Search

Government agencies develop invasive species management programmes assuming early detection is key to successful management. Some theoretical studies confirm this intuition, while others, which restrict sampling effort to be constant in time, suggest managers are investing too heavily in sampling to detect new local invader populations. We explore whether these optimal constant-effort strategies underplay the importance of early surveillance and determine how much changing sampling effort through time reduces total management costs. Using optimal control theory to calculate time-dependent surveillance policies that minimize the total cost of sampling, eradication, and damage by the invasive, we find that the best strategies often use intense early sampling, followed by reduced sampling effort. Intense early sampling can drastically reduce costs compared with constant-effort strategies if propagule pressure from outside the managed area is low. However, if new infestations arise from frequent independent introductions from an outside source, constant strategies are cost-effective. Synthesis and applications. For invasive species that are initially present, not frequently reintroduced into the managed area, and for which surveillance is reasonably cost-effective, we recommend a simple three-phase management programme that deploys intense early surveillance until the majority of undetected populations have been discovered, followed by medium effort until most of the heavily infested areas have been cleared of the invader and finally low long-term effort to prevent infestations caused by future introductions and spread from populations that escaped surveillance. This programme captures the majority of the economic benefits from varying sampling effort continuously through time.

Grapevine leafroll disease threatens the economic sustainability of the grape and wine industry in the United States and around the world. This viral disease reduces yield, delays fruit ripening, and affects wine quality. Although there is new information on the disease spatial-dynamic diffusion, little is known about profit-maximizing control strategies. Using cellular automata, we model the disease spatial-dynamic diffusion for individual plants in a vineyard, evaluate nonspatial and spatial control strategies, and rank them based on vineyard expected net present values. Nonspatial strategies consist of roguing and replacing symptomatic grapevines. In spatial strategies, symptomatic vines are rogued and replaced, and their nonsymptomatic neighbors are virus-tested, then rogued and replaced if the test is positive. Both nonspatial and spatial classes of strategies are formulated and examined with and without considering vine age. We find that spatial strategies targeting immediate neighbors of symptomatic vines dominate nonspatial strategies, increasing the vineyard expected net present value by 18% to 19% relative to the strategy of no disease control. We also find that age-structured disease control is preferred to non-age-structured control but only for nonspatial strategies. Sensitivity analyses show that disease eradication is possible if either the disease transmission rate or the virus undetectability period is substantially reduced.

This study evaluated whether experience of Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae) on a poinsettia cash crop, Euphorbia pulcherrima Willd. ex Koltz (Euphorbiaceae), influences the effectiveness of an eggplant trap crop, Solanum melongena L. (Solanaceae). Two whitefly strains were tested: one was reared on poinsettia (poinsettia-strain) and a second was reared on bean [Phaseolus vulgaris L. (Fabaceae)] (bean-strain). We first determined whether host experience altered the preference of adult whiteflies for eggplant and their survivorship on poinsettia. Then, we determined whether changes in the preference and/or survivorship influenced the effectiveness of the trap cropping. Adult whiteflies from both strains consistently redistributed and settled on an eggplant trap crop in significantly higher numbers compared to poinsettia. The adult survivorship of the poinsettia-strain whiteflies was slightly higher on poinsettia than on the bean-strain. In research greenhouse experiments, we found that the trap cropping consistently resulted in a decrease in the density of the poinsettia-strain whiteflies on the cash crop compared to that in monoculture. However, higher adult whitefly survivorship on eggplant than on poinsettia could compromise its effectiveness as a trap crop in poinsettia. The effectiveness of trap cropping, as reflected by the whitefly density reduction on a poinsettia cash crop, was significantly smaller than the attractiveness of the trap crop, as indicated by the whitefly abundance on an eggplant trap crop.

Habitat complexity can mediate interactions among predators and herbivores and influences arthropod population density and community structure. The abundance of many predatory mites (Acari: Phytoseiidae) is positively associated with abundance of non-glandular trichomes. We hypothesized that (1) increasing the complexity (trichome density mimicked with cotton fiber patches) of the habitat that predatory mites encounter on leaves would reduce adult dispersal from plants, and (2) increasing habitat complexity would reduce the time that mites spend walking. Typhlodromus pyri Scheuten retention on plants increased linearly in the presence of trichome mimics; mites placed on plants lacking leaf trichomes showed a behavioral response that led to active dispersal. Phytoseiid retention increased with both fiber patch size and fiber density within patches. Moving fiber patches from the underside of the leaf to the upper leaf surface did not change phytoseiid retention but did alter egg distribution, suggesting trichomes do not exclusively influence phytoseiid behavior. Phytoseiid activity level as measured by the amount of time spent walking did not decrease with the addition of fibers. Overall, increasing habitat complexity in the form of non-glandular trichomes strongly reduced T. pyri dispersal behavior; the predatory mites showed a consistent preference for complex trichome-rich habitat that was manifest both rapidly and in absence of predators. Hence, the frequently observed pattern of population-level accumulation of phytoseiids on trichome-rich plants appears to be driven by a behavioral response to the presence and abundance of non-glandular trichomes on the leaf surface manifested in the level of dispersal and/or retention. The primary implication of phytoseiid-habitat interactions for biocontrol programs is that where plants have no trichomes, T. pyri will not establish. Whether this behavioral response pattern is a general response of phytoseiids to leaf trichomes or varies with species is a question that remains unanswered.

