eliminate competitors by either combat or physiological suppression
(Salt, 1968; Fisher, 1971; Vinson and Iwantsch, 1980). As discussed in the
section on ‘Larval Adaptations to the Parasitoid Lifestyle’, most solitary
endoparasitic Hymenoptera and some Diptera produce first instars with
enlarged mandibles (Salt, 1961; Vinson and Iwantsch, 1980). First instars
are able to move through the host haemocoel and locate competitors,
possibly by perception of as yet unidentified chemical cues (S.B. Vinson,
personal communication). Once located, larvae fight with their mandibles
until one of the opponents is killed. Although ectoparasitoids lack
enlarged mandibles, some species are able to move across the surface of
hosts and kill competitors by eating them (Clausen, 1940). Ectoparasitoids
are also usually competitively superior to endoparasitoids, because endo-
parasitoid larvae cannot attack ectoparasitoids and ectoparasitoids tend to
consume hosts much more rapidly. Since parasitoid larvae lack fighting
mandibles after the first instar, older larvae use other strategies to
eliminate competitors. Some species outcompete younger larvae simply
by consuming available host resources before the younger parasitoid can
attain a large enough size to pupate. Others physiologically suppress the
development of younger larvae. Studies with the ichneumonidVenturia
canescensindicate that older larvae inhibit development of younger com-
petitors by reducing oxygen levels in the host (Fisher, 1963). Experiments
with other species, however, suggest that physiological suppression is
due to factors injected into the host by the ovipositing female (e.g. viruses)
or secretion of cytotoxic factors by the older larvae or teratocytes (Strand,
1986).
Gregarious parasitoids do not fight, but the presence of additional
larvae diminishes the amount of host resources available to any one
offspring. Competition is resolved by the race to consume host resources,
with older larvae usually having a clear advantage over younger larvae.
This is well illustrated by Strand and Godfray (1989), who used eye-
colour mutants of the ectoparasitoidBracon hebetorto show that survival
of larvae in a second clutch depends on the size of the first clutch and the
amount of time between ovipositions (Fig. 7.3).Larval specialization in polyembryonic parasitoids
One group of gregarious parasitoids that engage in larval fighting is
polyembryonic parasitoids from the family Encyrtidae (Hymenoptera)
(Strand and Grbic, 1997). Polyembryony is a form of clonal development
in which a single egg produces two or more genetically identical
offspring. Polyembryonic encyrtids lay their eggs into the egg stage of
Lepidoptera and offspring complete their development at the end of the
host’s larval stage. The wasp egg first develops into a single embryo,
which then proliferates into an assemblage of embryos, called a polygerm
or polymorula. The majority of embryos in the polymorula develop
into larvae when the host moults to its final instar. These so-called142 M.R. Strand