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Host Range in Dipteran Parasitoids

Session Leader: John Stireman III

Price (1980) estimated that approximately one half of all animals on earth are parasitic insects. His rather broad usage of the term parasite includes all insects in which individuals obtain the majority of their food from a single host (e.g. most phytophagous insects). However a large fraction (around 50%; Godfray 1994) of these parasites are true parasitoids that kill their host. There are several characteristics of parasites and parasitoids that may have contributed to their great taxonomic diversity including small size, the patchy distribution of host resources, and perhaps most importantly, the intimate interactions they experience with hosts (Price 1980; Futuyma and Moreno 1988; Thompson 1994). The close relationships that parasitoids enter with their hosts (especially endoparasitoids) are likely to favor specialization, which may lead to coevolution, high rates of speciation, and adaptive radiations (Price 1980).

There are over 18,000 species of described dipteran parasitoids, derived from probably more than one hundred different lineages in which the habit evolved (Eggleton and Belshaw 1992). The major groups of dipteran parasitoids are listed in the table below. Though this not the most speciose order in terms of parasitoids, no other insect order is comparable in the number of independent derivations of the parasitoid habit, and thus the independent groups with which to analyze the causes and consequences of the parasitoid lifestyle. As a whole, dipteran parasitoids attack an extremely wide range of hosts from ants to anurans (Eggleton and Belshaw 1992; Feener and Brown 1997). This may be due in part to the taxonomic diversity of parasitoid lineages and their ties to substrate-zone habitats. Moreover, this pattern is mimicked within several of the lineages that have developed the parasitoid habit such as the Bombyliidae, Sarcophagidae, Phoridae, and Tachinidae (see table below). These groups have not only diversified to exploit a wide range of hosts, but often contain individual species which themselves exhibit extreme polyphagy (e.g. the tachinid Compsilura concinnata reared from over 100 species in three orders; O'Hara 1985). In general, even relatively specialized groups such as the Pipunculidae, which are endoparasitoids of leafhoppers and planthoppers, and the Acroceridae, which only attack spiders, contain taxa that attack a wide range of hosts often spanning several subfamilies or families (Schlinger 1981, 1987; Skevington and Marshall 1997).

This general lack of host specificity in dipteran parasitoids presents some interesting questions. First, given that the vast majority are endoparasitic and must cope with the internal environment of the host including immune defense responses, how are they able to attack such a wide range of hosts? Also, since they do not appear to exhibit some of the characteristics of parasites (i.e. extreme specialization) that would predispose them towards adaptive radiations and diversification, why have they been so taxonomically successful? Finally, what evolutionary and ecological factors or host attributes determine host range in dipteran parasitoids?

These questions cannot be adequately addressed here, but I will present a few possibly relevant points. First, many dipteran parasitoid groups (Tachinidae, Acroceridae, Nemestrinidae, Bombyliidae, and some Sarcophagidae; Clausen 1940) have evolved various means of maintaining contact with atmospheric oxygen despite their endoparasitic habit, and thus cannot be "suffocated" by host hematocytes. Also, it may be significant that the ancestral habit of most dipteran parasitoid groups is thought to be saprophagy (Eggleton and Belshaw 1993). Second, a comparison of dipteran parasitoids with their sister groups by Weigmann et al. (1993) did not reveal an overall relationship between parasitism and taxonomic diversity (though this study lacked much important phylogenetic and ecological information). However, the groups which exhibited the greatest taxonomic diversity (Bombyliidae, Tachinidae) are also among the most polyphagous of dipteran parasitoids. In contrast several of the more host-specific groups such as Pyrgotidae and Conopidae are relatively depauperate in species. Finally, one general factor that has been mentioned as being an important determinant of host range in several dipteran parasitoid groups is the macro and micro-habitat of potential hosts and the associated manner of host location by adult females (Lawton 1986; Schlinger 1987; Yeates and Greathead 1997; Stireman unpub. data). As Feener and Brown (1997) state in their review of dipteran parasitoids, "Several lines of evidence suggest that host specificity is more often determined by the events leading up to oviposition, rather than events occurring after oviposition"

Major groups dipteran parasitoids

FamilyDescribed parasitoid speciesPrimary hosts
Sciomyzidae100?Gastropods: (snails/slugs)
Nemestrinidae300Orth.: Acrididae
Bombyliidae5000primarily Hym., Col., Dip.
Sarcophagidae750?Lep., Orth., Hom., Col., Gastropoda + others
Tachinidae8200+Lep., Hym., Col., Hem., Dip., + many others
Phoridae300??Hym., Dip., Col., Lep., Isop.,Diplopoda + others
Calliphoridae240?earthworms, gastropods

More about flies (some nice pics including a few parasitoid species)

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Copyright 1998, John Stireman III