Response of parasitoid egg load to host dynamics and implications for egg load evolution

Abstract

A theoretical debate about whether parasitoids should be time or egg limited now recognizes both as feasible, and interest has turned to determining the circumstances under which each might arise in the field, and their implications for parasitoid behaviour and evolution. Egg loads of parasitoids sampled from the field are predicted to show a negative response to host availability, but empirical support for this relationship is scarce. We measured how a parasitoid's egg load responded to seasonal fluctuations in host population density and recorded the predicted correlation. In early summer, parasitoids were at high risk of time limitation due to low host availability, and in late summer, their offspring were at greater risk of egg limitation due to high host availability. Despite clear seasonal changes in selection pressures on egg load and lifespan, the parasitoid showed no evidence of seasonal variation in its reproductive strategy. We made minor modifications to a previously published model to explore the effects of seasonal variation in host availability on optimal investments in eggs and lifespan and obtained several new results. In particular, under circumstances analogous to some of those observed in our field study, temporal stochasticity in reproductive opportunities can cause investments in eggs to increase, rather than decrease as previously predicted. Our model results helped to explain the parasitoid's lack of a seasonally varying reproductive strategy. Understanding the evolution of parasitoid egg load would benefit from a shift of research emphasis from purely stochastic variation in parasitoid reproductive opportunities to greater consideration of host dynamics.

Factors shaping life history traits of two proovigenic parasitoids

“…Despite early opinions that female parasitoids rarely deplete their eggs in nature Visser et al 1992;Sevenster et al 1998), recent empirical data combined with theoretical developments suggest that egg limitation in parasitoids, as well as in other insects, is likely to occur under field conditions (Heimpel & Rosenheim 1998;Casas et al 2000;Rosenheim et al 2008;Phillips & Kean 2017). Moreover, even in cases where egg limitation is rare, it may impose strong selection on female reproductive traits, due the disproportionately high reproductive contribution of egg-limited females (i.e.…”

Plant-Mediated Behavioural Avoidance of a Weevil Towards Its Biological Control Agent

“…Adult L. bonariensis were collected on 8 January 2018 at Lincoln (43.64397 o S, 172.44292 o E) during the peak emergence of the first summer generation adults (late December to early February) when parasitism rates were low ( Goldson et al, 1998 ; Phillips and Kean, 2017 ). This timing reduced the probability of weevil-parasitoid interactions prior to the experiment.…”

“…Weevils were collected again at Lincoln in late February and early March 2018 from which parasitoids were reared. At this time high field parasitism rates occur ( Goldson et al, 1998 ; Phillips and Kean, 2017 ). The samples were maintained in the conditions described above pending prepupal M. hyperodae emergence.…”

“…summer generation adults (late December to early February) when parasitism rates were low (Goldson et al, 1998;Phillips and Kean, 2017). This timing reduced the probability of weevilparasitoid interactions prior to the experiment.…”

Behaviour drives contemporary evolution in a failing insect-parasitoid importation biological control programme

“…Stable diapausing L. bonariensis field parasitism rate data (Goldson, 1981;Goldson et al, 1993) were extracted from both published (Ferguson et al, 1997;Barker, 2013;Goldson et al, 2014a;Phillips and Kean, 2017;Tomasetto et al, 2017) and unpublished records (AgResearch). These data were collected from the Northern Region (Locality: Ruakura), Central Region (Locality: Lincoln), and the Southern Region (Locality: Invermay).…”