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.

Herbivore induced release of plant volatiles mediating the foraging behavior of the aphid parasitoid Aphidius ervi was investigated using the pea aphid, Acyrthosiphon pisum, feeding on broad bean, Vicia faba. Behavioral responses were studied using an olfactometer and a wind tunnel. Volatiles obtained by air entrainment of aphid infested plants were more attractive to A. ervi than those from uninfested plants, in both behavioral bioassays. GC-EAG of both extracts showed a number of peaks associated with responses by A. ervi, but with some differences between extracts. Compounds giving these peaks were tentatively identified by GC-MS and confirmed by comparison with authentic samples on GC, using two columns of different polarity. The activity of pure compounds was further investigated by EAG and wind tunnel assays. Results showed that, of the compounds tested, 6-methyl-5-hepten-2-one was the most attractive for A. ervi females, with linalool, (Z)-3-hexen-1-yl acetate, (E)-β-ocimene, (Z)-3-hexen-1-ol, and (E)-β-farnesene all eliciting significantly more oriented flight behavior than a solvent control. Foraging experience significantly increased parasitoid responses to these compounds, with the exception of (E)-β-farnesene. Time-course GC analysis showed that feeding of A. pisum on V. faba induced or increased the release of several compounds. Release of two of these compounds (6-methyl-5-hepten-2-one and geranic acid) was not induced by the nonhost black bean aphid, Aphis fabae. During the analysis period, production of (E)-β-ocimene remained constant, but 6-methyl-5-hepten-2-one, linalool, geranic acid, and (E)-β-farnesene appeared during the first day after A. pisum infestation and increased in concentration with increasing time of aphid feeding.

Identification of Semiochemicals Released During Aphid Feeding That Attract Parasitoid Aphidius ervi

For over 100 years, arthropod pests and weeds have been controlled by the introducion of exotic natural enemies (21). For almost as long, entomologists have discussed the importance of genetics and the possibility of genetic manipulations in biological control, and today these remain subjects of considerable debate. Perhaps the first issue raised was that predators and parasites might be artificially selected for improved performance in the field (46, 47, 59). In contrast, a major concern for laboratory rearing of insects and mites used in biological control is that such rearing will promote genetic adaptation to laboratory conditions and thus worsen performance in the field (57. 58). Another concern is that natural enemies imported for biological control constitute only a limited sample of the genetic variation present in the species, and may therefore lack the ability to adapt to their new environment (64,65). Many of these issues. and possible solutions. have been discussed recently

Management of Genetics of Biological-Control Introductions

Strategies Involved in the Location of Hosts by the ParasitoidAphidius erviHaliday (Hymenoptera: Braconidae: Aphidiinae)

The responses of femaleAphidius ervi to odors from a host food plant (Vicia faba), host aphids (Acyrthosiphon pisum), nonhost aphids (Aphis fabae), and aphid-plant complexes were investigated in a specially designed wind tunnel and a Y-tube olfactometer. In single-target (no-choice) and two-target (dual-choice) experiments, plant volatiles played a crucial role in the host foraging behavior ofA. ervi. The odor from theA. pisum-plant complex elicited the strongest responses byA. ervi females, followed by the odor from plants previously damaged by the feeding ofA. pisum. There was a significantly weaker response to odor fromA. pisum in the absence of the plant and to undamaged plants. Similarly, mechanically damaged plants and plants infested with the nonhost aphidA. fabae did not elicit strong responses. A plant that had been damaged byA. pisum and subsequently washed with distilled water was as attractive as an unwashed, previously infested plant.Aphidius ervi probably overcomes the reliability-detectability problem by selectively responding to herbivore-induced, volatile, semiochemical cues emitted by the first trophic level and by distinguishing between the volatiles induced by host and nonhost aphids.

Relative importance of semiochemicals from first and second trophic levels in host foraging behavior ofAphidius ervi.

The host preference behaviour of the generalist aphid parasitoid Aphidius colemani was investigated using a Y-tube olfactometer. Female A. colemani showed a preference for the host-plant complex on which they had been reared, even though the same aphid host was involved, demonstrating a host plant preference. This preference was not evident when the parasitoids were dissected from their mummies prior to adult emergence. Host plant preference exhibited during host selection appeared to be induced by chemical cues encountered on the mummy case at the time of emergence, but preferences could be changed by subsequent foraging experiences. It is concluded that plant chemical cues play a major role in determining initial preferences through a process of emergence conditioning but that learning processes, involving cues encountered during oviposition in or contact with the host, can modify these initial preferences.

Wilf Powell

Papers

Identification of Semiochemicals Released During Aphid Feeding That Attract Parasitoid Aphidius ervi

Management of Genetics of Biological-Control Introductions

Strategies Involved in the Location of Hosts by the ParasitoidAphidius erviHaliday (Hymenoptera: Braconidae: Aphidiinae)

Relative importance of semiochemicals from first and second trophic levels in host foraging behavior ofAphidius ervi.

The role of plant chemical cues in determining host preference in the generalist aphid parasitoid Aphidius colemani.