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

(: Braconidae) has been documented parasitizing over 40 species of

Tephritidae (Diptera) (Rousse et al. 2007), where as 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 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 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 ,

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 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 by reducing the number of herbivorous attackers present on the plant (Dicke & van Loon 2000; Guerrieri et al.

2002; Turlings et al. 1995). Many plants release synonomes systemically, and tissue

damage at the site of emission is not required (Guerrieri et al. 2002; Hatano et al. 2008;

Turlings et al. 1995; Meiners et al. 2000). Systemic synomone release by the plant is

induced by chemicals from the saliva of herbivores (Van Loon et al. 2000; Turlings et al.

1995). Turlings et al. (1995) showed that these chemicals are often different from

Pomeranz 6 chemicals emitted by mechanical damage. It has also been shown that infested plants can communicate with un-infested plants through root exudates to increase the strength of the signal (Guerrieri et al. 2002).

Synonomes have been shown to be extremely specific. Powell et al. (1998) showed that A. ervi is able to differentiate between synonomes emitted from V. faba when infested with A. pisum versus infestation with Aphis fabae. Acyrthosiphon pisum is the natural host for A. ervi (Powell et al. 1998).

As mentioned above, synonomes are easily detected but the least reliable.

Detection of synonomes does not guarantee that the herbivorous insect is still present.

Tobacco is the host plant for Spartocera dentiventris and emits synonomes when infested. Naïve females of the egg parasitoid of S. dentiventris, Gryon gallardoi were shown to be attracted to just the smell of tobacco leaves. Experienced females showed greater preference for tobacco leaves that had S. dentiventris eggs on them (see above)(Rocha et al. 2008). Similar results were found with Fopius arisanus

(Hymenoptera: Braconidae). Fopius arisanus is a generalist parasitoid and is attracted to more plant synonomes than G. gallardoi, however; plants that were infested with fruit flies (Diptera: Tephritidae) were preferentially selected (Rousse et al. 2007).

Immunity

Many parasitoids are host specific, and can only successfully develop in permissive hosts (Schmidt et al. 2001). Host defenses include physical barriers such as the cuticle (Stanley & Miller 2006), and cellular defenses (Stanley & Miller 2006;

Schmidt et al. 2001). One of the main defense systems in insects against endoparasitoids

Pomeranz 7 is encapsulation. This poses an interesting question of how insects recognize phylogenetically related endoparasitoids as non-self. In bacterial and viral recognition, cell structure is markedly different from eukaryotic cell structure, but eukaryotic parasitoids have very similar cellular structure (Schmidt et al. 2001).

There are many techniques that hosts use to recognize non-self foreign bodies.

One common element of non-self recognition is lack of basement membranes found lining all insect body cavities. It has been shown that successful parasitoid eggs and

larvae are covered in basement membrane-like components (Schmidt et al. 2001). These components may act as camouflage for the parasitoid inside the host body cavity.

Other strategies of non-self recognition involve glycoproteins. In this system, specific carbohydrate patterns would be recognized as self. Some of the glycoproteins present on parasitoids would be incorrectly recognized as self, but the large degree of diversity in such proteins would allow hosts to recognize specific carbohydrate patterns as non-self (Schmidt et al. 2001).

When a body is successfully recognized as non-self, encapsulation can commence. In some lepidopteran species, one type of hemocyte cell is responsible for recognition and initial binding to foreign bodies. Other hemocyte cells then form layers on top of these original hemocytes (Schmidt et al. 2001; Stanley & Miller 2006). Other studies have shown that only a single type of hemocyte is required for successful encapsulation. In both of these strategies, host hemocyte cells recognize specific proteins on the parasitoid and surround it with layers of cells (Schmidt et al. 2001; Schmid-

Hempel 2005). Numerous studies have shown that eicosanoids are important in mediating encapsulation in response to bacterial challenges, and have also been implicated in

Pomeranz 8 response to parasitoids (for review see Stanley & Miller 2006). These encapsulations, known as nodules, are then subject to phenoloxidanse (PO), which is an important intermediate in sequestering or killing foreign bodies (Kanost et al. 2004).

Immune defenses in host insects are present at all life stages. Eggs of Manduca sexta (Lepidoptera: Sphingidae) have been shown to increase transcription of many immune related genes when parasitized by Trichogramma evanescens (Hymenoptera:

Thrichogrammatidae). Many of these proteins were similar to those transcribed when infected with bacterial microbes (Abdel-latief & Hilker 2008; Kanost et al. 2004). In one study of immune responses of M. sexta eggs, parasitoid development was successful in only 64% of the parasitized eggs. This is a large selection pressure on T. evanescens to suppress or avoid the immune responses of M. sexta (Abdel-latief & Hilker 2008).

