Jan P. Nyrop

Leaf pubescence and two-spotted spider mite webbing influence phytoseiid behavior and population density

Phytoseiid mites, both in agricultural and natural systems, can play an important role in the regulation of herbivorous mites. Host plant traits, such as leaf pubescence, may influence the dynamics between predator and prey. In this study, we examined the influence of leaf surface characteristics (leaf pubescence and two-spotted spider mite webbing) on the behavior of two species of predatory mites, the generalist Typhlodromus pyri and the spider mite specialist Phytoseiulus persimilis. In laboratory trials, T. pyri females consistently spent more time and deposited more eggs on leaf discs from trichome-rich apple varieties compared to relatively trichome-poor varieties. A similar result was found when the choice involved trichome-rich and trichome-poor apple varieties planted into the same pot where leaves were allowed to touch so that the mites could freely move from leaf to leaf. To further explore the effect of structure created by pubescence and to remove possible confounding effects of chemical cues, we added cotton fibers to trichome-free bean leaves. T. pyri females consistently spent more time and deposited more eggs on the side of a glabrous bean leaf with artificial cotton fibers versus the side without added fibers. When given a choice between two densities of cotton fibers, T. pyri females consistently selected the highest density of available fibers in which to to reside and oviposit. T. pyri also preferred cotton fiber configurations in which it could move underneath and access the plant surface. The artificial pubescent leaf was also used to test the effect of leaf hairs and two-spotted spider mite webbing on the behavior of P. persimilis. P. persimilis females preferred residing and ovipositing on surfaces with cotton fibers or two-spotted spider mite webbing than on bean leaf areas without these structures. When presented a choice between cotton fibers or webbing, the behavior of P. persimilis females depended on the cotton fiber density. In a mixed-variety apple orchard, we investigated the relationship between leaf pubescence and phytoseiid density under field conditions. We found a highly significant, positive relationship between density of trichomes on leaves and abundance of T. pyri, whereas spider mite prey numbers were uniformly low and unrelated to trichome density. These field results suggest that the behavioral responses found in our laboratory experiments have population consequences.

Neonicotinoid insecticide imidacloprid causes outbreaks of spider mites on elm trees in urban landscapes.

Background Attempts to eradicate alien arthropods often require pesticide applications. An effort to remove an alien beetle from Central Park in New York City, USA, resulted in widespread treatments of trees with the neonicotinoid insecticide imidacloprid. Imidacloprids systemic activity and mode of entry via roots or trunk injections reduce risk of environmental contamination and limit exposure of non-target organisms to pesticide residues. However, unexpected outbreaks of a formerly innocuous herbivore, Tetranychus schoenei (Acari: Tetranychidae), followed imidacloprid applications to elms in Central Park. This undesirable outcome necessitated an assessment of imidacloprids impact on communities of arthropods, its effects on predators, and enhancement of the performance of T. schoenei. Methodology/Principal Findings By sampling arthropods in elm canopies over three years in two locations, we document changes in the structure of communities following applications of imidacloprid. Differences in community structure were mostly attributable to increases in the abundance of T. schoenei on elms treated with imidacloprid. In laboratory experiments, predators of T. schoenei were poisoned through ingestion of prey exposed to imidacloprid. Imidacloprids proclivity to elevate fecundity of T. schoenei also contributed to their elevated densities on treated elms. Conclusions/Significance This is the first study to report the effects of pesticide applications on the arthropod communities in urban landscapes and demonstrate that imidacloprid increases spider mite fecundity through a plant-mediated mechanism. Laboratory experiments provide evidence that imidacloprid debilitates insect predators of spider mites suggesting that relaxation of top-down regulation combined with enhanced reproduction promoted a non-target herbivore to pest status. With global commerce accelerating the incidence of arthropod invasions, prophylactic applications of pesticides play a major role in eradication attempts. Widespread use of neonicotinoid insecticides, however, can disrupt ecosystems tipping the ecological balance in favor of herbivores and creating pest outbreaks.

Dynamics ofPanonychus ulmi andTyphlodromus pyri: factors contributing to persistence

We addressed the question of persistence of predator and prey in a biological control system by examining temporal patterns ofPanonychus ulmi (Koch) and its predator,Typhlodromus pyri Scheuten at two geographic locations and at two spatial scales. At the scale of an orchard, bothP. ulmi andT. pyri were persistent over the time frame of 6 years. At the scale of an individual tree,T. pyri appeared to be more persistent than its prey,P. ulmi. We used a simulation model of single populations ofP. ulmi andT. pyri to determine which of several aspects of the biology of the two species could contribute to such a pattern. Spatial incongruity between predator and prey was essential for persistence of both species. The generalist food habit ofT. pyri probably contributes to the persistence ofT. pyri on individual trees, and may cause occasional extinction ofP. ulmi at this spatial scale. The presence of alternate food is likely an essential element for successful biological control in this system. Cannibalism byT. pyri results in higher prey densities, that is, it is detrimental to the biological control ofP. ulmi, but has no effect on the relative persistence of the two species.

