs of the introduced technology were lower than the costs of current farmers’ practice. Given costs of labour and inputs and eggplant price fluctuations over years, farmers are recommended to use the improved nursery management practices. Also treating the field may depend on the capacity of farmers to invest in the additional labour and input. In fields with improved management, T. harzianum persisted at the end of the growing season and reduced populations of R. solanacearum and P. vexans. Also, seed preserved from improved management plots showed reduction of both pathogens compared to seed from farmers’ practice plots. The reduction of pathogens both in field soil and preserved seed for improved management plots may imply that a next crop will start with a lower level of disease pressure. Laboratory tests showed nursery raised seedlings from screenhouse experiments were always found infected with R. solanacearum, but soil application of T. harzianum alone or combined with hot water treatment of seed substantially reduced the infection. Given the positive results of our study, the technology should be made locally available to support eggplant IPM.

Mating behaviour often increases predation risk, but the vulnerability within mating pairs differs between the sexes. Such a sex difference is expected to lead to differences in responses to predation risk between the sexes. In the two-spotted spider mite Tetranychus urticae, males engage in pre-copulatory mate guarding because only the first mating results in fertilisation. We investigated (i) whether pre-copulatory pairs are more conspicuous to the predatory mite Phytoseiulus persimilis than solitary females, (ii) whether the vulnerability to the predator differs between sexes within the pre-copulatory pair, (iii) whether each sex of T. urticae responds to predation risk during pre-copulatory mate guarding and (iv) whether T. urticae's response to predation risk affects predator behaviour. Because T. urticae females are immobile during pre-copulatory mate guarding, we observed male behaviour to evaluate effects of predation risk. We found that the predators detect more pre-copulatory pairs than solitary females and that more females than males of the pre-copulatory pairs are preyed upon by the predators. The preference of spider mite males for pre-copulatory pairs versus solitary females was affected by whether or not the female had been exposed to predators during development. Male T. urticae exposed to predation risk did not alter their behaviour. These results suggest that only the most vulnerable sex, that is the female, responds to predation risk, which modifies male behaviour. Regardless of T. urticae females’ experience, however, P. persimilis detected more T. urticae pre-copulatory pairs than solitary females, suggesting that pre-copulatory mate guarding itself is dangerous for T. urticae females when these predators are present. We discuss our results in the context of sex-dependent differences in predation risk.

The harlequin ladybird, Harmonia axyridis, is an important natural enemy of aphids throughout the world, but is now also considered an invasive alien species. We performed a meta-analysis of published life history data to address the question whether invading populations in Europe and North America have life history parameters that differ from native populations in Asia, explaining the beetle’s invasion success in new territories. In this meta-analysis, we accounted for important covariables that are often reported in published studies such as temperature, food source (aphids or eggs of Ephestia kuehniella), strain (laboratory or field populations) and photoperiod. Temperature was a key factor having consistent large effects on development rate, survival and reproductive characteristics of H. axyridis. Food source, strain, and photoperiod had effects on some, but not all characteristics, and their overall effect across characteristics was minor. Individuals of invasive populations had a shorter pre-oviposition period and higher fecundity at low temperatures than those of native populations, and a greater longevity across all temperatures. No differences in survival were found between native and invasive populations, while differences in development rate were not consistent, with opposing results obtained according to the way development rate was measured in trials reported in the literature. Results of this meta-analysis support the hypothesis that the life history of the beetle has changed during its invasion into North America and Europe. Invasive populations had a shorter pre-oviposition period and higher fecundity at low temperatures, as well as a greater longevity across all temperatures than native populations. These differences may partially explain the invasive success of H. axyridis.

Under predicted global climate change, species will be gradually exposed to warmer temperatures, and to a more variable climate including more intense and more frequent heatwaves. Increased climatic variability is expected to have different effects on species and ecosystems than gradual warming. A key challenge to predict the impact of climate change is to understand how temperature changes will affect species interactions. Herbivorous insects and their natural enemies belong to some of the largest groups of terrestrial animals, and thus they have a great impact on the functioning of ecosystems and on the services these ecosystems provide. Here we studied the life history traits of the plant-feeding insect Plutella xylostella and its specialist endoparasitoid Diadegma semiclausum, when exposed to a daily heat pulse of 5 or 10°C temperature increase during their entire immature phase. Growth and developmental responses differed with the amplitude of the heat pulse and they were different between host and parasitoid, indicating different thermal sensitivity of the two trophic levels. With a +5°C heat pulse, the adult parasitoids were larger which may result in a higher fitness, whereas a +10°C heat pulse retarded parasitoid development. These results show that the parasitoid is more sensitive than its host to brief intervals of temperature change, and this results in either positive or negative effects on life history traits, depending on the amplitude of the heat pulse. These findings suggest that more extreme fluctuations may disrupt host-parasitoid synchrony, whereas moderate fluctuations may improve parasitoid fitness.

