Generalist Predator Community Research Articles

Intraguild predation is increased in areas of low prey diversity in a generalist predator community.

Niche differentiation and intraguild predation (IGP) can allow ecologically similar species to coexist, although it is unclear which coexistence mechanism predominates in consumer communities. Until now, a limited ability to quantify diets from metabarcoding data has precluded the use of sequencing data to determine the relative importance of these mechanisms. Here, we pair a recent metabarcoding quantification approach with stable isotope analysis to examine diet composition in a wolf spider community. We compare the prevalence of resource partitioning and IGP in these spiders and test whether factors that influence foraging performance, including individual identity, morphology, prey community and environmental conditions, can explain variation in diet composition and IGP. Extensive IGP is likely the primary coexistence mechanism in this community, and other factors to which foraging variation is often attributed do not explain diet composition and IGP here. Rather, IGP increases as prey diversity decreases. Foragers are driven to IGP where resource niches are limited. We highlight the need to examine how drivers of predator-prey interaction strengths translate into foraging in natural systems.

Seasonal and landscape effects on the biotic resistance of forest communities to experimental insect introductions

Natural enemies play an important role in the regulation of many forest insect populations. The hypothesis that these organisms also reduce invasion potential is one element of a concept known as biotic resistance. While many studies have shown that the abundances of natural enemies are affected by landscape structure and diversity, seasonality, and host tree species, this study tests the hypothesis that these factors affect the biotic resistance of forest communities to invasion by a non‐native tussock moth. At 20 sites on Vancouver Island, Canada, spanning a range of natural to urban forests, small populations of the rusty tussock moth, Orgyia antiqua, an exotic polyphagous tussock moth, were introduced. Introductions were repeated at three different periods in the year, on coniferous and deciduous host trees, and included both late‐instar larvae and pupae. The survival of these small populations was monitored in relation to four landscape variables measured around each site. Spring introductions had significantly lower mortality rates than either early or mid‐summer introductions. There was little difference in predation rates between coniferous and deciduous host trees. The amount and type of forest cover in the landscape had important, but seasonally dependent, effects on survival that likely reflect changes in the habitat requirements of a shifting community of generalist predators. Based on the results, this study concludes that landscapes with intermediate forest cover are the least resistant to invasion by early feeding species such as the rusty tussock moth.

Can Generalist Predators Control Bemisia tabaci?

Simple SummaryWhiteflies are major insect pests on a global scale. The use of insecticides is the primary tool for controlling them, but there are many problems relying on this strategy. However, natural enemies like predators and parasitic insects that attack whiteflies can help provide a sustainable pest management approach. This paper focuses on predators that feed on whiteflies as well as other insect pests and are called generalist predators. We provide a comprehensive view of generalist predator contributions and review the currently recognized generalist predators of whiteflies. There are many generalist predators in agricultural cropping systems that help control whiteflies. We highlight the need for conservation biological control programs through habitat management strategies and the use of selective insecticides, with an aim for more sustainable management of whiteflies in crops.The whitefly, Bemisia tabaci, has developed resistance to many insecticides, renewing interest in the biological control of this global pest. Generalist predators might contribute to whitefly suppression if they commonly occur in infested fields and generally complement rather than interfere with specialized natural enemies. Here, we review literature from the last 20 years, across US cropping systems, which considers the impacts of generalist predators on B. tabaci. Laboratory feeding trials and molecular gut content analysis suggest that at least 30 different generalist predator species willingly and/or regularly feed on these whiteflies. Nine of these predators appear to be particularly impactful, and a higher abundance of a few of these predator species has been shown to correlate with greater B. tabaci predation in the field. Predator species often occupy complementary feeding niches, which would be expected to strengthen biocontrol, although intraguild predation is also common and might be disruptive. Overall, our review suggests that a bio-diverse community of generalist predators commonly attacks B. tabaci, with the potential to exert substantial control in the field. The key challenge will be to develop reduced-spray plans so that generalist predators, and other more specialized natural enemies, are abundant enough that their biocontrol potential is realized.

Seasonality shapes the amplitude of vole population dynamics rather than generalist predators

Seasonality has been suggested as a necessary factor for the initiation of vole population cycles in Fennoscandia. This has been well described for a latitudinal gradient. Here, we used an elevational gradient as a proxy for winter length to study how the length of the winter season correlates with the amplitude of bank vole Myodes glareolus population cycles. In addition, we studied whether the small mammalian generalist predator community present locally could explain any elevational effects. We estimated the population size of 30 local bank vole populations. We found a strong effect of elevation on the amplitude of the population cycle with local populations at around 800 m elevation having 1.5 times greater densities than populations in the valley (ca 300 m elevation). A univariate model with elevation as predictor for amplitude was twice as likely to be the best model than models including generalist predators. Our results fit well with the theories of a positive effect of winter length on the amplitude of vole population cycles in Fennoscandia, irrespective of whether the seasonal effect corresponds to a latitudinal or elevational gradient. The mechanisms may be limited resources during winter rather than generalist predators.

