Differing Host Exploitation Efficiencies in Two Hyperparasitoids: When is a ‘Match Made in Heaven’?
Jeffrey A. Harvey
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Roel Wagenaar
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Rieta Gols
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R. Gols Department of Entomology, Wageningen University and Research Centre
, P.O. Box 8031, 6700 EH Wageningen,
The Netherlands
Host exploitation behavior in two hyperparasitoids, Lysibia nana and Gelis agilis, was compared in single cocoon clusters of their primary parasitoid host, Cotesia glomerata. L. nana reproduces sexually, is fully winged, does not host-feed and matures eggs quite rapidly after eclosion, whereas G. agilis possesses opposite traits. Cohorts of individual hyperparasitoid females of differing age and physiological state were given access to single cocoon clusters of C. glomerata that also varied in age. These results reveal that the reproductive biology of L. nana is well matched to exploit cocoon broods in C. glomerata, suggesting strong a co-evolutionary history with this host. By contrast, G. agilis is much less efficient at exploiting host cocoons and is probably a generalist species that attacks other hosts in nature.
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The evolution of patch- and host-exploitation behavior in parasitoid wasps has been
studied for many years (Hassell 1971; Mackauer and Vlkl 1993; Spataro and
Bernstein 2007). Parasitoids are model organisms for ecological and evolutionary
studies because many species can only develop on certain types of hosts which are
patchily distributed in nature (Godfray and Shimada 1999). Given this fact, female
parasitoids are under strong selection to optimize the exploitation of host patches in
order to optimize their fitness. One of the shortcomings of studies exploring patch
residence rules in parasitoids is that they often pay little attention to a range of
ecophysiological factors that influence foraging behavior. For example, reproductive
traits often differ profoundly amongst parasitoids attacking different stages of the
same host, and even in some instances the same stage (Price 1970, 1972; Harvey
2008; Harvey et al. 2009). Another constraint on parasitoid foraging is based on the
number of eggs that the parasitoid can immediately mobilize for oviposition (Jervis
et al. 2001). Many parasitoids are synovigenic and emerge with only a few (or even
no) ripe eggs and require up to 48 h or more to accumulate significant numbers
(Jervis et al. 2001, 2008). Depending on constraints imposed by host abundance and/
or availability, some parasitoids invest very little in reproduction and thus never
produce large complements of eggs (Ellers et al. 2000).
Another important factor influencing the evolution of foraging behavior in
parasitoids concerns the degree of host specialization exhibited by the parasitoid
species being studied. Although generalists have the advantage of exploiting a wider
range of resources, they are predicted to be less efficient in using one particular
resource compared to specialists (Stilmant et al. 2008). Highly specialized parasitoids
that attack one or only a few hosts in nature will exhibit behavior and biological
characteristics that are finely tuned to hosts with which they are strongly co-evolved
(Price 1970, 1972; Godfray 1994). By contrast, the foraging behavior of broad
generalists will probably be less tailored to a specific host species and therefore their
response to host patches may be less refined than that demonstrated by specialists.
In this study we compare and contrast the responses of two secondary idiobiont
hyperparasitoids to single broods of their primary parasitoid host, Cotesia glomerata
L. (Hymenoptera: Braconidae). C. glomerata is a gregarious primary koinobiont
endoparasitoid that attacks the larvae of cabbage butterflies (Pieridae). In turn,
cocoons of C. glomerata are attacked by a number of secondary hyperparasitoids,
including the solitary species Lysibia nana Gravenhorst (Hymenoptera:
Ichneumonidae) and Gelis agilis Fabricius (Hymenoptera: Ichneumonidae). Both
hyperparasitoids are quite closely related (they occur in the same subfamily, Cryptinae)
although L. nana is much more frequently recovered from cocoon clusters of C.
glomerata (Harvey 2008; Harvey et al. 2009). At eclosion, both species have no
mature eggs in their ovaries and thus are wholly synovigenic (Jervis et al. 2008).
However, the two species differ in some important respects. For example, L. nana
reproduces sexually and adults are fully winged, whereas G. agilis is an asexually
reproducing species whose females are wingless. Furthermore, L. nana acquires all
of its resources for reproduction during larval development, whereas adult G. agilis
must host-feed to optimize egg production (Jervis and Kidd 1986; Harvey 2008).
Finally, although the adult wasps are of equivalent size, eggs of G. agilis are much
larger than those of L. nana (Harvey 2008).
Cotesia glomerata typically produces single broods of tightly clustered cocoons
containing an average of 2030 wasps per brood (Harvey 2000; Gu et al. 2003).
These clusters represent an aggregated and high value resource for solitary
hyperparasitoids, and it h (...truncated)