| Scientists have debated the relative advantages of specialist and generalist biological control agents. The two species of ladybeetle that I study, Coccinella septempunctata L. and Harmonia axyridis Pallas, potentially show attributes of both---they are specialists on specific prey when that prey is common, but have the capacity to act as generalists when their focal prey is rare. Demonstrating that individual insects shift between prey species (e.g., act as generalists) requires one to develop a means of inferring insect movement. Chapter 1 of this thesis develops one such technique, utilizing stable carbon isotopes, and examines its overall utility. Chapter 2 uses this technique to examine how H. axyridis and C. septempunctata use different crops, and finds evidence that H. axyridis moves between crops, consistent with the patterns of adult abundance I observed in the field (e.g., there is evidence that H. axyridis acts as a generalist). Chapter 3 examines the associations between each ladybeetle species and a different aphid species (e.g., how each species acts as a specialist). As a whole, the thesis finds evidence of each ladybeetle species acting as a specialist on a different species of aphid, but also finds evidence of one ladybeetle, H. axyridis, acting as a generalist when densities of its focal prey, soybean aphids, are low. While there is circumstantial evidence (presented elsewhere) that suggests that C. septempunctata may also have the potential to behave as a generalist, the thesis as a whole suggests that there are differences between C. septempunctata and H. axyridis in this regard, notably in the likelihood of each species to consume aphids from C4-photosynthetic crops. |