Frequency, efficiency, and physical characteristics of predation by generalist predators of brown marmorated stink bug (Hemiptera: Pentatomidae) eggs

The native generalist predator community of Halyomorpha halys, an invasive species in the United States and Europe, has not been well defined to date. The aims of the current study were to determine whether generalist predators that are commonly found in mid-Atlantic orchards and vegetable crops are capable of feeding on H. halys eggs and, if so, to systematically characterize the appearance of feeding damage in order to link it to sentinel egg mass surveys. Over 25 field-collected and commercially available arthropod predator taxa, including adults and immatures, were evaluated as potential predators of H. halys eggs in laboratory trials, and a photographic library of egg mass damage was developed. In addition, over 400 sentinel egg masses were deployed in tree fruit and vegetable crops, and direct observations were made of predator taxa in situ. We found that the most frequent and efficient predators of H. halys eggs were katydids and ground beetles, and to a lesser extent earwigs, jumping spiders, and crickets. Egg damage for each taxon studied was ascribed to one of four egg damage syndromes: complete chewing, incomplete chewing, stylet sucking, and punctured sucking. The first two are caused by predators with chewing mouthparts and are differentiated by the presence of irregular chorion debris left behind in the case of incomplete chewing compared with nothing left behind or an entire segment of the egg mass completely removed for complete chewing. The latter two are caused by predators with piercing/sucking mouthparts or chelicerae and are separable by the presence of a feeding sheath for stylet sucking compared to a simple hole or slit in otherwise hollowed eggs for punctured sucking via chelicerae. Complete and incomplete chewing were the two feeding syndromes most frequently documented in the field and with the greatest number of eggs consumed per egg mass. Taxa evaluated in laboratory trials were reliably found in tree fruit and vegetable crops. Overall, our work contributes to the identification of key egg mass predators of H. halys in specialty crop agroecosytems.

Complementary suppression of aphids by predators and parasitoids

Biocontrol practitioners have long debated the relative merits of specialist parasitoids versus generalist predators as biological control agents. Predators and parasitoids exhibit a broad array of ecological differences that might lead them to play different, and perhaps complementary, roles in pest suppression. However, predators often feed on parasitoids with the potential to disrupt biological control. We compared suppression of the woolly apple aphid (Eriosoma lanigerum) when attacked solely by the specialist parasitoid Aphelinus mali, to aphid suppression when the parasitoid was joined by a diverse community of generalist predators (primarily earwigs, syrphids, predatory bugs and spiders). In a series of exclusion-cage experiments conducted on apple trees (Malus domestica) placed in the field, we found that the parasitoid alone significantly slowed aphid population growth. However, consistently flat or declining aphid densities were seen only when the parasitoid was paired with the generalist predator guild. This suggested that predators and parasitoid exerted complementary impacts, with the most effective aphid suppression achieved when the two natural enemy classes were present together. Consistent with this observation, percentage aphid parasitism was not consistently reduced in the presence of predators; this implies relatively weak predator–parasitoid interference. Altogether, our results suggest that the conservation of a diverse community of natural enemies, including both predators and parasitoids, is an effective way to strengthen suppression of this, and perhaps other, aphid species.

Biological control in winter: novel evidence for the importance of generalist predators

Summary The role of generalist predators in pest control has been neglected because generalists are not able to track pest populations. Generalist predators are suggested to be important in spring before specialist predators become active. Here, we show that some generalist predators are important even during winter, when the majority of arthropod pests and their enemies are dormant. We quantified the role of winter‐active generalist predators on the suppression of pear psylla during winter using a discrete nonlinear model of an intraguild predation system. To parameterize our model, we conducted a series of experiments on (i) functional responses, (ii) prey preferences and (iii) ontogenetic development and made observations on the population densities of spiders and potential prey. We ran the model for different winter scenarios, that is, for very cold and very warm winters. Synthesis and applications. We found that winter‐active predators considerably suppressed the pear psylla population. Predators exerted a stronger effect in a warmer winter than in a colder one. Orchard growers thus should avoid use of non‐selective pesticides during this period and instead aim to support the community of generalist predators. Our results suggest more attention should be given to encouraging generalist predator populations in other systems, even in periods when crops are not producing.

