Influence of host size on the clutch size and developmental success of the gregarious ectoparasitoid Eulophus pennicornis (Nees)(Hymenoptera: Braconidae) attacking larvae of the tomato moth Lacanobia oleracea (L.) (Lepidoptera: Noctuidae)

Abstract

SUMMARY The relationship between clutch size, host size and progeny survival in the gregarious ectoparasitoid Eulophus pennicornis was investigated in a number of scenarios. When naive parasitoids were exposed singly to Lacanobia oleracea hosts, clutch size was strongly correlated with the size of the host. However, survival of parasitoid offspring was negatively affected by the size of the host such that, in larger hosts, greater wasp larval and pupal mortality was recorded. As a result, no gain in realised fecundity was achieved through parasitizing L. oleracea larvae of mass >0.4 g over hosts of mass between 0.2–0.3 g. When exposed to populations of mixed stadium hosts (larvae in the fourth, fifth and sixth instars) during the entire lifespan of the wasp, host size and clutch size were correlated in early ovipositions (first three ovipositions). However, as the wasps aged, the relationship was much less apparent. When the parasitoid was restricted to foraging upon populations of sixth instar hosts only, no relationship between host size and clutch size was apparent. Exposure of the parasitoid to mixed and fixed stadium host populations showed that final(sixth) stadium hosts were the most frequently parasitized (ca. 96% of parasitized hosts) and that the average numbers of eggs laid per wasp, and the number of hosts parasitized, was significantly lower when the parasitoid was provided with fourth or fifth instar hosts only. The results indicate that the reproductive success of E. pennicornis does not increase with increasing host size or greater resource availability above a certain threshold, and that the physiological status of the host at the time of parasitism is the governing factor determining oviposition decisions and parasitoid survival. We conclude that E. pennicornis has been selected to preferentially utilize those hosts that maximize progeny survival and to adapt clutch size to the size of such hosts. We hypothesize that the major driver leading to the evolution of this strategy is the ability of the parasitoid to physiologically regulate the host.

The Biological Effects of Venom Derived from the Ectoparasitic Wasp Eulophus pennicornis (Nees) (Hymenoptera: Eulophidae): Evidence for Dual Endocrine Regulation~!2008-08-08~!2009-05-30~!2010-02-11~!

“…Previous work [8,14] has shown that early sixth stadium hosts (0 -48 h post ecdysis) are preferentially parasitized by E. pennicornis and suggest that juvenilization, and the capacity to induce it, have been selective drivers in this parasitoid preferentially parasitizing hosts of this age. The powerful juvenilizing effect that a component of E. pennicornis's venom induces in the host lends credence to the fact that it is probably critical for the wasp to ensure that hosts do not become physiologically committed to pupation once they have been parasitized.…”

“…Furthermore, the observable effects of venom factors on various aspects of host development have not been reported in any detail, particularly with respect to injection into different host stages, and the response of larvae to different doses. These points are of particular relevance as E. pennicornis has the ability to parasitize a range of stages of L. oleracea (fourth, fifth and sixth stadium larvae) and to vary its clutch precisely in proportion to the size of the host [14]. Furthermore, the reproductive success of this parasitoid appears linked to both its ability to identify the highest quality hosts and to successfully regulate their development [8,14].…”

“…These points are of particular relevance as E. pennicornis has the ability to parasitize a range of stages of L. oleracea (fourth, fifth and sixth stadium larvae) and to vary its clutch precisely in proportion to the size of the host [14]. Furthermore, the reproductive success of this parasitoid appears linked to both its ability to identify the highest quality hosts and to successfully regulate their development [8,14]. These observations are best illustrated by the fact that late-parasitized hosts (i.e.…”

“…These observations are best illustrated by the fact that late-parasitized hosts (i.e. those within 3-4 days of pupation) continue to develop following parasitism, and form prepupae, a developmental event that leads to very poor parasitoid larval survival; whereas larvae parasitized at the start of the final stadium prove to be optimal and are preferentially parasitized by this wasp [14].…”

Clutch size decisions of Cotesia glomerata, a gregarious parasitoid of Pieris brassicae

“…Many studies conclude that clutch size increases with host size. Host size is in many instances a good predictor of nutritional quality of the host (Bell et al 2005;Hardy et al 1992;Klein et al 1991;Zaviezo and Mills 2000); however, there are exceptions where parasitoids rely not simply on host size to assess nutritional quality (Dorn and Beckage 2007;Hackermann et al 2007). These studies support the above expectations that superparasitism should be a more frequent event in the field when fewer but larger hosts are available (Gu et al 2003).…”