FLEXIBLE LARVAL GROWTH ALLOWS USE OF A RANGE OF HOST SIZES BY A PARASITOID WASP

Recent optimality models of host-parasitoid associations have assumed that host quality varies with host size or age at parasitism. This is based on the fact that larger hosts provide more resources, making siz e a reliable indicator of the amount of resources available for parasi toid development. Few studies have examined this in parasitoids that a llow their hosts to continue development after parasitism (koinobiont parasitoids). In this study we compared growth trajectories of the koi nobiont ichneumonid endoparasitoid Venturia canescens developing in fo ur larval instars of one of its hosts, the moth Plodia interpunctella. Hosts were reared with excess food and parasitized as late second, th ird, fourth, and fifth instars. Hosts were dissected at intervals afte r parasitism, and host and parasitoid dry mass determined. The survivo rship, development time, and size of eclosing adult wasps were also mo nitored. Hosts parasitized by Venturia continued to grow and become pr epupae, although their maximum masses were progressively smaller the e arlier that the hosts were parasitized: second-instar (L2) hosts grew to 70% of control (unparasitized) size, while L4 and L5 hosts reached control size. The maximum larval dry mass, as well as eclosing adult s ize, varied with host instar at parasitism. Venturia larvae spent prol onged periods as first instars when developing from L2 hosts and grew most rapidly during early stages in L5 larvae. The data shaw that for Venturia, host duality is not a direct function of host size at parasi tism. Furthermore, the pattern of development shown by Venturia differ s markedly from that shown by aphidiid koinobiont parasitoids.