Trap cropping, though promising, has had little evaluation in greenhouses. This study evaluated eggplant, Solanum melongena L. (Solanaceae), as a trap crop for two whitefly species, Trialeurodes vaporariorum (Westwood) and Bemisia argentifolii Bellows & Perring (both Hemiptera: Aleyrodidae), on greenhouse poinsettia, Euphorbia pulcherrima Willd. ex Koltz (Euphorbiaceae). Because the two whitefly species co-occur in greenhouses, a common trap crop for both whiteflies is desirable. When adults were provided a choice between eggplant and poinsettia in a cage, 60% of B. argentifolii and 98% of T. vaporariorum were observed on eggplant after 3 days. However, when adults were given eggplant after first settling on poinsettia, only 38% of B. argentifolii were later found on eggplant, whereas 95% of T. vaporariorum moved to eggplant. In a greenhouse experiment, eggplant did not affect either the spatial distribution or density of adult B. argentifolii on poinsettias. In contrast, eggplant changed the spatial distribution of T. vaporariorum within 3 days by attracting and retaining the adults. However, the attractiveness of eggplant did not result in a reduced number of T. vaporariorum on poinsettias compared with poinsettias in monoculture. Adult T. vaporariorum mortality was high on poinsettias and this likely caused adult density on poinsettias in monoculture to decrease as fast as that under trap cropping. A simulation model was developed to examine how adult whitefly mortality on poinsettia influences trap cropping effectiveness. When whitefly mortality was high, simulated densities were similar to the experimental data. This reveals an unexpected factor, pest mortality on the main crop, that can influence the relative effectiveness of trap cropping. Our results indicate that eggplant will not work unilaterally as a trap crop for B. argentifolii. For T. vaporariorum, attraction to eggplant might be useful as a trap crop system when whitefly mortality on the main crop is not high.

In New York, onion [Allium cepa L. (Alliaceae)] fields often border woods or other vegetable fields. Because onion maggot adults, Delia antiqua (Meigen) (Diptera: Anthomyiidae), spend a significant portion of their time outside of onion fields, surrounding habitat may affect patterns of fly activity and oviposition within onion fields. To better understand these patterns throughout the onion-growing season, first-, second-, and third-flights maggot adult (male and female) activity was monitored using yellow sticky cards. Half of the monitored fields bordered woods, whereas the other half bordered other onion or vegetable fields. Within all fields, yellow sticky cards were placed at five distances along a transect beginning at the onion field edge extending into the field center. First-flight male and female adult activity was greatest along onion field edges and was especially high along edges bordering woods. This pattern diminished during the second flight and was absent during the third. To determine if spatial patterns of onion maggot oviposition by first-flight onion maggots were similar to first-flight adult activity patterns, potted onion plants were placed in onion fields that bordered or did not border woods in late May and early June 2003. The number of eggs laid in the soil at the base of each plant was recorded. Unlike spatial patterns of first-flight adult activity, oviposition patterns were not affected by bordering habitat or distance from the field edge. Based on the activity of onion maggot adults in onion fields, future and existing control strategies should consider targeting first-flight adults along field edges rather than across entire fields, especially in fields that border woods. In contrast, based on spatial patterns of oviposition within onion fields, controls targeting onion maggot larvae should be applied on a fieldwide basis.