Aphids can cause major environmental problems in urban areas. One important problem is the annual outbreaks of lime aphid, Eucallipterus tiliae (L.) (Hemiptera: Aphididae), which spoil the surroundings of lime trees by depositing honeydew. To date no environmentally friendly method has been demonstrated to yield effective control of lime aphids. Attempts are made in some cities to control lime aphids by releasing larvae of the native two-spot ladybird beetle, Adalia bipunctata (L.) (Coleoptera: Coccinellidae). However, it is known that adult ladybird beetles disperse soon after release, and there is little indication they provide control of the aphids. Here, we demonstrate experimentally that releases of a flightless strain of A. bipunctata, obtained from natural variation in wing length, can reduce the impact of honeydew from lime aphid outbreaks on two species of lime in an urban environment. Both larvae and adult beetles were released, and we discuss the contribution of the flightless adults to the decline in honeydew.

A central issue in the evolutionary ecology of species interactions is coevolution, which involves the reciprocal selection between individuals of interacting species. Understanding the importance of coevolution in shaping species interactions requires the consideration of spatial variation in their strength. This is exactly what the, recently developed, geographic mosaic theory of coevolution addresses. Another major development in the study of population ecology is the introduction of the population genomics approach in this field of research. This approach addresses spatial processes through molecular methods. It is of particular interest that population genomics is especially applicable to natural populations of non-model species. We describe how population genomics can be used in the context of the geographic mosaic of coevolution, specifically to identify coevolutionary hot-spots, and to attribute genetic variation found at specific loci to processes of selection versus trait remixing. The proposed integration of the population genomics approach with the conceptual framework of the geographic mosaic of coevolution is illustrated with a few selected, particularly demonstrative, examples from the realm of insect--plant interactions.

In the Danish region of Kvaerkeby, a mutation in an, as yet, unknown single autosomal gene has resulted in a dominant resistance (R-) allele in the flea beetle Phyllotreta nemorum L. (Coleoptera: Chrysomelidae: Alticinae). It enables the beetle to overcome the defences of Barbarea vulgaris ssp. arcuata (Opiz.) Simkovics G-type (Brassicaceae) and use it as a host plant. In this study, we investigated the pleiotropic effects associated with the presence of this particular R-allele in female P. nemorum. These females had the R-allele backcrossed into the genetic background of non-resistant beetles. The effects were investigated under both favourable and stressful conditions (cold shock). The presence of the R-allele in a non-resistant genetic background caused a very high mortality in resistant individuals during the early stages of development under both conditions, but it did not affect the adult life-history traits longevity, body size and fecundity, under both conditions. Regardless of temperature treatment, resistant females in general were found to lay significantly more eggs. Developmental stability, as measured by tibia length fluctuating asymmetry, was not correlated with overall developmental stress in this study.

1 Linking new developments in genomics with ecology provides interesting novel tools to address ecological questions in ways that have not been possible up to now. In this paper we address this issue for the ecology of infochemicals and plant?insect interactions in a food web context. 2 Plants are at the basis of most terrestrial food webs and insects are a dominant animal group interacting with plants. Insect?plant communities are characterized by direct and indirect interactions, many of which are mediated by infochemicals. Plants respond to insect herbivory with the production of volatiles that attract the enemies of the herbivores, such as insect predators and parasitoids. Moreover, the plant volatiles may be exploited by any organism in the environment and this results in many more infochemically mediated interactions. Thus, a food web is overlaid with an infochemical web. 3 In the past, several manipulative tools have been developed to investigate the role of infochemicals. The rapid advancement of molecular genetics and `-omics? technologies results in interesting new tools. A recent development is the use of well-characterized genotypes that are modified in the mechanisms underlying the induced plant volatiles. These genotypes produce precisely manipulated phenotypes that often differ in only a single gene, and can be used to investigate the effect of particular genes on specific interactions. Moreover, these genotypes can be introduced into a natural community to assess the effects of the genetic change and its resulting phenotypic change on interactions with the entire natural community. 4 Furthermore, with the progress in microarray technology it becomes possible to assess the expressed genotype of plants in the field, which can be exploited to investigate expressed genetic variation under field conditions. 5 These developments are expected to be only the beginning of a successful integration of -omics technologies, such as transcriptomics and metabolomics, with community ecology into the new research field of ecogenomics. 6 In this review we present the current status and discuss the prospects for the future of an ecogenomic approach to the role of herbivore-induced plant volatiles in insect?plant community ecology.

