No signs of inbreeding depression in Mastrus ridens (Hymenoptera: Ichneumonidae), a parasitoid of codling moth
Mastrus ridens (Hymenoptera: Ichneumonidae) corresponds to one of the two dominants parasitoids of codling moth in its region of origin in Central Asia (southern Kazakhstan and north-western China). It was introduced from Kazakhstan to California in the mid 1990’s within the context of a classical Biological Control program, and from there to other parts of the world, in some cases failing to establish. Traditionally biocontrol practitioners have not included other disciplines in the understanding of failures of deliberate introductions. Because the success of an introduced population may be limited by genetic processes, we carried out new collections in its area of origin and imported laboratory colonies from New Zealand. Then we generated two populations: a mix of New Zealand lab colony with wild individuals from Kazakhstan (NZK), and the lab colony from New Zealand (NZNZ). Insects from these populations were reared with codling moth as a host, and under two protocols: (1) inbred: mating brothers and sisters and (2) outbred: mating females randomly, but excluding brothers. Longevity, parasitism and reproduction were compared for the populations and type of mating.
Mean females longevity was 16.9 days, and there was no effect of population or crossing (p = 0.55). Of the 502 total larvae offered, 220 were parasitized (43.8%), and 92 of the 128 females followed (71.9%) had at least one successful parasitization. Successful females parasitized on average 2.4 larvae (range 1 – 6) in their lifetime, with no difference between treatments (p = 0.96). Mean parasitism by females was 58.8%, with no population effect (p = 0.18) or crossing type (p = 0.97), but a significant population x crossing effect (p = 0.03), with NZNZ outbred being larger than NZK outbred, but no differences between inbred females. On average, successful females generated 7.4 progeny in their lifetime (2.5 females and 4.9 males), with no differences between treatments (p ANOVAs: females = 0.79; males = 0.27; total = 0.61). Progeny sex ratio was male biased (0.65 males/total progeny). Mean progeny per parasitized larva was 3 (1 females + 2 males), with no difference between treatments (p = 0.89). Overall, these results suggest there is no inbreeding depression in M. ridens. More experiments are underway with a new set of individuals. Funding: FONDECYT 1131145