Theoretical developments are helping us to comprehend the basic parameters governing the dynamics of the interactions between generalist predators and their many pest and nonpest prey. In practice, however, inter- and intraspecific interactions between generalist predators, and between the predators and their prey, within multispecies systems under the influence of rapidly changing biotic and abiotic variables are difficult to predict. We discuss trade-offs between the relative merits of specialists and generalists that allow both to be effective, and often complementary, under different circumstances. A review of manipulative field studies showed that in approximately 75% of cases, generalist predators, whether single species or species assemblages, reduced pest numbers significantly. Techniques for manipulating predator numbers to enhance pest control at different scales are discussed. We now need to find ways of disentangling the factors influencing positive and negative interactions within natural enemy communities in order to optimize beneficial synergies leading to pest control.

Can generalist predators be effective biocontrol agents

Introduction to the practice of statistics, (3rd edition), by David S. Moore and George P. McCabe. Pp. 825 (with appendices and CD-ROM). £27.95. 1999. ISBN 0 7167 3502 4 (W. H. Freeman).

Farmers are facing serious plant protection issues and phytosanitary risks, in particular in the tropics. Such issues are food insecurity, lower income in traditional low- input agroecosystems, adverse effects of pesticide use on human health and on the environment in intensive systems and export restrictions due to strict regulations on quarantine pests and limits on pesticide residues. To provide more and better food to populations in both the southern and northern hemispheres in a sustainable manner, there is a need for a drastic reduction in pesticide use while keeping crop pest and disease damage under control. This can be achieved by breaking with industrial agriculture and using an agro- ecological approach, whose main pillar is the conserva- tion or introduction of plant diversity in agroecosystems. Earlier literature suggest that increasing vegetational biodiversity in agroecosystems can reduce the impact of pests and diseases by the following mechanisms: (1) resource dilution and stimulo-deterrent diversion, (2)

https://link.springer.com/content/pdf/10.1007%2Fs13593-011-0022-4.pdf

Plant species diversity for sustainable management of crop pests and diseases in agroecosystems: a review

A framework for evaluating ecosystem services provided by cover crops in agroecosystems

We consider how fungi that form symbiotic associations with plants interact with insect herbivores attacking the same plants. Both endophytes and mycorrhizae have significant impacts on herbivores with which they are in relatively intimate contact, but weaker effects on those from which they are spatially separated. Generalist insects are usually adversely affected by the presence of endophytes and mycorrhizae, whereas specialist insects may often benefit. Effects on feeding guilds vary according to type of fungi; for example, aphids are often negatively affected by endophytes but respond positively to mycorrhizae, and leafchewers are usually negatively affected by both types of fungi. There is a strong taxonomic bias in the literature and many interactions remain little studied; laboratory studies predominate over field studies. Although some patterns emerge, there is a large amount of specificity and context dependency in the outcome of interactions, reflecting the influence of fungal and host genotype, fungal, host, and insect species, and environmental factors. Whereas some of the mechanisms underpinning these interactions are relatively well characterized, others remain unclear and await elucidation by molecular and metabolomic techniques.

Impacts of plant symbiotic fungi on insect herbivores: mutualism in a multitrophic context.

The ecological guild concept has been of great interest to arachnologists, and the different manner in which spiders forage for a common resource—prey arthropods—has led to numerous attempts to classify them into guilds. However, questions have been raised about the validity of guilds and the taxon-centered basis of their definition. Here, we propose an alternative approach to guild classification, using quantitative analysis of ecological characteristics of spider families. While generalizations may not apply to all species within a taxon, results from this approach suggest eight major spider guilds similar to earlier guild assignments by some authors and provide a reasonable framework for future studies. We used this classification in a comparison of spider guild composition across several major crops (from published studies). While total species richness varied widely among crops, the proportion of the total species within each guild was remarkably even across crops. The relative abundance of guilds (based on numbers of individuals) varied greatly, which may reflect availability of resources within a crop type. Patterns of similarity in guild composition suggest the possibility of plant habitat structure as an influence on the spider community. Further detailed analyses of spider guilds in various crops have been constrained by both a lack of comparable quantitative data and the paucity of behavioral and natural history infor- mation available for many taxa. As recent studies have shown that assemblages of spiders can impact pest populations and reduce crop damage, a better understanding of spider guild composition and variation in spider community structure among crops is essential in future studies of the arthropod fauna in agroeco- systems.