Parasitoids have also developed defenses in response to host-immune system selection pressures. Ectoparasitoids develop on the outside of the host, and are thus incapable of being encapsulated (Schmidt et al. 2001). These ectoparasitoids have to combat other host immune systems including physical barriers (i.e. host cuticle). They generally kill their host quickly by secreting proteases into the hosts’ body cavity

(Schmidt et al. 2001). Endoparasitoids exhibit passive defenses by developing in areas of the host body that are removed from hemocyte contact, such as the nerve ganglions

(Schmidt et al. 2001). Other strategies include protein coatings that are not recognized as foreign (see above) or that host hemocytes are unable to adhere to (Schmidt et al. 2001).

Active defenses include the parasitoid utilization of polydnaviruses, hemocyte suppression factors such as serine protease inhibitor (Schmidt et al. 2001), teratocytes and venom (Dahlman 1990; Kaeslin et al. 2005). Polydnaviruses are injected along with the

Pomeranz 9 parasitoid egg or larvae and suppress the host’s immune system (Schmidt et al. 2001).

Inhibition of PO in hosts has been observed as a direct cause from certain polydnaviruses

(Dahlman 1990). The function of teratocytes is not fully understood (Dahlman 1990;

Kaeslin et al. 2005). It has been implicated in PO inhibition, hemocyte suppression and prevention of encapsulation (Dahlman 1990).

Unique Chemicals

The parasitoid-host interaction between P. triangulum and H. rutilans (see above) offers another unique example of chemical utilization. Females of P. triangulum are parasitoids of honey bees (Hymenoptera: Apidae). Philanthus triangulum builds a nest burrow with many honeybee bodies stored inside. The larva of P. triangulum eats the stored honeybee bodies and develops safely underground (Strohm et al. 2008). This system obviously provides a large food source that is susceptible to exploitation by other species. Females of H. rutilans are specialists that invade the P. triangulum nests and lay their own eggs on the stored honeybees. When these eggs hatch the larvae eat the P. triangulum larvae and the stored honeybee. In order to do this successfully H. rutilans uses a form of chemical camouflage (Strohm et al. 2008). Using gas chromatography- mass spectrometery (GC-MS) Strohm et al. (2008) showed that the cuticle of H. rutilans closely resembles the cuticle of P. triangulum.

Many species of Eucharitid (Hymenoptera) are parasitoids of eusocial insects such as ants. In many cases the Eucharitid larvae develops in an ant larvae and emerges as an adult inside the nest. Mating then occurs outside of the ant nest. In order to

Pomeranz 10 do this, chemical camouflage similar to that mentioned above is utilized (Lachaud &

Perez-Lachaud 2009).

Conclusion

Parasitoids utilize many different chemicals for host location, recognition host

immune system suppression and camouflage. Synonomes are generally used for long-

range habitat location, and kairomones are used for short-range host location and

recognition (Afsheen et al. 2008; Powell et al. 1998), however; exceptions are common.

Kairomones such as sex and aggregation pheromones can be used to convey long-range

information to parasitoid species (Wertheim et al. 2003). Many important non-chemical

cues not discussed in this paper are important for oviposition FAPs and host selection.

The host immune system/parasitoid interface is a very complex and interesting

interaction. The evolutionary pressures exerted by both parties should theoretically be driving constant adaptation. This is an area of research that needs to be further studied.

Unique chemicals have allowed certain parasitoids to be well adapted to exploit their natural hosts. Chemical camouflage is a clever evolutionary solution for parasitoids living in a chemical world.

Pomeranz 11 References

Abdel-latief, M. & Hilker, M., 2008. Innate immunity: Eggs of Manduca sexta are able to respond to parasitism by Trichogramma evanescens. Insect Biochemistry and Molecular Biology [Insect Biochem. Mol. Biol.]. Vol. 38, 38(2), 136-145.

Afsheen, S. et al., 2008. Differential attraction of parasitoids in relation to specificity of kairomones from herbivores and their by-products. Insect Science [Insect Sci.]. Vol. 15, 15(5), 381-397.

Dahlman, D.L., 1990. Evaluation of teratocyte functions: An overview. Archives of Insect Biochemistry and Physiology [ARCH. INSECT BIOCHEM. PHYSIOL.]. Vol. 13, 13(3-4), 159-166.

Dicke, M. & van Loon, J.J.A., 2000. Multitrophic effects of herbivore-induced plant volatiles in an evolutionary context. Entomologia Experimentalis et Applicata [Entomol. Exp. Appl.]. Vol. 97, 97(3), 237-249.

Dicke, M. & Sabelis, M.W., 1988. Infochemical terminology: Based on cost-benefit analysis rather than origin of compounds?. Functional Ecology [FUNCT. ECOL.]. Vol. 2, 2(2), 131-139.

Guerrieri, E. et al., 2002. Plant-to-plant communication mediating in-flight orientation of Aphidius ervi. Journal of Chemical Ecology [J. Chem. Ecol.]. Vol. 28, 28(9), 1703-1715.