This study examines the three-way interaction between symbionts, insect herbivores and their host plants during adaptation to resistant crop varieties. We conducted a long-term selection study (20 generations of continuous rearing) with a monophagous phloem-feeder, the brown planthopper [Nilaparvata lugens (Stål)], on several resistant rice (Oryza sativa L.) varieties. Planthopper fitness and the abundance of yeast-like symbionts (YLS) were monitored throughout the selection process. N. lugens populations collected from six regions in the Philippines adapted to the resistant varieties as noted by increasing body size and increased egg-laying. Adaptation was partially through physiological and behavioral changes apparent during feeding: Planthoppers on resistant plants had relatively high levels of xylem feeding compared with planthoppers on susceptible plants. YLS densities were highly dependent on the host rice variety. However, there were no consistent trends in YLS density during host plant switching and virulence adaptation: Compared to densities in planthoppers on the standard susceptible variety Taichung Native 1 (TN1), YLS densities were consistently higher on PTB33 (resistant), similar on IR62 (resistant) and IR65482 (moderately resistant) but lower on IR22 (susceptible). Furthermore, YLS densities often remained the same despite improved planthopper fitness over generations. Our results do not support the hypothesis that changes in YLS density mediate planthopper adaptation to resistant varieties. However, slight reductions in YLS densities toward the end of selection on TN1, IR22 and IR62 may indicate that YLS have lower functional significance where varieties and environmental conditions are constant between generations.

Proximate factors affecting animal behavior include stimuli generated by conspecifics. In spider mites of the genus Tetranychus (Acari: Tetranychidae), males guard pre-reproductive quiescent females, because only the first mating results in fertilization. In a dual-choice experiment, more adult males of T. urticae were attracted to females guarded by a male than to solitary females. Because spider mites are known to perceive volatiles, we hypothesized that guarded and solitary females differ in the volatile blends emitted. To test this hypothesis, headspace volatiles of guarded females, solitary females, and solitary males were collected, respectively. GC/MS analysis detected octanal, methyl salicylate, ethyl 4-ethoxybenzoate, and methyl cis-dihydrojasmonate in all of the groups. Orthogonal Projection to Latent Structures Discriminant Analysis (OPLS-DA) of the blends clearly discriminated guarded females from solitary females, supporting our hypothesis. Individual compounds did not show significant difference in emission rates for guarded females vs. solitary females, suggesting that differences lay in the total blend composition. OPLS-DA did not discriminate between the blends emitted by guarded females and solitary males. In conclusion, the differences in the volatile blends are likely to mediate male discrimination between guarded and solitary females.

We report a case of genetic linkage between the two major loci underlying different wing traits in the two-spot ladybird beetle, Adalia bipunctata (L.) (Coleoptera: Coccinellidae): melanism and winglessness. The loci are estimated to be 38.8 cM apart on one of the nine autosomes. This linkage is likely to facilitate the unravelling of the genetics of these traits. These traits are of interest in the context of the evolution of intraspecific morphological diversity, and for the application of ladybird beetles in biological control programs.

Since the establishment of Harmonia axyridis in Europe, populations of native ladybirds have decreased. Overwintering survival is one of the aspects of the biology of H. axyridis that may contribute to its firm establishment in and invasion of a new area. In this study winter survival of five wild H. axyridis populations was assessed under natural and semi-natural conditions, with a focus on the potential influence of location and orientation on winter survival. Overwintering survival of H. axyridis in the Netherlands is high: 70.8–88.2%. When overwintering at one central site, populations sampled at five locations showed statistically significant different mortality rates. Furthermore, winter survival of H. axyridis at the sample sites was higher when beetles were hibernating at the southwestern sides of buildings, where most aggregations of ladybirds were found. Survival was higher at sheltered sites compared to exposed sites. Harmonia axyridis has a comparable or higher overwintering survival than most common native ladybird species. A high overwintering survival results in a large post-hibernation population in spring, leading to a rapid population build-up. Thus, the high winter survival probably contributes to the success of the exotic H. axyridis .

