Most biological control systems involve a diverse community of natural enemies. We investigated how specialist and generalist natural enemies differ as biological control agents of pea aphids (Acyrthosiphon pisum), and how interactions among natural enemies affect successful control. In alfalfa, pea aphids are attacked by a specialist parasitoid wasp, Aphidius ervi, and a guild of generalist predators primarily made up of Nabis and Orius bugs, coccinellid and carabid beetles, and web-building spiders. In three field experiments, we manipulated the parasitoid, then the generalist predator guild, and finally both classes of natural enemy, and recorded resulting impacts on pea aphid population control. The parasitoid caused little immediate reduction in aphid population growth but caused a large decline after a delay corresponding to the generation time of the parasitoid. In contrast, the generalist guild caused an immediate decline in the aphid population growth rate. However, the generalists did not exert density-dependent control, so aphid densities continued to increase throughout the experiment. The third field experiment in which we simultaneously manipulated parasitoids and predators investigated the possibility of “nonadditive effects” on aphid control. Densities of parasitoid pupae were 50% lower in the presence of generalist predators, indicating intraguild predation. Nonetheless, the ratio of parasitoids to aphids was not changed, and the impact of the two types of natural enemies was additive.

We constructed a stage-structured model of aphid, parasitoid, and predator dynamics and fit the model to data from our field experiments. The model supports the additivity of parasitoid and predator effects on aphid suppression but suggests that longer-term experiments (32 d rather than 20 d) would likely reveal nonadditive effects as predation removes parasitoids whose response to aphid densities occurs with a delay. The model allowed us to explore additional factors that could influence the additivity of parasitoid and predator effects. Aphid density-dependent population growth and predator immigration in response to aphid density would likely have little influence on the additivity between parasitism and predation. However, if a parasitoid were to show a strong Type II functional response, in contrast to A. ervi whose functional response is nearly Type I, interactions with predators would likely be synergistic. These analyses reveal factors that should be investigated in other systems to address whether parasitism and predation act additively on host densities.

Corresponding Editor: E. Evans.

https://www.jstor.org/stable/pdfplus/3108000.pdf

Interactions between specialist and generalist natural enemies: parasitoids, predators, and pea aphid biocontrol

Publisher Summary This chapter discusses the resistance of plants to insects. Resistance is the ability of a certain variety to produce a larger yield of good quality than other varieties at the same initial level of infestation and under similar environmental conditions. Three basic components or mechanisms are involved in plant resistance. First, plants may be nonpreferred for oviposition, shelter, or food, primarily, because of the lack of or presence of chemical or physical factors. Second, resistant plants may affect the biology of the insect adversely that is called antibiosis. Third, resistant plants may be tolerant, surviving under levels of infestation that would kill or severely injure susceptible plants. Several problems may occur at various stages in the development of any host plant resistance program. Some of these problems include: proper finance, entomological problems, and problems in technique development. Resistance is most often found in off types or exotic wild species, and considerable crossing and selection is required to move the resistant genes into a desirable agronomic background. The greatest use of resistant varieties would undoubtedly be as one component part of pest management system.

Resistance of plants to insects.

Two broad classes of arthropod natural enemies attack insect herbivores: specialists and generalists. The tight dynamical linkage of specialist natural enemies and their prey may make the specialists able to respond numerically to, and perhaps suppress, herbivore outbreaks. Because generalist predators may attack not only herbivores, but also the herbivores' specialist natural enemies, generalist predators may disrupt control of herbivore populations rather than contribute to it. We examined interactions between pea aphids (Acyrthosiphon pisum), a specialist parasitoid wasp (Aphidius ervi) that attacks the aphids, and a common generalist predator, carabid beetles (primarily Pterostichus melanarius). In two field experiments in alfalfa, we manipulated carabid densities to measure their direct (through predation) and indirect (through intraguild predation on A. ervi) impact on aphid population dynamics. The first experiment was initiated when plants were short (following cutting), and carabid predation of a...