Guild structure of spiders in major crops

Promoting generalist predators in agriculture via habitat manipulation has gained much interest in biocontrol research. Straw shelters have been used by Chinese farmers for .2,000 yr to provide temporary spider refugia during cyclic farming disturbances. This method, however, has not been systematically investigated on larger scales in western-style agriculture. Our prelim- inary observations indicated a significant decrease in the abundance of spiders (76%) and their egg sacs (75%), after conventional tillage of soybeans (Glycine max (L.) Merr.). We hypothesized that providing alternative habitats in tilled fields could conserve predatory assemblages following this major disturbance. We used modular habitat refugia constructed of chicken wire loosely filled with bedding straw to provide temporary habitats for epigeic predators in a soybean field. Refugia held 5—36 times the spider density compared with open field, and the production of spider egg sacs was enhanced 18—87 times. Almost 60% more spider species were found in refugia than in open field. Abundance of harvestmen, carabids, and staphylinid beetles also significantly increased in habitat refugia. Increased habitat cover and provision of alternative prey in habitat refugia may have caused this dramatic predator increase. Soybean seedlings grown withi n1mo fhabitat refugia suffered 33% less insect damage compared with plants at control locations. Decrease in seedling damage, however, did not significantly increase soybean seed production. Applications of discrete habitat refugia may provide an alternative to habitat manipulation techniques to conserve and augment arthropod predators in agroecosystems.

https://www.fcnym.unlp.edu.ar/catedras/controlbiologico/Bibliografia%20PDF/TP%201%20Refugios%20para%20ara.pdf

Modular Habitat Refugia Enhance Generalist Predators and Lower Plant Damage in Soybeans

We tested the hypothesis that if the quality or quantity of vegetation in small forest patches is greater than in large patches, then vegetation attributes may contribute to greater densities of wh...

Effects of vegetation structure and edge habitat on the density and distribution of white-footed mice (Peromyscus leucopus) in small and large forest patches

Mutualistic symbionts are widespread in plants and may have strong, bottom-up influences on community structure. Here we show that a grass‐endophyte mutualism shifts the composition of a generalist predator assemblage. In replicated, successional fields we manipulated endophyte infection by Neotyphodium coenophialum in a dominant, non-native plant (Lolium arundinaceum). We compared the magnitude of the endophyte effect with manipulations of thatch biomass, a habitat feature of known importance to spiders. The richness of both spider families and morphospecies was greater in the absence of the endophyte, although total spider abundance was not affected. Thatch removal reduced both spider abundance and richness, and endophyte and thatch effects were largely additive. Spider families differed in responses, with declines in Linyphiidae and Thomisidae due to the endophyte and declines in Lycosidae due to thatch removal. Results demonstrate that the community impacts of non-native plants can depend on plants mutualistic associates, such as fungal endophytes.

Plant–fungus mutualism affects spider composition in successional fields

We gathered dietary data for several harvestman species in a soybean field and adjacent hedgerow habitats to assess the extent of their polyphagy. A total of 1032 harvestmen, predominantly Leiobunum spp., were observed during almost 50 search hours over two seasons. More harvestmen and increased foraging activity were observed in the hedgerow than in the soybean field, and in both habitats harvestmen were more active at night. Earthworms (Oligochaeta: Lumbricidae) were the prey items most frequently recorded as being consumed by all observed harvestman species in both habitats (47.1%), while fruit and other plant material constituted 18.1% of the records. Feeding trials conducted with female harvestmen, Hadrobunus maculosus (Wood), revealed that these arachnids were not capable of subduing live earthworms, suggesting that consumption of these in the field might be limited to scavenging. Implications of harvestman foraging for local food web dynamics are discussed.

Alan B. Cady

Papers

Guild structure of spiders in major crops

Modular Habitat Refugia Enhance Generalist Predators and Lower Plant Damage in Soybeans

Effects of vegetation structure and edge habitat on the density and distribution of white-footed mice (Peromyscus leucopus) in small and large forest patches

Plant–fungus mutualism affects spider composition in successional fields

Diet composition and significance of earthworms as food of harvestmen (Arachnida: Opiliones).