Keep them coming, baby: Determinants of the field oviposition rate of the soybean aphid parasitoid Binodoxys communis (Hymenoptera: Braconidae)

The ability of parasitoids to suppress host populations is linked to their per-capita oviposition rate. A release-and-recapture study featuring the soybean aphid parasitoid B. communis was conducted in soybean plots in the summers of 2007 and 2008. Parasitoids were recaptured between 9 AM and 4 PM on two consecutive days following each release and females were dissected to determine their egg load. Control females caught between 9 AM and 11 AM were individually caged for eight hours with a clean soybean plant in the field to estimate the baseline field egg maturation rate. A comparison of the wasps caught in the morning and caged for eight hours with females caught in the afternoon provided an estimate of the field oviposition rate of B. communis. In 2007, females caught in the morning and the afternoon had similar egg loads of 81.5 +/- 7.1 and 85.5 +/- 9.1, while caged females without host access had an egg load of 134.3 +/- 14.4, resulting in an estimated oviposition rate of approximately six eggs per hour. Females matured approximately the same number of eggs as they laid over the course of the day, allowing egg loads of field-collected parasitoids to remain relatively constant. Parasitoid size and age, aphid density and temperature played minor roles in determining the oviposition rate. These results suggest that egg maturation has the capability to stave off egg limitation in B. communis foraging in the field, potentially facilitating the ability of this parasitoid to suppress populations of the soybean aphid.