https://www.jstor.org/stable/pdfplus/2680190.pdf

Generalist predators disrupt biological control by a specialist parasitoid

The factors responsible for determining the host-plants and feeding sites of aphids, and the various probing activities (the role of the labium, stylet insertion, surface saliva deposition, the behaviour of the aphid, virus transmission) are examined. There is a brief review of stylet structure and movement and the possible sensory nature of these organs, followed by a detailed review of the characteristics of aphid stylet paths in plant tissues. The penetration of epidermis and vascular tissues is treated separately while that within the intermediate tissues is covered in relation to leaves and stems, roots, trees, galls and excised tissue as well as in separate sections on Aphis fabaeScopoli and Myzus persicae (Sulzer). Stylet destinations and behaviour in the sieve tubes are discussed together with general features such as rate and depth of penetration, guidance to the feeding site, effects of tissue hardness and stylet withdrawal. The ingestion rate of plant sap is reviewed and its constitution and importance examined together with the significance of artificial diets. The salivary secretions including sheaths and tracks, their functions and their role in the transference of material between aphid and host are dealt with. The nature of the physical and internal damage resulting from aphid feeding is briefly covered, and also some plant-insect interrelations. The aphid species whose stylets have been examined in plant tissue are listed.

Plant penetration by feeding aphids (Hemiptera, Aphidoidea): a review

Plant Resistance to Insects

Various selection criteria were effective in isolating pea aphid, Macrosiphum pisi (Harr.), resistant plants from susceptible alfalfa, Medicago satire L., populations and these selection criteria also appeared suitable for the evaluation of progeny from resistant plant selections. Selection of vigorous surviving seedlings after infestation with large aphid populations in early seedling stage was a practical and rapid method for use in the selection of resistant plants. Selection and evaluation for resistance were also accomplished on the basis of differential plant stunting at various growth stages and on basis of aphid colonization on individual plants. Although some of the techniques presented were applicable to field conditions under natural aphid infestation, the advantage of greenhouse screening and evaluation in the seedling stage is evident. The general agreement of the results obtained with the various techniques employed indicates their validity and usefulness as a basis for selecting and evaluating resistant plants.

Selection and Evaluation of Pea Aphid-Resistant Alfalfa Plants

Substantial yield losses have occured from greenbug, Schizapus graminum (Randani), infestations of the sorghum crop over a wide geographical area Economic populations of chinch bugs, Blissus Leucopterus (Say), are more sporadic and localised than greenbug infestations that occur
most years Extensive iosses have been attributed to the A strain of Maize Dwarf Mosaic Virus (MDMV)

Breeding for resistance to greenbug, chinch bug and maize dwarf mosaic in sorghum

Greenbugs, Schizaphis graminum (Rondani), mixed in corncob grits were effectively transferred to wheat, sorghum, and pearl millet plants in both field and greenhouse by a mechanical insect dispenser (inoculator or bazooka). Numbers of greenbugs released were determined by the ratio of greenbugs: grit. Over 90% of greenbugs survived in grits for 2, 6, 12, and 24 h at 35, 25, 15, and 5°C, respectively. Infestations produced by greenbug releases from the inoculator were more uniform than those obtained by mass infestation methods. Preliminary observations suggest that the technique also may be useful to infest plant with other aphid species and some Hemiptera.

Mechanical Insect Dispenser for Infesting Plants with Greenbugs (Homoptera: Aphididae)

Alate greenbugs were trapped at Hays, Kans., from 1974 to 1979, inclusive. There were two distinct flight periods each year. The first flight peaked during the first week of June, and the second during the last week of July (1974 to 1977) or the third week of August (1978 and 1979). Flight patterns indicated that early planting of both wheat and sorghum should be avoided. More greenbugs were trapped from SE than NW during the spring but not during the summer. More greenbugs were collected from heights of 3 to 6 m than from 1.5 m, but that might be attributed to type of ground cover. As the green vegetation increased, the numbers of greenbugs collected decreased; however, strips of green wheat did not provide practical protection to adjacent bordering areas.

H. L. Hackerott

Papers

Selection and Evaluation of Pea Aphid-Resistant Alfalfa Plants

Breeding for resistance to greenbug, chinch bug and maize dwarf mosaic in sorghum

Mechanical Insect Dispenser for Infesting Plants with Greenbugs (Homoptera: Aphididae)

Dispersal of Alate Biotype C Greenbugs in Kansas