Macrocentrus grandii is a polyembryonic parasitoid, with embryos that divide asexually within the host (European corn borer, Ostrinia nubilalis) to produce broods of clonal offspring. From a biological control standpoint, polyembryony seems advantageous because each parasitized host yields multiple parasitoids with minimal time and egg investment. When we observed M. grandii in the field, however, we found that the parasitoid virtually always invested additional time and, if possible, stings into hosts that it had already stung, apparently reducing some of the advantages of polyembryony. We therefore investigated and found support for two potential benefits that can be gained by self-superparasitism in this system. First, multiple stings allowed production of mixed-sex broods: 27% of multiply-stung versus 0% of singly-stung hosts produced mixed-sex broods. Second, multiple stings increased mean parasitoid progeny produced per host, primarily by reducing the chance of complete brood failure. Our results indicate substantial benefit for a second sting, but little benefit for three or more stings, even though M. grandii was sometimes observed to invest more than two stings within a single host. However, we also found that within-host larval competition is prevalent, suggesting that supernumerary stings may pay off in competition against conspecific larvae. Such additional investment within a single host would be particularly beneficial when hosts, rather than eggs, are limiting, but would decrease the overall efficacy of M. grandii as a biological control agent.
Bibliographical noteFunding Information:
We thank A. Eriksen for extensive laboratory and field assistance, and M. Hunter for comments on a previous version of this manuscript. This research was supported by the National Center for Environmental Research (NCER) STAR Program, EPA award U-91578501-2, NSF award DMS-0083468, the University of Minnesota Center for Community Genetics, fellowships from the University of Minnesota graduate school, and the Center for Insect Science through NIH Training Grant #2 K12 Gm00708-06.
- Clutch size
- Foraging behavior
- Host utilization
- Intraspecific competition
- Macrocentrus grandii
- Ostrinia nubilalis