Parasitoid-host interactions

Ultimately, the success of a parasitoid-host interaction depends on the ability of the parasitoid to affect certain biochemical processes within the constraints imposed by the host. 

Effect on the Host Endocrine System. Several publications document an endocrine basis of several parasitoid-host interactions (18, 20, 49, 50). However, most of the mechanisms involving the endocrine interactions remain to be clarified. At best, we can describe the qualitative effects but not the regulatory factors responsible for the interactions. Until the characterization of... 

Teratocytes and Serosa. Teratocytes, which originate from the disintegration of an embryonic membrane or serosa (= trophamnion) of some parasitic Hymenoptera, appear to have a significant role in parasitoid-host interactions (, 77-80, Dahlman, D. L. Arch. Insect Biochem. Physiol., in press). A discussion of teratocytes is... 

A long nonoccluded filamentous virus, morphologically distinct from the polydnaviruses, was characterized from the reproductive tract of the parasitoid C, marginiventris. Though the role of the filamentous virus in the parasitoid-host interaction is unknown, the virus was reported to replicate in hypodermal and tracheal matrix cells of the host larvae (113). 

Tetratrophic interactions between a host plant, a phytophagous pest (primary host), a hymenopteran parasitoid or symbiont (secondary host) and a hymenopteran hyperparasitoid (which parasitizes the secondary host) are of considerable importance, because hyperparasitism can significantly reduce populations of economically beneficial parasitoids [11]. Hyperparasitoids use host-marking (=spacing) pheromones, sex pheromones [12], and host-detection cues [42], but they also show additional chemically mediated interactions with the other partners. These include detection of the primary host s secretions by the hyperparasitoid [43], detection of plant volatiles by the hyperparasitoid [44], and detection of the hyperparasitoid s secretions by the primary host [45] or by the secondary host. The latter causes the secondary host to avoid locations where the hyperparasitoid is foraging [46]. 

Schuster, M. F. and Calderon, M. (1986). Interactions of host plant resistant genotypes and beneficial insects in cotton ecosystems. In Interactions of Plant Resistance and Parasitoids and Predators of Insects, eds. D. J. Boethel and R. D. Eikenbary, pp. 84-97. New York John Wiley Sons. 

Chapters in this volume consider how plants use chemicals to defend themselves from insect herbivores the complexity of floral odors that mediate insect pollination tritrophic interactions of plants, herbivores, and parasitoids, and the chemical cues that parasitoids use to find their herbivore hosts the semiochemically mediated behaviors of mites pheromone communication in spiders and cockroaches the ecological dependence of tiger moths on the chemistry of their host plants and the selective forces that shape the pheromone communication channel of moths. 

The external cuticle of insects is covered by a waxy layer composed of mixtures of hydro-phobic lipids that include long-chain alkanes, alkenes, wax esters, fatty acids, alcohols, aldehydes, and sterols. The primary purpose of this layer is to maintain water balance and prevent desiccation, as described in Chapter 6, but many of the cuticular lipid components have important secondary roles as intraspecific contact chemical signals (pheromones). These roles include species and sex recognition during reproductive interactions, and nestmate recognition and other colony organization functions in social insects. Thus, these compounds are essential mediators of insect behaviors. Cuticular compounds are also exploited by parasitoids and predators as interspecific contact cues (kairomones) to aid in host location. 

Hawkins BA. Pattern and Process in Host-Parasitoid Interactions. Cambridge University Press Cambridge, UK, 1994. 

Many factors contribute to the interactions of parasitoids and their host insects (10). Table I summarizes data of parasitic Hymenoptera that regulate the biochemistry, development, and behavior of their host. This is an attempt to list those species that have been subjected to thorough studies and is not intended to be a complete list. 

The adult female parasitoid may transfer one or more type(s) of regulatory factor(s) to the host during parasitism. Some genera of braconid and scelionid families produce teratocytes. These cells originate from an embryonic membrane in the parasitoid egg and are released into the host hemocoel. Certain parasitoid females of the families Ichneumonidae and Braconidae contain symbiotic viruses that are injected into the host with the egg. Diverse functions have been attributed to the teratocytes and the symbiotic viruses. Both play a major role in the interaction of the parasitoid with their respective hosts. 

Effect on the Composition of the Host. The primary purpose for the association of the parasitoid and the host is for the parasitoid to use the host as a source of nutrients (for reviews of parasitoid nutrition see 23, 24). However, the effect of parasitism extends beyond the depletion of host nutrients by the feeding parasitoid. Clearly the interaction between the parasitoid and the host is bidirectional, with the parasitoid responding to host substances and the host responding to the parasitoid. 

Parasitoid Response to the Host. Simultaneous with the effects of parasitism on the host, are the effects of the host on the parasitoid. The response of the parasitoid to host parameters influences the way the parasitoid interacts with the host. This creates a continuum between the "conformers and the "regulators. ... 

The mechanisms whereby parasitoids use kairomones to locate hosts are obviously of crucial importance in predator-prey interactions and may be divided in two categories depending on whether they are the result of long range or cole-range chemoreception. In some cases, olfaction can be influenced by the parasitoid s previous experience (551). A number of kairomones used by parasitoids as aids in host location have been identified. Hemolymph, cuticule, frass scale, mandibular gland and feces can be sources of such kairomones, and long-chain hydrocarbons are the main chemical stimuli responsible (150, 552-569). 

Comparison of the pheromone systems of closely related species, has revealed that the attraction and the pheromone-receptor interaction found between them correlates with the degree of relatedness (Lanier and Wood, 1975 Priesner, 1979a, b). However, such a cross attraction could be disadvantageous for related species living in the same area, and under such circumstances an additional interspecific mechanism - interruption - is found which serves to maintain the isolation between sympatric species (Birch, Chapter 12). Thus, one species may produce an additional pheromone compound which interrupts the attraction of the other species. Chemical compounds which function as pheromones may also serve as cues for predators, parasitoids and prey (Vincent, Chapter 8). In these cases the predator or parasites have developed capabilities for recognizing the pheromones of their prey or host insects. Predators may also produce the attractants of their prey (Weaver, 1978 Eberhard, 1977). Conversely, some insect species have developed defensive compounds which are avoided by both insect and vertebrate predators, (cf. Eisner, 1970 Blum, 1978 Huheey, Chapter 10). Finally, during... 

The potential host community can be perceived by parasitoids through any of the above stimuli from the host, the food or shelter of the host, organisms associated with the host, or interactions between these factors. Plants appear to play an important role in potential host community location, partly because plants are the source of food for most hosts. It has been contended that the evolution of the parasitoid habitat in Hymenoptera may have stemmed from a previous plant-parasite relationship (Malyshev, 1968). If true, such a relationship would provide insight into the role of plants in the host selection process. The importance of plants in host community location is further supported by the observation that there is less tendency for parasitoids to select phylogen-etically related hosts than unrelated hosts found on the same plant (Cross and Chesnut, 1971). 

Community-wide effects on endophytes may be extended beyond herbivores to their associated parasites and predators in other trophic levels. For example, Neotyphodium endophytes of Lolium multiflorum, lower plant quality, reduce the densities of aphid herbivores, and indirectly reduce the rate of parasitism on aphids by parasitoids [105]. The potential for alteration of the interactions of grasses with herbivores and their natural enemies will clearly depend on many factors, including the frequency of infected hosts within the commxmity, the types and relative levels of bioprotective alkaloids, and the spatial distribution of grasses and herbivores. 

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