We monitored numbers of Coleomegilla maculata DeGeer and Harmonia axyridis (Pallas) eggs, larvae, and adults in an Illinois cornfield during anthesis. Larvae and adults were collected from the field and their gut contents were examined. Also, daily pollen deposition was recorded over the sample period, and prey biomass per plant was calculated for each of four replicate plots of field corn. The numbers of egg clutches and adults per plant were compared with daily pollen deposition, and pre- and postanthesis daily pollen deposition, densities of egg clutches, and adult densities were compared. Also, we compared the numbers of larvae and adults collected per plot and pollen and prey densities in the plots. Finally, the proportions of larvae and adults that had fed on corn pollen or prey were compared between predator species. We found that the number of coccinellid egg clutches increased significantly after anthesis and that the densities of adults of both species were not significantly different pre- and postanthesis. Larval and adult populations of H. axyridis were significantly correlated with prey densities, but C. maculata were not. All instars of C. maculata and H. axyridis occurred in corn during anthesis, and corn pollen was found in the guts of all four instars and in adults of both species. Dissections revealed that the majority of C. maculata larvae and adults had pollen in their guts, and a minority of H. axyridis larvae and a single adult had pollen in their guts. Conversely, the majority of H. axyridis larvae and adults had insect prey in their guts, and <40% of C. maculata larvae and adults had prey in their guts. Potential mechanisms for the numerical increases of coccinellids observed during anthesis and the implications of pollen feeding for risk assessment of transgenic insecticidal corn to C. maculata are discussed.
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