Diversity and phenology of the generalist predator community in apple orchards of Central Washington State (Insecta, Araneae)

AbstractPredatory insects and spiders were collected from apple orchards in two geographic regions of Central Washington State, United States of America to assess seasonal phenology and diversity of the generalist predator community. Arthropods were collected from orchard canopy every 3–7 days over two growing seasons (March–October) at seven organically managed and two insecticide-free orchards. Over 35 000 specimens and 80 species of spiders (Araneae), ladybeetles (Coleoptera: Coccinellidae), lacewings (Neuroptera), and predatory true bugs (Hemiptera) were collected. Composition of insect and spider communities differed between the two geographic regions. Indicator species analysis identified several species that had a significant association with one of the two regions. Counts of the most common taxa were examined in detail on a calendar date basis to determine seasonal phenology of adult and immature stages. We observed substantial differences among taxa in number of generations, seasonal timing of first appearance in orchards, overwintering stages, and seasonal occurrence of the adult and immature life stages in orchards. Understanding seasonal phenology of natural enemies in orchards is a core requirement in integrated pest management programs for apple pests, and results of this study provide this information for the generalist predator community of orchards in the Pacific Northwest.

Lack of strong refuges allows top-down control of soybean aphid by generalist natural enemies

Refuges have been shown to be important mediators of predator–prey interactions, and in particular, have been proposed as a potential mechanism allowing herbivore populations to reach outbreak levels. However, very little research on the role of refuges has been conducted in systems dominated by generalist predators. We investigated the existence of refuges from predation for the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae) at multiple scales. This species invaded North America and in spite of previous studies demonstrating strong suppression by generalist natural enemies, its populations periodically cause significant economic losses. Using naturally occurring populations of soybean aphid and its natural enemies, we tested for the presence of A. glycines spatial and dynamic refuges at the within-field, single plant, and within-plant scale. At the within-field level, we found only weak and transient spatial patterns in aphid populations suggesting the lack of spatial refuges at this scale. Similarly, at the plant level we found no individual colonies that escaped predation and aphid suppression was 9- to 28-fold greater in comparison with caged controls regardless of initial aphid density. When high aphid populations were exposed to predation they were rapidly reduced to levels close to the average field density and showed reduced per capita growth rates, indicating an absence of dilution of predation risk at increased aphid density. Finally, we found a significant shift in the distribution of aphids to the lower portions of the plant in the presence of generalist predators, suggesting a partial refuge from predation at the within-plant scale. Overall, we found the naturally occurring community of generalist predators to exert strong top-down suppression of soybean aphid populations at multiple scales, and no evidence that the presence of refuges at the scales studied can lead to outbreak populations. The partial refuge from predation at the within-plant scale revealed in our study may have important consequences for the within-season population dynamics of A. glycines, since it may be associated with low plant quality tradeoffs, and therefore warrants further research.

Integrating Chemical and Biological Control of European Corn Borer in Bell Pepper

Using multiple locations and a series of field trials over 2 yr, we evaluated an integrated pest management program for Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) management in peppers involving biorational chemistries, inundative releases of Trichogramma ostriniae (Pang & Chen), and conservation of generalist predators. In small plot trials, three biorational insecticides (spinosad, indoxacarb, and methoxyfenozide) provided comparable control of O. nubilalis as two broad-spectrum conventional insecticides (acephate and lambda-cyhalothrin). However, lambdacyhalothrin at most locations, and indoxacarb at one location, resulted in outbreaks of green peach aphids. We also observed significant effects on the generalist predator community: beneficial communities in methoxyfenozide-treated plots were most similar to untreated controls, and acephate-treated plots were the least similar. Management systems comparing untreated controls, inundative release of T. ostriniae with methoxyfenozide applied when lepidopterans exceeded thresholds, or weekly applications of acephate or lambda-cyhalothrin, showed no effects on marketable fruit or percentage of fruit damaged, but the conventional insecticide approach caused aphid flares. Inundative releases of T. ostriniae and biorational chemistries provide a more environmentally sound approach to managing O. nubilalis in peppers, due, in part, to conservation of generalist predators.

