Chemicals Involved with Parasitoid-Host Interactions
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Justin Pomeranz BSPM 507 [email protected] Spring 2009 Chemical Warfare: Chemicals Involved with Parasitoid-Host Interactions Abstract Parasitoids are important natural enemies for many species. Parasitoids show a strong degree of variation in their specialization for host species. Fopius arisanus (Hymenoptera: Braconidae) has been documented parasitizing over 40 species of Tephritidae (Diptera) (Rousse et al. 2007), where as Hedychrum rutilans (Hymenoptera: Chrysididae) only parasitizes one species, Philanthus triangulum (Strohm et al. 2008). Host location and recognition are obviously of vital importance for evolutionary fitness. Parasitoids have developed many cues for host location and recognition. Chemical signals, such as herbivory induced plant synomones (Guerrieri et al. 2002) and kairomones produced directly from the host, such as; aggregation chemicals (Wertheim, Vet, and Dicke 2003) or its by-products (Hatano et al. 2008), are important for long- range location. Also of importance are chemicals used to subdue the immune system of the host (Abdel-latief and Hilker 2008, Schmid-Hempel 2005). Chemical camouflage has also proven to be an important aspect for some parasitoid species (Strohm et al. 2008). Introduction Parasitoid insects are very common in nature, with 100,000 already described, primarily in the orders Hymenoptera and Diptera. There are estimated numbers of parasitoid species reaching up to 1 million (Kaeslin et al. 2005). Unlike parasites, which require their host to continue living for development, parasitoids are insects whose larvae survive by developing on or in their insect hosts, eventually causing death (Kaeslin et al. 2005). This has obvious population control aspects and many parasitoids have been studied as possible biological control agents. It is important to understand their behavior in order to understand the full implications parasitoids will have on both beneficial and pest species (Lachaud & Perez-Lachaud 2009). Parasitoid insects can be classified as endoparasitoids, which develop inside the body cavity of their host; and ectoparasitoids, which develop on the outside of their host. Both of these strategies require the parasitoid to bypass the hosts’ immune system (Schmidt et al. 2001). This creates a red queen type arms race between the hosts’ immune system and the parasitoids’ survival strategies. Being capable of suppressing a hosts’ immune system leads many parasitoids to become extreme specialists, sometimes only exploiting a single species for development(Afsheen et al. 2008; Schmid-Hempel 2005; Hatano et al. 2008; Strohm et al. 2008). There are, however, examples of generalist parasitoids (Hatano et al. 2008; Schmid-Hempel 2005; Strohm et al. 2008). This obviously puts a large amount of importance on correct location and recognition of hosts. Chemicals are the primary location cues (Meiners & Hilker 1997). Semiochemicals are chemicals that are emitted and only recognized by conspecifics. Allelochemicals are chemicals that are emitted by one species and can be recognized by Pomeranz 2 individuals of other species (Hatano et al. 2008). Many allelochemicals were originally semiochemicals that other insects have evolved to recognize. Kairomones and synonomes are two classes of allelochemicals commonly used by parasitoids in location. Kairomones are chemicals that benefit the receiver, and convey a disadvantage to the donor (Dicke & Sabelis 1988). Kairomones can be either volatile or non-volatile (Afsheen et al. 2008). Examples of kairomones include cuticular hydrocarbons of the European beewolf, Philanthus triangulum (Hymenoptera: Crabonidae) (Kroiss et al. 2008), aphid honeydew and aphid alarm pheromones (Hatano et al. 2008). Synonomes are classified as chemicals that benefit both the receiver and the donor (Dicke & Sabelis 1988), for example Vicia faba emits synonomes when infested with Acyrthosiphon pisum (Hemiptera: Aphididae) that attract A. pisum’s natural enemy, the parasitoid Aphidius ervi (Hymenoptera: Braconidae) (Guerrieri et al. 2002). To the best of the authors’ knowledge, all synonomes are volatile chemicals. Allelochemicals are not always given equal preference by parasitoid species. Kairomones are the most reliable signal, but the least detectable, whereas synonomes are the most detectable but the least reliable (Meiners & Hilker 1997). This causes the parasitoids to weight chemical stimuli differently depending on the context. Kairomones from Spartocera dentiventris (Hemiptera: Coreidae), present in the secretions used to glue their eggs to tobacco leaves, were shown to attract the egg parasitoid Gryon gallardoi (Hymenoptera: Scelionidae) more strongly then synonomes emitted from the host plant. However