Females of the larval parasitoid of Drosophila, Asobara citri, from sub-Saharan Africa, defend patches with hosts by fighting and chasing conspecific females upon encounter. Females of the closely related, palearctic species Asobara tabida do not defend patches and often search simultaneously in the same patch. The effect of patch defence by A. citri females on their distribution in a multi-patch environment was investigated, and their distributions were compared with those of A. tabida. For both species 20 females were released from two release-points in replicate experiments. Females of A. citri quickly reached a regular distribution across 16 patches, with a small variance/mean ratio per patch. Conversely, A. tabida females initially showed a clumped distribution, and after gradual dispersion, a more Poisson-like distribution across patches resulted (variance/mean ratio was closer to 1 and higher than for A. citri). The dispersion of A. tabida was most probably an effect of exploitation: these parasitoids increasingly made shorter visits to already exploited patches. We briefly discuss hypotheses on the adaptive significance of patch defence behaviour or its absence in the light of differences in the natural history of both parasitoid species, notably the spatial distribution of their hosts.

The concept of ‘‘enemy-free space’’ (EFS) refers to ways of living that reduce or eliminate the vulnerability of a species to natural enemies. It has been invoked to explain host shifts of phytophagous insects. A demonstrated cause of EFS is escape from enemies in time, through phenological mismatching of herbivore development and enemy occurrence, leading to low percentages of predation/parasitism of herbivores occurring at a certain time. The mere measurement of percentage parasitism, however, is not sufficient to demonstrate EFS in certain cases. Here we present such a case, where parasitism was studied of a phytophagous insect (Phyllotreta nemorum), using two different host plant species in the field: an atypical, relatively rarely used, plant (Barbarea vulgaris), and a more widely used one (Sinapis arvensis). At one location we found a paradoxical result: on each separate sampling day throughout the season the percentage of parasitism of P. nemorum using a patch of B. vulgaris was not significantly different from, or even significantly higher than on a nearby patch of S. arvensis. The overall season-wide proportion parasitism of the flea beetle cohort using the B. vulgaris patch, however, was lower. We conclude that, in the year and at the location we studied, the patch of B. vulgaris provided enemy-free space to the herbivore in the form of a temporal refuge, and that the importance of enemy-free space in the use of an atypical host plant should be evaluated on the basis of season-wide sampling, including estimation of host population size.

The flea beetle Phyllotreta nemorum L. (Coleoptera: Chrysomelidae) is genetically polymorphic for resistance against the defences of one of its host plants, Barbarea vulgaris R.Br. (Brassicales: Brassicaceae). Whereas resistant flea beetles are able to use B. vulgaris as well as other cruciferous plants as food, non-resistant beetles cannot survive on B. vulgaris. This limitation to host plant use of non-resistant beetles could potentially lead to asymmetric gene flow and some degree of genetic isolation between the different resistance-genotypes. Therefore, we studied the extent of genetic differentiation at neutral allozyme loci between samples of flea beetles that were collected at different locations and first tested for resistance phenotype. Since earlier work has shown a weak, but significant, effect of geographical distance between the samples on their genetic differentiation, in the present study variation at the neutral allozyme loci in P. nemorum was partitioned between geographical distance and resistance-phenotype. Both sources independently contributed statistically significantly to population differentiation. Thus, there appears to be a limitation to genetic exchange between the resistant and non-resistant flea beetles when corrections are made for their geographic differentiation. This is consistent with the presence of some degree of host race formation in this flea beetle.

The challenge of using ladybird beetles for biological control of insect pests such as aphids is that the adult beetles tend to fly away from the host plants. Therefore, flightless ladybirds might improve biocontrol. There are several artificial ways to obtain flightless beetles, but it may be preferable to use natural variation in flight ability. We investigated, for the first time, biocontrol by inundative augmentation of natural flightless morphs of the ladybird beetle Adalia bipunctata. Microcosm experiments using single leaves with one of three species of aphid revealed no differences in consumption behavior between flightless and winged beetles. Monitoring for 48 h of single, caged pepper plants infested with aphids of Myzus persicae nicotianae or Aulacorthum solani showed that flightless beetles had a longer residence time on the plants than winged beetles. This only translated into significantly better biocontrol of M. persicae. Despite their difference in residence time, both beetle morphs reduced the population growth of A. solani. This is probably explained by the tendency of A. solani to drop from the plant upon disturbance, and we predict that flightless beetles may outperform winged ones in the long term. Overall, our results provide a proof of principle that natural flightless A. bipunctata can improve biocontrol of aphids by ladybird beetles. However, we recognize that the effect of biocontrol will vary with the species of aphid used and that further examination in long term and large scale experiments is required.

