The rhizosphere encompasses the millimeters of soil surrounding a plant root where complex biological and ecological processes occur. This review describes recent advances in elucidating the role of root exudates in interactions between plant roots and other plants, microbes, and nematodes present in the rhizosphere. Evidence indicating that root exudates may take part in the signaling events that initiate the execution of these interactions is also presented. Various positive and negative plant-plant and plant-microbe interactions are highlighted and described from the molecular to the ecosystem scale. Furthermore, methodologies to address these interactions under laboratory conditions are presented.

The Role of Root Exudates in Rhizosphere Interactions with Plants and Other Organisms

▪ Abstract Many agroecosystems are unfavorable environments for natural enemies due to high levels of disturbance. Habitat management, a form of conservation biological control, is an ecologically based approach aimed at favoring natural enemies and enhancing biological control in agricultural systems. The goal of habitat management is to create a suitable ecological infrastructure within the agricultural landscape to provide resources such as food for adult natural enemies, alternative prey or hosts, and shelter from adverse conditions. These resources must be integrated into the landscape in a way that is spatially and temporally favorable to natural enemies and practical for producers to implement. The rapidly expanding literature on habitat management is reviewed with attention to practices for favoring predators and parasitoids, implementation of habitat management, and the contributions of modeling and ecological theory to this developing area of conservation biological control. The potential to int...

Habitat Management to Conserve Natural Enemies of Arthropod Pests in Agriculture

Traditionally, measurement of the acute toxicity of pesticides to beneficial arthropods has relied largely on the determination of an acute median lethal dose or concentration. However, the estimated lethal dose during acute toxicity tests may only be a partial measure of the deleterious effects. In addition to direct mortality induced by pesticides, their sublethal effects on arthropod physiology and behavior must be considered for a complete analysis of their impact. An increasing number of studies and methods related to the identification and characterization of these effects have been published in the past 15 years. Review of sublethal effects reported in published literature, taking into account recent data, has revealed new insights into the sublethal effects of pesticides including effects on learning performance, behavior, and neurophysiology. We characterize the different types of sublethal effects on beneficial arthropods, focusing mainly on honey bees and natural enemies, and we describe the methods used in these studies. Finally, we discuss the potential for developing experimental approaches that take into account these sublethal effects in integrated pest management and the possibility of integrating their evaluation in pesticide registration procedures.

The Sublethal Effects of Pesticides on Beneficial Arthropods

Host plant quality is a key determinant of the fecundity of herbivorous insects. Components of host plant quality (such as carbon, nitrogen, and defensive metabolites) directly affect potential and achieved herbivore fecundity. The responses of insect herbivores to changes in host plant quality vary within and between feeding guilds. Host plant quality also affects insect reproductive strategies: Egg size and quality, the allocation of resources to eggs, and the choice of oviposition sites may all be influenced by plant quality, as may egg or embryo resorption on poor-quality hosts. Many insect herbivores change the quality of their host plants, affecting both inter- and intraspecific interactions. Higher-trophic level interactions, such as the performance of predators and parasitoids, may also be affected by host plant quality. We conclude that host plant quality affects the fecundity of herbivorous insects at both the individual and the population scale.

Host plant quality and fecundity in herbivorous insects

Parasitoids and predators of herbivores have evolved and function within a multitrophic context. Consequently, their physiology and behavior are in­ fluenced by elements from other trophic levels such as their herbivore victim (second trophic level) and its plant food (first trophic level) (126) . Natural enemies base their foraging decisions on information from these different trophic levels, and chemical information plays an important role. This review is restricted to the ecology of chemical information from the first and second trophic levels. The importance of so-called infochemicals, a subcategory of semiochemicals, in foraging by parasitoids and predators has been well documented (e.g. reviewed in 31, 78, Il l , 183, 185) , and we do not intend to repeat the details. But because of a lack of testable hypotheses, all this research is conducted rather haphazardly: the total puzzle of infochemical use has not been solved for any natural enemy species. Here we approach the use of infochemicals by natural enemies from an evolutionary and ecological standpoint. Our basic concept is that information from the first and second trophic levels differs in availability and in reliability, a difference that shapes the way infochemicals are used by a species. We generate hypotheses on (a)

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Ecology of infochemical use by natural enemies in a tritrophic context.

