Agricultural intensification has resulted in a simplification of agricultural landscapes by the expansion of agricultural land, enlargement of field size and removal of non-crop habitat. These changes are considered to be an important cause of the rapid decline in farmland biodiversity, with the remaining biodiversity concentrated in field edges and non-crop habitats. The simplification of landscape composition and the decline of biodiversity may affect the functioning of natural pest control because non-crop habitats provide requisites for a broad spectrum of natural enemies, and the exchange of natural enemies between crop and non-crop habitats is likely to be diminished in landscapes dominated by arable cropland. In this review, we test the hypothesis that natural pest control is enhanced in complex patchy landscapes with a high proportion of non-crop habitats as compared to simple large-scale landscapes with little associated non-crop habitat. In 74% and 45% of the studies reviewed, respectively, natural enemy populations were higher and pest pressure lower in complex landscapes versus simple landscapes. Landscape-driven pest suppression may result in lower crop injury, although this has rarely been documented. Enhanced natural enemy activity was associated with herbaceous habitats in 80% of the cases (e.g. fallows, field margins), and somewhat less often with wooded habitats (71%) and landscape patchiness (70%). The similar contributions of these landscape factors suggest that all are equally important in enhancing natural enemy populations. We conclude that diversified landscapes hold most potential for the conservation of biodiversity and sustaining the pest control function.

https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2006.3530

Sustainable pest regulation in agricultural landscapes: a review on landscape composition, biodiversity and natural pest control

Optimal plant defense should incorporate any mechanisms that influence the feeding behavior of potential pests. From a diverse collection of examples suggesting that the defense of a plant may be improved in the company of specific neighbors, we discuss a framework of operational mechanisms that begin to clarify some aspects of the recognized influence of species diversity on herbivory. Neighbors serve as insectary plants for herbivore predators and parasites, and influence herbivore feeding behavior by repelling, masking, attracting, and decoying. Insectary plants lower the numerical response of herbivores by increasing the efficiency of their predators and parasites. Repellent plants primarily lower functional response by causing the predator to fail to locate or reject its normal prey. Attractant-decoy plants dilute herbivore impact by drawing off herbivores, either increasing or decreasing their numerical and functional response (or either). The concept of gene conservation guilds adds diversionary and delaying tactics to the adaptation-counteradaptation view of plant-herbivore coevolution. The useful life of a given gene for resistance may best be extended by mechanisms that disrupt genetic tracking (specialization) by herbivores. Some plants may remain inedible not because their chemistry or morphology represents an evolutionary impasse, but because they live in an environment that provides acceptable options of variable quality. Feeding environments that provide little or no choice promote specialization by forcing physiological adaptation. Conversely, the evolutionary momentum of specializing herbivores may be lowered by enhancing their susceptibility, either by selection against virulent individuals, or by decreasing the exposure frequency of susceptible genotypes. The latter mechanism of conserving susceptible individuals takes advantage of herbivore behavioral sensitivity to variable plant quality. Direct selection against virulent genotypes requires temporal cycling of the herbivore population between resistant and nonresistant hosts. Both events may occur within defense guilds that provide acceptable feeding options of similar but distinctive quality.

https://science.sciencemag.org/content/sci/193/4247/24.full.pdf

Plant Defense Guilds

Vegetation management and biological control in agroecosystems

▪ Abstract Many terrestrial communities include omnivorous arthropods that feed on both prey and plant resources. In this review we first discuss some unique morphological, physiological, and behavioral traits that enable omnivores to exploit such dissimilar foods, and we explore possible evolutionary pathways to omnivory. We then examine possible benefits and costs of omnivory, describe the relationships between omnivory and other high-order complex trophic interactions, and consider the stability level of communities with closed-loop omnivory. Finally, we explore some of the implications of omnivory for crop damage and for biological, chemical, and cultural control practices. We conclude that the growing realization of the ubiquity of omnivory in nature may require a change in our view of the structure and function of ecological systems.

Omnivory in terrestrial arthropods: mixing plant and prey diets.

The enemies hypothesis holds that predatory insects and parasitoids are more effective at controlling populations of herbivores in diverse systems of vegetation than in simple ones. Eighteen studies that tested the enemies hypothesis are reviewed. Of those studies reporting mortality from predation or parasitism, nine found higher mortality rates in diverse systems; two found a lower mortality rate; and two found no difference. The mechanisms that are thought to underlie the enemies hypothesis and directions for future research are discussed. Evidence suggests that the enemies hypothesis and the resource concentration hypothesis (which predicts that herbivores more easily find, stay in, and reproduce in monocultures of host plants than in polycultures) are complementary mechanisms in reducing numbers of herbivores in diverse agricultural systems.

/pdf/enemies-hypothesis-a-review-of-the-effect-of-vegetational-58zbj83grl.pdf

Enemies Hypothesis: A Review of the Effect of Vegetational Diversity on Predatory Insects and Parasitoids

SUMMARY

A wide range of natural enemies, including predators, parasites and entomophagous fungi were observed to feed on the stinging nettle aphid, Microlophium carnosum, populations of which increased rapidly in late April and early May. Patches of stinging nettles thus served as an important alternate feeding site for some beneficial natural enemies before pest aphids appeared on cultivated plants. Anthocoridae, Miridae and Coccinellidae were the most abundant specific predators sampled on nettles; only the Coccinellidae appeared to disperse over a defined period to other habitats. The hymenopterous parasites Aphidius ervi and Ephedrus lacertosus parasitized up to 10% of M. carnosum populations in June: two species of the fungus Entomophthora occurred spasmodically.



Cutting patches of nettles in May or June had the most striking effect on the species and numbers of Coccinellidae. Cutting in mid-June might increase the numbers and impact of natural enemies on nearby pest infestations.

The role of the perennial stinging nettle, Urtica dioica, as a reservoir of beneficial natural enemies

SUMMARY

The Cowpea seed moth, Cydia ptychora, was able to complete its full life-cycle on a wide range of tropical grain legumes. In glasshouse tests, the adult moth showed oviposition p references between grain legume species but not between cowpea varieties, although there were differences in larval damage to the seeds of the latter. On cowpea, most eggs were laid on 8–10-day-old pods and also in particular micro-habitats; such behaviour was seemingly influenced by the mode of pod carriage.



The moth undergoes no known diapause during the dry season in southern Nigeria and cowpea plants in a suitable condition are infested throughout the year. Wild leguminous hosts were not identified during the present study.

R. M. Perrin

Papers

The role of the perennial stinging nettle, Urtica dioica, as a reservoir of beneficial natural enemies

Studies on host‐plant selection by the cowpea seed moth, Cydia ptychora