One component of developing a systematic approach for deployment of trap crops is to understand how the trap crop modifies pest behavior. Glossy-leafed collards, Brassica oleracea L. var. acephala (Brassicaceae), were evaluated as a potential trap crop for diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), because they are attractive to P. xylostella adults and are a poor host for P. xylostella larvae compared to cabbage, Brassica oleracea L. var. capitata . We used large field plots to measure the changes in adult, egg, and larval P. xylostella densities in cabbage when the trap crop was planted in the field. Furthermore, we planted the trap crop in dispersed and concentrated spatial arrangements to determine the impact of trap crop arrangement on the behavior of P. xylostella . In 2002, results showed that the presence of collards within a cabbage field reduced larval density on cabbage. In 2003, neither trap crop arrangement had a significant impact on P. xylostella larval density on cabbage. Adult moths aggregated in proximity to collards in 2002, but not in 2003. Egg and larval data in both years in all treatments showed that total oviposition was highest near a central release point, indicating that females lay many eggs before dispersing very far when suitable host plants are available. The mean direction of P. xylostella movement and oviposition from a central release point was not consistent or correlated to wind direction. Plant size of the trap crop in relation to the main crop and environmental factors may have been responsible for the inconsistent effectiveness of the trap crop.

A cornerstone of spatial ecology is the quantification of the patchy nature of animal and plant populations in space. By using spatial covariance, total covariance, and quantile variance, we found that patchiness of Japanese beetle grub populations varied more between years than between fairways at a central New York golf course. We also observed that populations tended to shrink and swell around patches with perennially low density, that locations with perennially low grub density were associated with high soil organic matter content, and that locations with frequently high grub density were associated with intense adult beetle activity in the vicinity.

Insect populations tend to be patchy, and the nature of the patches is a critical component of ecology. Predator-prey interactions, coexistence of competing species, survival of rare species as habitat is destroyed, and damage to crops are just a few examples of spatially-dependent ecological processes. For want of tractable quantitative approaches, understanding of spatial ecology has lagged far behind recognition of its importance. We assert that a quantitative foundation of a spatial ecology involves the reification of patches as objects of study. We introduce two new measures of patch dynamics: total covariance for comparing degrees of patchiness between populations, and quantile variance for quantifying the constancy of dispersion patterns through time. These new measures, in combination with the long-established spatial covariance from geostatistics, comprise a rudimentary toolbox for reification of patches and empirical field studies in spatial ecology.

To determine whether diverse populations and biotypes of Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae) can be mass-produced and stored under uniform conditions, we assessed reproduction in five North American populations, four from the carnea biotype and one from the mohave biotype. Non-diapause adults from the carnea biotype varied significantly in only one aspect of reproduction, viz., the preoviposition period, whereas the duration and rate of oviposition, as well as fecundity, did not vary significantly. Relative to the carnea populations, the mohave population typically had delayed oviposition and a lower rate of oviposition; however, its oviposition period was long, and thus its fecundity differed only from that of the most productive carnea populations. The shelf-life (= survival during storage, and subsequent survival and reproduction after storage) was evaluated for four populations of the carnea biotype. Survival of diapausing adults during the first 18 weeks of storage was high among all four populations; adults from three populations also showed high survival after 35 weeks of storage. A high incidence of females from all four populations laid some fertile eggs after being stored for up to 31 weeks or longer. After six or more weeks of storage, average fecundity was high and ranged from ∼400–900 eggs/female. Duration of storage (up to 39 weeks) was a significant determinant of fecundity in two of the four populations. In practice, diverse populations of C. carnea can be efficiently mass-produced (with or without storage) under uniform conditions in the insectary. Thus, the choice of which populations to mass-produce and use in augmentation can be based primarily on the characteristics (e.g., climate, habitat, crop) of the intended area of release, rather than on considerations concerning mass-rearing.

The North American Free Trade Agreement (NAFTA) and the General Agreement on Tariffs and Trade (GATT) have focused attention on risk assessment of potential insect, weed, and animal pests and diseases of livestock. These risks have traditionally been addressed through quarantine protocols ranging from limits on the geographical areas from which a product may originate, postharvest disinfestation procedures like fumigation, and inspections at points of export and import, to outright bans. To ensure that plant and animal protection measures are not used as nontariff trade barriers, GATT and NAFTA require pest risk analysis (PRA) to support quarantine decisions. The increased emphasis on PRA has spurred multiple efforts at the national and international level to design frameworks for the conduct of these analyses. As approaches to pest risk analysis proliferate, and the importance of the analyses grows, concerns have arisen about the scientific and technical conduct of pest risk analysis. In January of 1997, the Harvard Center for Risk Analysis (HCRA) held an invitation-only workshop in Washington, D.C. to bring experts in risk analysis and pest characterization together to develop general principles for pest risk analysis. Workshop participants examined current frameworks for PRA, discussed strengths and weaknesses of the approaches, and formulated principles, based on years of experience with risk analysis in other setting and knowledge of the issues specific to analysis of pests. The principles developed highlight the both the similarities of pest risk analysis to other forms of risk analysis, and its unique attributes.