the combination of eggs and tobacco leaves together were more attractive than either alone (Rocha et al. 2008). This shows that G. gallardoi uses chemical stimuli in ways that will maximize its fitness. Pomeranz 3 Other secondary cues not included in this discussion include color (Powell et al. 1998; Shi et al. 2009; Hatano et al. 2008), shape, sound and movement (Hatano et al. 2008; Powell et al. 1998). This paper is meant to give a broad overview of the chemicals involved between parasitoid-host interactions. Listing the names and descriptions of all known chemical identities is beyond the scope of this paper. Chemicals used in signaling, immunity/ immune suppression and other unique chemicals are discussed below. Kairomones There are many known kairomones in natural systems. Most kairomones increase in importance as distance between parasitoid and host decrease (Afsheen et al. 2008), although some long distance pheromones such as sex and aggregation pheromones (Wertheim et al. 2003) can attract parasitoids from some distance. Aphidius ervi uses synonomes for long-range habitat identification (see below). Once suitable habitats have been found, A. ervi utilizes kairomones emitted from its host A. pisum for host recognition and oviposition FAPs (Powell et al. 1998). Wertheim et al. (2003) showed that an increase in the aggregation pheromone of the fruit fly Drosophila melanogaster (Diptera: Drosophilidae) increased the number of its parasitoid, Leptopilina heterotoma (Hymenoptera: Eucoilidae), which arrived at the location of pheromone, but did not increase L. heterotoma’s search pattern. Like A. ervi, L. heterotoma required other cues to induce fixed action patterns (FAPs) for search and oviposition behaviors (Wertheim et al. 2003; Powell et al. 1998). Pomeranz 4 The beewolf Philanthus triangulum illustrates another example of kairomones. Philanthus triangulum creates nest hydrocarbons that play a role in nest-mate recognition. The specialized parasitoid Hedychrum rutilans (Hymenoptera: Chrysididae) has been shown experimentally to spend more time at an air vent that emits these nest hydrocarbons. This is thought to the most important chemical cue in correct host recognition in this parasitoid-host interaction (Kroiss et al. 2008). There is a similar host recognition strategy of Formica lemani (Hymenoptera: Formicidae) by its parasitoid Microdon mutabilis (Diptera: Syrphidae). Females of M. mutabilis exhibited oviposition FAPs in the laboratory when exposed to cuticular extracts of F. lemani. These FAPs were not observed when presented with extracts from other sympatric ant species. Microdon mutabilis is an extreme example in that it is specially adapted to specific populations of F. lemani. When presented with extracts from other populations of F. lemani oviposition FAPs were not observed (Schoenrogge et al. 2008). Another important aspect of kairomones is correct life-stage recognition (Afsheen et al. 2008). It has been observed that the egg parasitoid Oomyzus gallerucae (Hymenoptera: Eulophidae) of Xanthogaleruca luteola (Coleoptera: Chrysomelidae) recognized kairomones in X. luteola’s feces that started search FAPs for egg locations. Other kairomones specific to the eggs were recognized by O. gallerucae and initiated oviposition FAPs (Meiners & Hilker 1997). Larval parasitoids have been shown to utilize both volatile and non-volatile kairomones. Lepidopteran larvae have glands near their mandibles that secrete volatile kairomones that attract their respective parasitoids. Contact with host by-products such as Pomeranz 5 frass, silk or saliva elicit search FAPs in the generalist larval parasitoid Cotesia marginiventris (Hymenoptera: Braconidae) (Afsheen et al. 2008). Pupal parasitoids often rely more heavily on contact kairomones. Dhalbominus fuscipennis (Hymenoptera: Chalcidoidea) is a pupal parasitoid of Gilpinia hercyniae (Hymenoptera: Diprionidae). In a lab setting, volatiles from G. hercyniae did not attract D. fuscipennis. An extract from the outer cocoon layer of G. hercyniae caused antennal drumming of D. fuscipennis and indicated host recognition (Afsheen et al. 2008). Synonomes Many plants defend themselves from herbivorous arthropod attack by recruiting natural enemies (Guerrieri et al. 2002). Plant volatiles emitting from infested plants probably originally evolved to deter herbivores or as antibiotics in response to pathogenic or herbivorous infestation (Turlings et al. 1995). It is likely that parasitoids evolved recognition of these volatiles secondarily (Turlings et al. 1995). The secondary evolution of parasitoid recognition allowed these volatiles to be used as synonomes (see Dicke and Sabelis, 1988 for definition). These chemicals emitted by the plants benefit the natural enemies by directing them to food sources, and benefit the plants