The flea beetle, Phyllotreta nemorum L. (Coleoptera: Chrysomelidae), is an intermediate specialist feeding on a small number of plants within the family Brassicaceae. The most commonly used host plant is Sinapis arvensis L., whereas the species is found more rarely on Cardaria draba (L.) Desv., Barbarea vulgaris R.Br., and cultivated radish (Raphanus sativus L.). The interaction between flea beetles and Barbarea vulgaris ssp. arcuata (Opiz.) Simkovics seems to offer a good opportunity for experimental studies of coevolution. The plant is polymorphic, as it contains one type (the P-type) that is susceptible to all flea beetle genotypes, and another type (the G-type) that is resistant to some genotypes. At the same time, the flea beetle is also polymorphic, as some genotypes can utilize the G-type whereas others cannot. The ability to utilize the G-type of B. vulgaris ssp. arcuata is controlled by major dominant genes (R-genes). The present investigation measured the frequencies of flea beetles with R-genes in populations living on different host plants in 2 years (1999 and 2003). Frequencies of beetles with R-genes were high in populations living on the G-type of B. vulgaris ssp. arcuata in both years. Frequencies of beetles with R-genes were lower in populations living on other host plants, and declining frequencies were observed in five out of six populations living on S. arvensis. Selection in favour of R-genes in populations living on B. vulgaris is the most likely mechanism to account for the observed differences in the relative abundance of R-genes in flea beetle populations utilizing different host plants. A geographic mosaic with differential levels of interactions between flea beetles and their host plants was demonstrated.

Eretmocerus species (Hym. Aphelinidae) are solitary parasitoids of Bemisia tabaci (Gennadius). Mate finding and mating behavior of two species, E. mundus and E. eremicus, were studied under laboratory conditions. We used three populations of Eretmocerus: typical arrhenotokous populations of E. eremicus ( from USA) and E. mundus ( from Spain), and an atypical thelytokous population of E. mundus ( from Australia). We studied the intra- and interspecific responses of males to volatile and nonvolatile components of the female sex pheromones, mating behavior, and hybridization between populations and species. In both arrhenotokous populations, males reacted to volatile pheromones by walking toward conspecific virgin females. Males also reacted to nonvolatile pheromones by spending more time on and around patches on leaves of poinsettia plants that had been exposed to virgin females. Males of E. eremicus showed the same reaction to the nonvolatile sex pheromone of E. mundus females, but E. mundus males did not show any reaction to the nonvolatile sex pheromone of E. eremicus. There was no response of males of both species to thelytokous females of E. mundus. In both species three phases were distinguished in the mating behavior: premating, mating, and postmating. The duration of the phases differed between the three populations. Successful copulation between the two Eretmocerus species did not occur. In contrast, we recorded some successful copulations between Australian males and Spanish females of E. mundus, but they did not produce any hybrid females.

The view of (insect) populations as assemblages of local subpopulations connected by gene flow is gaining ground. In such structured populations, local adaptation may occur. In phytophagous insects, one way in which local adaptation has been demonstrated is by performing reciprocal transplant experiments where performance of insects on native and novel host plants are compared. Trade-offs are assumed to be responsible for a negative correlation in performance on alternative host plants. Due to mixed results of these experiments, the importance of trade-offs in host plant use of phytophagous insects has been under discussion. Here we propose that another genetic mechanism, the evolution of coadapted gene complexes, might also be associated with local adaptation. In this case, however, transplant experiments might not reveal any local adaptation until hybridization takes place. We review the results we have obtained in our work on the host plant use of the flea beetle Phyllotreta nemorum L. (Coleoptera: Chrysomelidae: Alticinae), and propose a hypothesis involving coadapted genes to explain the distribution of genes that render P. nemorum resistant to defences of one of its host plants, Barbarea vulgaris R. Br. (Cruciferae).

After establishment of Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae) in Europe, population densities of native ladybird species have decreased. The post-hibernation onset of female reproduction, a key characteristic influencing population dynamics and competition with related species, was studied. Hibernating individuals were collected and transferred to outdoor cages to continue overwintering. Every two weeks a sample of individuals was transferred to long-day, warm conditions. Intensity of dormancy was studied by determining the pre-oviposition period and ovarian development. Pre-oviposition periods were short throughout our observations, indicating that Harmonia axyridis was not in diapause but in a quiescent state. H. axyridis becomes active rapidly when temperature rises in spring but is not active earlier in the year than native species. Neither the mode of overwintering, nor the onset of spring activity can explain the invasion success of H. axyridis.