Hatano, E. et al., 2008. Chemical cues mediating aphid location by natural enemies. EUROPEAN JOURNAL OF ENTOMOLOGY, 105(5), 797-806.

Kaeslin, M. et al., 2005. Stage-dependent strategies of host invasion in the egg-larval parasitoid Chelonus inanitus. JOURNAL OF INSECT PHYSIOLOGY, 51(3), 287- 296.

Kanost, M.R., Jiang, H. & Yu, X., 2004. Innate immune responses of a lepidopteran insect, Manduca sexta. Immunological Reviews [Immunol. Rev.]. Vol. 198, 198, 97-105.

Kroiss, J., Bordon, S. & Strohm, E., 2008. Hydrocarbons in the nest material of a solitary digger represent a kairomone for a specialized . Behaviour [Anim. Behav.]. Vol. 76, 76(5), 1555-1563.

Lachaud, J. & Perez-Lachaud, G., 2009. Impact of natural parasitism by two eucharitid wasps on a potential biocontrol agent ant in southeastern Mexico. BIOLOGICAL CONTROL, 48(1), 92-99.

Pomeranz 12 Meiners, T. & Hilker, M., 1997. Host location in Oomyzus gallerucae (Hymenoptera: Eulophidae), an egg parasitoid of the elm leaf beetle Xanthogaleruca luteola (Coleoptera: Chrysomelidae). OECOLOGIA, 112(1), 87-93.

Meiners, T., Westerhaus, C. & Hilker, M., 2000. Specificity of chemical cues used by a specialist egg parasitoid during host location. ENTOMOLOGIA EXPERIMENTALIS ET APPLICATA, 95(2), 151-159.

Powell, W. et al., 1998. Strategies involved in the location of hosts by the parasitoid Aphidius ervi Haliday (Hymenoptera : Braconidae : Aphidiinae). BIOLOGICAL CONTROL, 11(2), 104-112.

Rocha, L., Sant'Ana, J. & Redaelli, L.R., 2008. Discrimination of Spartocera dentiventris (Berg, 1884) (Hemiptera: Coreidae) eggs by Gryon gallardoi (Brethes, 1913) (Hymenoptera: Scelionidae). Brazilian Journal of Biology [Braz. J. Biol.]. Vol. 68, 68(1), 161-167.

Rousse, P. et al., 2007. The host- and microhabitat olfactory location by Fopius arisanus suggests a broad potential host range. Physiological Entomology [Physiol. Entomol.]. Vol. 32, 32(4), 313-321.

Schmid-Hempel, P., 2005. Natural insect host-parasite systems show immune priming and specificity: puzzles to be solved. Bioessays [Bioessays]. Vol. 27, 27(10), 1026-1034.

Schmidt, O., Theopold, U. & Strand, M., 2001. Innate immunity and its evasion and suppression by hymenopteran endoparasitoids. BIOESSAYS, 23(4), 344-351.

Schoenrogge, K. et al., 2008. Host Recognition by the Specialist Hoverfly Microdon mutabilis, a Social Parasite of the Ant Formica lemani. Journal of Chemical Ecology [J. Chem. Ecol.]. Vol. 34, 34(2), 168-178.

Shi, J. et al., 2009. Pollination by deceit in Paphiopedilum barbigerum (Orchidaceae): a staminode exploits the innate colour preferences of hoverflies (Syrphidae). Plant Biology, 11(1), 17-28.

Stanley, D.W. & Miller, J.S., 2006. MINI REVIEW: Eicosanoid actions in insect cellular immune functions. Entomologia Experimentalis et Applicata [Entomol. Exp. Appl.]. Vol. 119, 119(1), 1-13.

Strohm, E. et al., 2008. A cuckoo in wolves' clothing? Chemical mimicry in a specialized cuckoo wasp of the European beewolf (Hymenoptera, Chrysididae and Crabronidae). FRONTIERS IN ZOOLOGY, 5. Available at: http://0- apps.isiknowledge.com.catalog.library.colostate.edu/full_record.do?product=WO S&search_mode=GeneralSearch&qid=16&SID=4EpfOe8poHcodnpnKKJ&page= 1&doc=3 [Accessed March 3, 2009].

Pomeranz 13

Turlings, T. et al., 1995. How caterpillar-damaged plants protect themselves by attracting parasitic wasps. Proceedings of the National Academy of Sciences of the United States OF, 92(10), 4169-4174.

Van Loon, J.J.A., De Boer, J.G. & Dicke, M., 2000. Parasitoid-plant mutualism: parasitoid attack of herbivore increases plant reproduction. Entomologia Experimentalis et Applicata [Entomol. Exp. Appl.]. Vol. 97, 97(2), 219-227.

Wertheim, B., Vet, L.E.M. & Dicke, M., 2003. Increased risk of parasitism as ecological costs of using aggregation pheromones: laboratory and field study of Drosophila- Leptopilina interaction. Oikos [Oikos]. Vol. 100, 100(2), 269-282.

Pomeranz 14