Conservation and manipulation of aphid parasitoids

Pheromones in the management of beneficial insects

The oviposition attack behaviour of the aphid parasitoid Aphidhis etvi Haliday is regulated by both chemical and physical cues. The host aphid Acyrthosiphon pisum (Harris) sealed into the tip of a glass capillary tube elicited oviposition attack responses from naive female parasitoids, demonstrating the role of visual cues. Colour seems to be an important cue because responses were elicited by other green aphids sealed into glass capillaries but not by the brown form of the natural host Sitobion avenae (F.). This was confirmed by placing paint pigments into the glass capillary tips. The parasitoids responded to yellow pigments, one of which elicited repeated oviposition attack responses from all females tested, but they did not respond to green pigments. The spectrum of reflected light from the yellow pigments was very similar to that from green Ac. pisum, with a high proportion of the total energy in the yellow-orange waveband. Aphid comicle secretion also elicited oviposition attack responses, which were not caused by its pale yellow-green colour- because the level of response was unaffected under red light conditions. The response to yellow pigments disappeared under red light. Responses to comicle secretion declined with time after its collection from the aphids, but olfactometer tests showed that this was not due to a loss of bioactive volatiles but that the kairomone involved is a very short range or contact cue. Host exuviae appear to provide a much more stable source of the oviposition kairomone as they retain their activity for many days. This work confrrms the existence of an aphid cuticular kairomone, as suggested by previous studies, and attempts are now being made to isolate the relevant chemical compound(s) from aphid exuviae.

Physical and chemical cues regulating the oviposition behaviour of Aphidius ervi Haliday (Hymenoptera: Braconidae)

The lily beetle (Lilioceris lilii, Chrysomelidae) feeds on Lilium, Fritillaria and Cardiocrinum plants and is a serious pest in gardens and amenity plantings in parts of Northern Europe and North America. Previous studies have indicated that within the beetle's host range there is variation in susceptibility, although thorough field investigation is lacking. Therefore a 3-year field trial was carried out to assess the susceptibility of six different Lilium to the beetle. The trial was laid out over two replicate blocks, with each block divided into a six by six grid, giving 36 plots; each plot contained nine Lilium of the same type. Lily types were allocated to plots within a block according to a quasi-complete Latin square. The plots were assessed weekly during the growing season over 3 years for plant damage and presence of the beetle. For each plot a damage index for each year was calculated as the average damage score per scoring visit. As beetle presence was low in the first year the damage index, mean adult, larval and egg presence over the final 2 years were analysed using ANOVA. The results indicate that there are significant differences in beetle occurrence and damage on the different Lilium in the trial, however there is also an interaction between damage index and year. The species L. regale and L. 'Golden Joy' gave consistently lower damage index/occurrence means than the hybrids L. 'Tiber', L. 'Brindisi', L. 'Conca d'Or' and L. 'Eliganzer'. We suggest that future host susceptibility trials should use a standard lily such as L. regale against which others can be compared, should take measurements of both beetle occurrence and damage and be carried out over several years to get reliable results. As there are differences in susceptibility of lilies there is a possibility to use lilies more prone to attack; this could be as trap plants for the beetle.

Wilf Powell

Papers

Conservation and manipulation of aphid parasitoids

Pheromones in the management of beneficial insects

Physical and chemical cues regulating the oviposition behaviour of Aphidius ervi Haliday (Hymenoptera: Braconidae)

Susceptibility of six Lilium to damage by the lily beetle, Lilioceris lilii (Coleoptera; Chrysomelidae)

Pushing insects around: the use of 'push-pull' strategies in integrated pest management