Floral resources impact longevity and oviposition rate of a parasitoid in the field

The use of floral resource subsidies to improve herbivore suppression by parasitoids requires certain trophic interactions and physiological changes to occur. While the longevity and fecundity of parasitoids are positively affected by nectar subsidies in laboratory studies, the impacts of floral subsidies on the fecundity and longevity of freely foraging parasitoids have not been studied. We studied the longevity and per capita fecundity of naturally occurring Diadegma insulare foraging in cabbage plots with and without borders of flowering buckwheat, Fagopyrum esculentum, as well as relationships between longevity, fecundity, sugar feeding and parasitism rates on larvae of the diamondback moth, Plutella xylostella. Relative longevity was estimated by counting broken setae on the fringe of the forewing. Floral borders increased the longevity of males and females in adjacent cabbage plots. The egg maturation rate of D. insulare was estimated by comparing egg loads of females collected early in the day with egg loads of females held without hosts in field cages throughout the day. Females in buckwheat cages matured 2.7 eggs per hour while females in control cages resorbed 0.27 eggs over the same time period. The fecundity of females collected in the afternoon was estimated by comparing their actual egg load to the estimated egg load in the absence of oviposition for females in a given plot. Females foraging in buckwheat plots had marginally fewer eggs remaining in their ovaries, and laid marginally more eggs than females in control plots. Females from both treatments carried 30-60 eggs by the afternoon and therefore were time-limited rather than egg-limited. Plots where a greater proportion of females had fed on sugar had longer-lived females. This suggests that feeding enhanced longevity of D. insulare. However, plots with longer-lived and more fecund females did not exhibit higher parasitism rates, although the power of these tests were low.