Flexible Larval Growth Allows Use of a Range of Host Sizes by a Parasitoid Wasp

Abstract

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 size a reliable indicator of the amount of resources available for parasitoid development. Few studies have examined this is parasitoids that allow their hosts to continue development after parasitism (koinobiont parasitoids). In this study we compared growth trajectories of the koinobiont ichneumonid endoparasitoid Venturia canescens developing in four larval instars of one of its host, the moth Plodia interpunctella. Hosts were reared with excess food and parasitized as late second, third, fourth, and fifth instars. Hosts were dissected at intervals after parasitism, and host and parasitoid dry mass determined. The survivorship, development time, and size of eclosing adult wasp were also monitored. Hosts parasitized by Venturia continued to grow and become prepupae, although their maximum masses were progressively smaller the earlier 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 size, varied with host instar at parasitism. Venturia larvae spent prolonged periods as first instars when developing from L2 hosts and grew most rapidly during early stages in L5 larvae.The data show that for Venturia, host quality is not a direct function of host at parasitism. Furthermore, the pattern of development shown by Venturia differs markedly from that shown by aphidiid koinobiont parasitoids.