Parasitoid host location

Some of the early work on parasitoid host location involved non-living host food. Thorpe and Jones (1937) found that Venturia canescens is attracted to the odor of oatmeal. Meat odor was found to be attractive to Alysia manducator and Nasonia Mormoniella) vitripennis (Laing, 1937). Since the source and nature of the responsible chemicals was not determined, it may be that microorganisms were involved. 

Some aspects of plant variation could interfere with the impact of natural enemies. Some enemies may be unable to associate microhabitat cues (e.g., chemical, physical, color, position) with prey or host location. For these enemies, prey or host feeding on restricted tissues will tend to appear widely spaced and they may not be readily encountered. It appears to me that many, if not most, parasitoids and predators can be found to use one or more cues. This negative effect could be counteracted by increased encounter rates during herbivore searching movements. 

McCall, P. J., Turlings, T. C. J., Lewis, W. J. and Tumhnson, J. H. (1993). Role of plant volatiles in host location by the specialist parasitoid Microplitis croceipes Cresson (Braconidae, Hymenoptera). Journal of Insect Behavior 6 625-639. 

Meiners, T. and Hilker, M. (1997). Host location in Oomyzus gallerucae (Hymenoptera Eulophidae), an egg parasitoid of the elm leaf beetle Xanthogaleruca luteola (Coleoptera Chrysomelidae). Oecologia 112 87-93. 

Meiners, T., Westerhaus, C. and Hilker, M. (2000). Specificity of chemical cues used by a specialist egg parasitoid during host location. Entomologia Experimentalis et Applicata95 151-159. 

Steinberg, S Dicke, M. and Vet, L. E. M. (1993). Relative importance of infochemicals from 1st and 2nd trophic level in long-range host location by the larval parasitoid Cotesia glomerata. Journal of Chemical Ecology 19 47-59. 

Baarlen, P. V., Topping, C. J. and Sunderland, K. D. (1996). Host location by Gelis festinans, an eggsac parasitoid of the linyphiid spider Erigone atra. Entomologica Experimentalis etApplicata 81 155-163. 

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. 

Plant-derived compounds such as (3Z)-hexenyl acetate, (3Z)-hexen-l-ol, (2 )-hexenal, (31 )-hexenal, 4-methyl-3-heptanol, (llZ)-hexadecenal, (9Z)-hexadecenal, and (7Z)-hexadecenal are used by parasitoids to locate the host habitat (Elzen et al., 1984 Kennedy, 1984 Turlings et al., 1991 Wickremasighe and Emden, 1992). (2E)-Hexenal was strongly inhibitory to tomato seed germination at concentrations as low as 6.9 pM (Bradow and Connick, 1990). 

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). 

Weseloh, R.M. Host Location by Parasitoids. In D.A. Nordlung, R.L. Jones, and W.J. Lewis eds., Semiochemicals Their Role in Pest Control, p. 79-95. New York John Wiley Sons 1981. 

Host sex pheromones can serve as cues to host location for several parasitoid species (Kennedy, 1979 Sternlicht, 1973). Prokopy and Webster (1978) found that the marking pheromone of Rhagoletis pomonella stimulates oviposition probing of Opius lectus. 

The great diversity of parasitoids and the complexity of parasitoid-host relationships is probably influenced by attempts of potential host insects to escape their predators, parasites and parasitoids. There is considerable speculation on the role that parasites (parasitoids) play in herbivore evolution. There are many examples where a host on different plants is attacked by different parasitoid species (see Vinson, 1981). As discussed by Zwolfer and Kraus (1957), and Vinson (1981), plants play an important role in the host selection process, probably by providing cues to the location of a potential host community. Theoretically, a host could escape a particular parasitoid by attacking a plant lacking those stimuli used by the parasitoid to locate the potential host community. This idea is supported by the observation that there is less tendency for parasitoids to select phylogenetically related hosts than to favor a range of hosts on a particular plant (Askew and Shaw, 1978 Cross and Chesnut, 1971). 

Weseloh, R. M. (1981) Host location by parasitoids. In Semiochemicals their Role in Pest Control (Nordlund, D. A., Jones, R. L. and Lewis, W. J., eds) pp. 79-95. John Wiley, New York. 

Birkett, M.A., et al. (2003) Volatiles from whitefly-infested plants elicit a host-locating response in the parasitoid, Encarsia formosa. J. them. Ecol. 29, 1589-1600... 

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]. 

Table 2.1. Examples of predators and parasitoids that use induced plant odors to locate their prey or hosts... 

Moreover, many parasitoids and predators, whether they are generalists or not, can find their hosts or prey on a variety of plant species and each of these has its own characteristic basic odor blend. Therefore, natural enemies that use plant odors to locate their prey will need to determine which odors are most reliably associated with a certain prey at a certain time. 

Powell, W., Pennacchio, F., Poppy, G. M. and Tremblay, E. (1998). Strategies involved in the location of hosts by the parasitoid Aphidius ervi Haliday (Hymenoptera Braconidae Aphidiinae). Biological Control 11 104-112. 

Chemical cues from spiders are also used by non-arachnid predators to locate their prey. For example, the ichneumonid wasp Gelisfestinans parasitizes the spider Erigone atra, which lives in wheat fields. Contact with the silk of its host elicits increased searching behavior from the parasitoid, whereas contact with silk from other spider species does not, indicating a high degree of specificity (Baarlen et al.,... 

Compared to parasitoid wasps, only a few species of dipteran parasitoids are reported to respond to HIPVs148-153 Host-habitat location remains largely unknown in parasitoid flies. 

In many cases, the volatile compounds emitted from leaves as a result of insect damage allow insect parasitoids and predators to distinguish between infested and uninfested plants, and therefore help to locate hosts or prey [230]. In the majority of plants reported so far, there are remarkable similarities in the structure of VOCs that are emitted from insect-damaged leaves [231]. This structural uniformity suggests the activation of a common set of biosynthetic pathways shared by a wide range of plants, and that the products are detectable by a broad spectrum of insect parasitoids and predators. For instance, nicotine is one of the most broadly effective plant defence metabolites known because it poisons acetylcholine receptors and is thus toxic to most heterotrophic organisms with neuromuscular junctions. 

The use of single semiochemicals in the field is not usually sufficient for pest control, but by combining semi-ocheinicals, substantial protection can be achieved using an integrated approach called the push-pull system. The push is located in the crop and may comprise antifeedants, nonhost semiochemicals, attraction of predators or parasitoids, oviposition deterrents, or epideictic pheromones. The pull component comprises lures or trap areas away from the crop containing the sex pheromone, host odors, oviposition... 

The proximity of the serosa and teratocytes with the developing parasitoid, and their simultaneous presence in the host, make it difficult to determine which is the source of the parasitoid regulatory factors. It is also possible that regulatory factors are synthesized in one tissue or location and released to the host at another location via a different tissue. These points remain to be determined in most cases. 

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