1.Trap cropping, the use of alternative host plants to reduce pest damage to a focal cash crop or other managed plant population, can be a sustainable strategy for pest control, but in practice it has often failed to reach management goals. Of the few successful trap cropping examples at a commercial scale, nearly all have included supplemental management strategies that reduce pest dispersal off the trap crop. In contrast, the trap cropping literature has focused extensively on trap plant attractiveness. 2.To test whether the dispersal of insects off trap plants is as important as the anecdotal evidence suggests, we developed a simple model to understand how a trap plant's spatial configuration within a field, its attractiveness and its ability to retain pests affect pest density on a target cash crop. 3.The model predicts that when trap crop retention is low, trap cropping is ineffective, and small increases in retention offer little improvement. However, when retention is high, small differences in retention dramatically affect trap cropping efficacy. In contrast, when the attractiveness of a trap crop is high, further increases in attractiveness have little effect on trap cropping efficacy. 4.Placing trap plants close together is most often detrimental to pest management because it leaves large portions of the field without nearby traps. However, planting the trap crop in rows often does not clump the landscape enough to cause this detrimental effect. 5.Synthesis and applications. The predictions from our model confirm the anecdotal evidence that trap cropping failures may be attributed to a focus on attraction at the expense of retention. A very high retention rate is required for effective reduction of pest densities. Therefore, additional practices that prevent insects from dispersing back into the cash crop may be essential for effective trap cropping designs. These techniques include trap vacuuming, trap harvesting, sticky traps, planting a high proportion of trap plants or applications of pesticides or natural enemies to the trap crop. © 2012 The Authors. Journal of Applied Ecology

Fourteen commercial leek fields with first and second generation Leek moth, Acrolepiopsis assectella Z., (LM) injury were sampled during 1986 in The Netherlands. For both generations, plant injury was more prevalent in the perimeter of the field than in interior portions. A sequential sampling program for use in making treatment decisions for LM was developed. An economic analysis of the value of sampling information derived from the sequential sampling program was performed. The parameters used for the analysis were crop yield and value, expected level of LM infestation, potential loss of value due to LM infestation, effectiveness of insecticide application, and cost of sampling. Due mainly to the high value of the crop and low cost of treatment, analysis indicates that there is little difference between a sampling-based management plan and prophylactic application of insecticides in terms of pest control costs. Additionally, such a sampling-based management plan is relatively insensitive to changes in the parameters used in the model. Thus, development of a threshold linked to a sampling procedure will not reduce pest control costs. Use of the sampling-based management plan will also not significantly increase pest control costs and will likely result in reduced insecticide use compared with a prophylactic treatment program. Zusammenfassung Bewertung einer Entscheidungsregel fur Bekampfung von Lauchmottenbefall in Porree In den Niederlanden wurde 1986 der durch den Befall der ersten und zweiten Generation der Lauchmotte (Acrolepiosis assectella Z.) verursachte Schaden auf 14 kommerziell genutzten Porreefeldern ermittelt. Fur beide Generationen wurde an den Feldrandern ein deutlich hoherer Befall als in dem inneren Bereich eines Feldes festgestellt. Zur Bekampfung der Lauchmotte wurde als Entscheidungsmodell ein sequentielles Probenahmeverfahren entwickelt. Der Informationsgehalt der sequentiellen Befallserhebung wurde einer okonomischen Bewertung unterzogen. Hierbei wurden folgende Parameter zugrunde gelegt: Erntemenge und Ertrag, erwarteter Befallsgrad sowie hierdurch verursachter moglicher Ertragsverlust, Wirkungsgrad einer Insektizidbehandlung und Kosten der Stichprobennahme. Die Analyse der einzelnen Parameter ergab nur eine geringe Kostendifferenz zwischen einer gezielten Bekampfung nach sequentieller Probenahme und prophylaktischen Insektizidspritzungen, bedingt durch den hohen Wert der Kultur und die niedrigen Kosten einer Bekampfung. Daruber hinaus ist das Verfahren einer gezielten Lauchmottenbekampfung gegeuber Veranderungen der obengenannten Parameter relativ unempfindlich. Deshalb wird die Entwicklung eines Schwellenwertes der mit Probenahmeverfahren verbunden ist die Bekampfungskosten nicht reduzieren. Die Anwendung der Probenahmeplans wurde Bekampfungskosten auch nicht erhohen und wurde wahrscheinlich Insecktizidverbrauch reduzieren im Vergleich zu einen prophylaktischen Behandlungsprogramm.