Non-target effects of the multicolored Asian lady beetle, Harmonia axyridis, on monarch butterfly, Danaus plexippus, populations

The multicolored Asian lady beetle, Harmonia axyridis (Pallas), was recently identified as a potential hazard to monarch butterflies, Danaus plexippus (L.). In response, a quantitative risk assessment for the potential impact of H. axyridis on D. plexippus in Minnesota corn, Zea mays L., and soybean, Glycine max L., fields was developed using a risk analysis and simulation software package. This assessment was conducted as an examination of the worst-case scenario for the impact of H. axyridis on D. plexippus . Habitat specific recruitment of D. plexippus eggs was determined empirically. Subsequently, the estimated abundance of D. plexippus in each habitat was reduced by stage-specific mortality rates. Two classes of mortality acted on D. plexippus populations. First, mortality induced by H. axyridis was a function of D. plexippus exposure to H. axyridis, and the predation rate of D. plexippus by H. axyridis. Exposure was determined through season-long monitoring of predator and prey populations on common milkweed, Asclepias syriaca L., in corn and soybean fields. The predation rate, as a function of exposure to H. axyridis , was determined through a series of caged field predation studies. Second, mortality due to causes other than H. axyridis were obtained from the literature. Exposure and subsequent risk varied by habitat, with low risk of H. axyridis impacting D. plexippus populations developing in corn fields, but moderate to high risk to those in soybean fields. Predicted proportionate rates of D. plexippus mortality attributable to H. axyridis were greater in soybean compared to corn fields. Production of third instar D. plexippus in corn and soybean fields with H. axyridis in these systems was 89.9% and 53.1%, respectively, of what it would have been without H. axyridis in these systems. To evaluate the landscape level risk of H. axyridis impacting D. plexippus, further data on the recruitment of D. plexippus in other habitats and exposure estimates specific to the additional habitats are needed.