whereas a low JH titre during ecdysteroid release results in develop-
mental reprogramming and metamorphosis. As discussed in the section
on ‘The Impact of Host Quality on Fitness’, many larval endoparasitoids
oviposit into early-instar hosts and progeny remain in the first instar
and delay their development until the host attains a larger size. Studies
with several parasitoid–host systems indicate that the parasitoid larva
recognizes when the host attains this larger size by moulting to a feeding
second instar in response to the endocrine signals in the host that control
initiation of metamorphosis (Lawrence and Lanzrein, 1993; Beckage,
1997).
Parasitoids also induce changes in host endocrine state by altering
rates of hormone biosynthesis and catabolism or by inactivating
hormones (Beckage, 1997). For example, endoparasitic braconids, such as
Cotesia congregata,Glyptapanteles liaridis and Microplitis demolitor,
arrest the development of their lepidopteran hosts during their final instar
and inhibit metamorphosis. This occurs because JH titres remain elevated
in parasitized hosts, and is of benefit to the parasitoid because larvae are
unable to emerge from the host if it pupates (Beckage and Riddiford, 1982;
Balgopalet al., 1996; Schopfet al., 1996). Studies with the braconid
Diachasmimorpha longicaudatussuggest that elevation of host JH titres
is due to secretion of JH by the parasitoid larva (Lawrenceet al., 1990),
while other studies indicate that the parasitoid larva and factors like
polydnaviruses and teratocytes interact to arrest host development.
Development of the lepidopteranManduca sexta, for example, is partially
arrested when injected with polydnavirus fromC. congregata, but PDV-
injected larvae always develop further than naturally parasitized hosts. In
contrast, co-injection of PDV and parasitoid eggs results in arrested host
development, which is very similar to that of parasitized hosts (Alleyne
and Beckage, 1997). Developmental arrest and/or inhibition of pupation
can also occur as a result of alterations in ecdysteroids. As examples,
polydnavirus from the ichneumonid Campoletis sonorensis causes
degeneration of prothoracic glands in last-instar host larvae (Doveret al.,
1987), while teratocytes from the braconidToxoneuron(=Cardiochiles)
nigriceps inactivate 20-hydroxyecdysone released from the host’s
prothoracic glands (Pennachioet al., 1994).Parasitoid-induced Changes in Host Behaviour
Many arthropods exhibit changes in behaviour in response to the risk of
parasitism or as a direct consequence of being parasitized (Moore, 1995).
Among the best examples of parasitoids as a selective force in host
populations is in field crickets, Teleogryllus oceanicus, attacked by
tachinids that prefer to forage at dusk (Zuket al., 1993). In turn, cricket
populations where tachinids occur sing primarily during scotophase,
whereas cricket populations in areas without tachinids sing during both
scotophase and dusk. Crickets also exhibit alterations in male aggressionInteractions between Larval Parasitoids and Their Hosts 139