Use of a New Immunomarking Method to Assess Movement by Generalist Predators Between a Cover Crop and Tree Canopy in a Pear Orchard

Poor understanding of natural enemy movement between cover crop and crop habitats has limited effective use of cover to enhance biological control of pest arthropods in agricultural systems. To address this problem, we used an egg-albumin immunomarker sprayed on a legume cover crop to monitor movement by generalist predators from the cover crop into the canopy of pear trees in a Central Washington State pear orchard. Generalist predators in four taxa (Heteroptera, Coccinellidae, Chrysopidae, Araneae) were collected from both cover crop and pear tree canopy, and tested using enzyme-linked immunonoassay (ELISA) methods to determine if the marker was present. In the three years of study, > 90% of arthropods collected from the cover crop had acquired the marker, indicating that we had excellent coverage and marking of arthropods with the solution. Between 17 and 29% (depending upon the year of study) of the approximately 2000 specimens collected from the tree canopy were also found to be marked, which is evidence that a percentage of the generalist predator community in the tree canopy had originated from or had visited the legume cover crop. These specimens included spiders and immature insects, which presumably colonized the tree either by walking up the tree trunk or (for spiders) by ballooning into the canopy. Some care must be taken in applying this technology or in interpreting results, due to unknown effects of the marker on arthropod behavior, and to questions of whether the marker might transfer from marked to unmarked insects as they interact in the field. Despite these caveats, the technology described here provides a new tool for assessing the effects of habitat management on movement of generalist predators in crop systems.

ENHANCING PLANT DIVERSITY FOR IMPROVED INSECT PEST MANAGEMENT IN NORTHERN CALIFORNIA ORGANIC VINEYARDS

ENHANCING PLANT DIVERSITY FOR IMPROVED INSECT PEST MANAGEMENT IN NORTHERN CALIFORNIA ORGANIC VINEYARDS

Contrasting Trophic Cascades Generated by a Community of Generalist Predators

Contrasting Trophic Cascades Generated by a Community of Generalist Predators

Cyclic dynamics in field vole populations and generalist predation

Summary1. A geographical gradient in the relative impact of generalist and specialist predators on small rodent populations has been hypothesized to be responsible for the gradient in cyclicity found in Fennoscandia. Population oscillations resulting from weasel–vole interactions are said to be dampened by the increasing stabilizing impact of generalist predators in southern Fennoscandia resulting from: (i) a greater abundance and diversity of predators sustained by alternative prey; (ii) the absence of significant snow cover leading to constant exposure of voles to generalist predators; and (iii) a heterogeneous habitat that makes dispersing voles more vulnerable to predators.2. Changes in the abundance of field voles (Microtus agrestisL.) in a man‐made spruce forest in northern England were recorded during 1984–98 using sign indices at 14–18 sites calibrated with capture–recapture estimates of vole density.3. Field vole populations exhibited cyclic dynamics which were in many ways similar to those reported from Fennoscandia, including population declines taking place during the breeding season and long periods with no recovery in numbers following population crashes.4. The density dependence structure of the time series was explored by means of partial autocorrelation functions, which suggested second‐order density dependence. Analyses based on two density estimates per year (spring and autumn) reveal significant negative values for lags of 1, 1·5 and 2 years, suggesting that the time‐lag might be somewhat shorter than 2 years.5. Estimates of predation on field voles by red foxes and tawny owls at high vole density were above the value predicted for this site and for the whole generalist predator community by a published model assuming that predation by generalist predators stabilizes vole populations. However, empirical estimates of the parameter used both for designing and testing the model are inherently imprecise.6. A qualitative evaluation of the three variables (see1) correlated to the Fennoscandian gradient and assumed to contribute to variations in generalist predation pressure did not support the hypothesis that low predation rates by generalist predators are necessary for vole dynamics to be dominated by the destabilizing impact of weasel–vole interactions. The specialist/generalist predation hypothesis must therefore be modified to account for the regular population cycles occurring in northern Britain.

Fox predation on cyclic field vole populations in Britain

The diet of the red fox Vulpes vulpes L. was studied during three winter periods in spruce plantations in Britain, during which time the cyclic field vole Microtus agrestis L. populations varied in abundance. Field voles and roe deer Capreolus capreolus L. were the two main prey species in the diet of the red fox. The contribution of lagomorphs to fox diet never exceeded 35% and species of small mammal other than field voles were of minor importance. The contribution of field voles was dependent on vole density. The non-linear density dependent relationship with a rather abrupt increase of field voles in fox diet when vole density exceeded ca 100 voles ha -1 was consistent with a prey-switching response. The contribution of field voles to fox diet during the low phase of population cycles was lower in Kielder Forest than in other ecosystems with cyclic vole populations. The number of foxes killed annually by forestry rangers was consistent with the evidence from other studies that foxes preying on cyclic small rodents might show a delayed numerical response to changes in vole abundance. Estimates of the maximum predation rate of the fox alone (200-290 voles ha -1 of vole habitat year -1 ) was well above a previously predicted value for the whole generalist predator community in Kielder Forest. Our data on the functional response of red foxes and estimates of their predation rates suggest that foxes should have a strong stabilising impact on vole populations, yet voles show characteristic 3-4 yr cycles.