Host size and sex effects on parasitism by the Irish strain of Microctonus aethiopoides in the laboratory and the field

The impact of host size and sex on parasitism by the Irish strain of the hymenopteran parasitoid Microctonus aethiopoides Loan was investigated in the laboratory and in the field. Field data showed that while there was a large overlap, female adults of the target host Sitona obsoletus Gmelin averaged heavier weights than males (7.0 mg vs. 5.4 mg, P < 0.001). Male and female adult weights varied monthly: this was likely due to the emergence of a new weevil generation and environmental factors such as rainfall. While host sex and weight had no obvious impact on overall parasitism rates in S. obsoletus in the field, a no-choice cage study revealed that M. aethiopoides had lower parasitism rates (74% vs. 91%, P < 0.001) in larger weevils compared to small weevils. Additionally, parasitoid larval development was faster in female hosts compared to males, irrespective of size. While undetectable in field observations, differential development rates would enhance Irish M. aethiopoides population resilience. By breaking the previous generation synchrony, the population would be less vulnerable to being decimated by a single adverse event.