Associative effects of the parasitoid Microctonus aethiopoides on target and non-target hosts

Abstract

The effect of Microctonus aethiopoides Loan exposure on the survival, feeding and fecundity of its target host, Sitona discoideus Gyllenhal, and two native non-target hosts, were studied with special reference to any observed non-parasitism effects.

Exposure to M. aethiopoides resulted in some (<10%) associative mortality in S. discoideus in the first experiment but no significant mortality was detected in the second. Similarly, while significant levels of associative mortality were detected in N. cervina in their first exposure to M. aethiopoides, subsequent exposures resulted in some associative mortality but not significantly more than the unexposed populations. This may have been due to the discovery of previously undetected levels of failed parasitism in the later experiments.

The fecundity of exposed but unparasitised S. discoideus and N. Cervina populations were reduced after M. aethiopoides exposure. Variation in the number of eggs produced by females made defining a significant effect impossible however.

M. aethiopoides was found to oviposit more in female N. cervina than male N. cervina. Significant sexual dimorphism exists in N. cervina, especially regarding weevil elytral width. This lead to a pair of experiments looking at the effects of N. cervina elytral width (as a measure of body size) on the parasitism success of M. aethiopoides. While smaller male weevils showed no relationship between parasitism success and size, female N. cervina were found to support development of M. aethiopoides when they were larger, suggesting the existence of a threshold host size for the parasitoid's success.

Exposure of N. cervina to M. aethiopoides in laboratory experiments was found to result in levels of failed parasitism between 17-35%. Parasitism usually seemed to fail soon after parasitoid eclosion. To test whether this was an artefact of laboratory exposure, exposure at simulated field densities was undertaken in field cages. Although parasitism levels were lower than in the laboratory, the incidence of failed parasitism was 33%. This suggests that the parasitism behaviour of M. aethiopoides did not differ between laboratory and field environments.

This study found that while associative mortality in the laboratory may be a small effect in comparison to actual parasitism, it can be significant. A more important consideration may be the effects on reproduction, although the experiments undertaken could not accurately define the significance of this effect. The incidence of failed parasitism that may remain undetected was certainly the most significant direct effect of the parasitoid apart from successful parasitism.