Leaf pubescence mediates the abundance of non-prey food and the density of the predatory mite Typhlodromus pyri

Plants with leaves having numerous trichomes or domatia frequently harbor greater numbers of phytoseiid mites than do plant with leaves that lack these structures. We tested the hypothesis that this pattern occurs, in part, with Typhlodromus pyri because trichomes increase the capture of pollen or fungal spores that serve as alternative food. Using a common garden orchard, we found that apple varieties with trichome-rich leaves had 2–3 times more pollen and fungal spores compared to varieties with trichome-sparse leaves. We also studied the effects of leaf trichome density and pollen augmentation on T. pyri abundance to test the hypothesis that leaf trichomes mediate pollen and fungal spore capture and retention and thereby influence phytoseiid numbers. Cattail pollen (Typha sp.) was applied weekly to mature ‘McIntosh’ and ‘Red Delicious’ trees grown in an orchard and, in a separate experiment, to potted trees of the same varieties. ‘McIntosh’ trees have leaves with many trichomes whereas leaves on the ‘Red Delicious’ trees have roughly half as many trichomes. With both field-grown and potted trees, adding cattail pollen to ‘Red Delicious’ trees increased T. pyri numbers compared to ‘Red Delicious’ trees without pollen augmentation. In contrast, cattail pollen augmentation had no effect on T. pyri populations on ‘McIntosh’ trees. Augmentation with cattail pollen most likely supplemented a lower supply of naturally available alternative food on ‘Red Delicous’ leaves and thereby enhanced predator abundance. These studies indicate that larger populations of T. pyri on pubescent plants are due, in part, to the increased capture and retention of pollen and fungal spores that serve as alternative foods.

Influence of ground cover on dynamics of Amblyseius fallacis Garman (Acarina : Phytoseiidae) in New York apple orchards

Abstract Four studies were conducted between 1989 and 1992 to determine the influence of vegetation beneath apple trees on the dynamics of the mite predator Amblyseius fallacis (Garman). Withholding herbicide applications in the spring to allow ground cover to grow beneath trees did not result in greater numbers of A. fallacis in trees during the summer compared with plots where herbicides were applied and ground cover was greatly reduced. A survey of 23 commercial orchards revealed that A. fallacis overwintered on trees in many of these sites (14). Typhlodromus pyri (Scheuten) and Typhlodromus occidentalis (Nesbitt) were also found in the trees. In two of the orchards phytoseiids and European red mites, Panonychus ulmi (Koch), were sampled the following spring and summer. Amblyseius fallacis were found in the trees at bloom (10 May) and there was no evidence that they first increased in density in the ground cover and then colonized trees. Overwintering predator populations were repeatedly sampled in six orchards during autumn, winter, and spring 1991–1992. Amblyseius fallacis and T. pyri overwintered in high numbers in the trees. Amblyseius fallacis was also found in high numbers in the ground cover. Typhlodromus pyri also overwintered in the ground cover, but in lower numbers. Both predator species suffered high overwintering mortality, with that of T. pyri apparently less than that of A. fallacis . Treatment of ground vegetation with a pyrethroid insecticide in the spring did not affect arboreal A. fallacis densities during the summer; however, treatment of trees with the insecticide greatly reduced A. fallacis densities. In New York, ground cover in apple orchards directly beneath the trees does not appear to play a large role in the dynamics of A. fallacis .

Integrated apple pest management in New York State using predatory mites and selective pesticides

A 3-year demonstration study was conducted in four western New York apple orchards to evaluate current approaches of sustainable pest management in representative commercial orchards. Pests that could be tolerated were regulated by natural antagonists, including predatory mites that were introduced to supplement endemic populations, while those with lower tolerance levels were managed with a schedule of selective pesticides, e.g., insect growth regulators and horticultural mineral oil. Pesticide application decisions (timing and materials) were made on the basis of current state extension guidelines, which involved a combination of protectant and threshold-based sprays determined through timely scouting and sampling procedures. Only non-toxic or minimally toxic pesticides to the principal mite and aphid predators were applied in the orchards. By the third season, effective conservation biological control of European red mite was achieved in all orchards, and fruit quality at harvest was equal or superior to that in comparison blocks managed using the growers’ conventional practices.

Spatial ecology of the Japanese beetle, Popillia japonica.

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.

SAMPLING FOR IPM DECISION MAKING: WHERE SHOULD WE INVEST TIME AND RESOURCES?

ABSTRACT Guides for making crop protection decisions based on assessments of pest abundance or incidence are cornerstones of many integrated pest management systems. Much research has been devoted to developing sample plans for use in these guides. The development of sampling plans has usually focused on collecting information on the sampling distribution of the pest, describing this sampling distribution with a mathematical model, formulating a sample plan, and sometimes, but not always, evaluating the performance of the proposed sample plan. For crop protection decision making, classification of density or incidence is usually more appropriate than estimation. When classification is done, the average outcome of classification (the operating characteristic) is frequently robust to large changes in the sampling distribution, including estimates of the variance of pest counts, and to sample size. In contrast, the critical density, or critical incidence, about which classifications are made, has a large influence on the operating characteristic. We suggest that rather than investing resources in elaborate descriptions of sampling distributions, or in fine-tuning sample size to achieve desired levels of precision, greater emphasis should be placed on characterizing pest densities that signal the need for management action and on designing decision guides that will be adopted by practitioners.

Attraction of Trialeurodes vaporariorum and Bemisia argentifolii to eggplant, and its potential as a trap crop for whitefly management on greenhouse poinsettia

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.

A Plant-Level, Spatial, Bioeconomic Model of Plant Disease Diffusion and Control: Grapevine Leafroll Disease

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.