The two-spotted lady beetle fails to avoid an introduced intraguild predator

Background/Question/Methods   The importation of the multicolored Asian lady beetle, Harmonia axyridis, has altered the lady beetle guild of North America. One interaction in which H. axyridis engages is intraguild predation, the consumption of other lady beetles. As H. axyridis is an arboreal species, it is native arboreal lady beetles that are most likely to fall victim to H. axyridis. In the area of Spokane, Washington, the most abundant native arboreal lady beetle is the two-spotted lady beetle, Adalia bipunctata. To begin to quantify the potential for H. axyridis and A. bipunctata to interact, we conducted a survey of lady beetles found on willow trees. In addition to our field survey, we performed a laboratory experiment to assess whether or not larval A. bipunctata can recognize and avoid areas with high densities of H. axyridis larvae based on indirect cues such as chemical track recognition. For the laboratory experiment, larval A. bipunctata were video recorded for one hour as they foraged in experimental arenas that had been treated with H. axyridis larvae or left untreated as controls.

Results/Conclusions   During both 2008 and 2009, A. bipunctata and H. axyridis were the most abundant lady beetles found in our field surveys. During 2008, we found 58 A. bipunctata, 39 H. axyridis, and 5 coccinellids belonging to other species. In the expanded 2009 survey, we found 147 A. bipunctata, 200 H. axyridis, and 34 other individual coccinellids. A contingency table analysis of the data from both years found that the presence of A. bipunctata was positively associated with the presence of H. axyridis on willow trees (chi-squared = 53.16, df = 1, p < 0.001). However, in our laboratory experiment, the movement velocity and average acceleration of A. bipunctata larvae did not differ between control arenas and arenas that had contained H. axyridis. The lack of behavioral change to indirect cues suggests that A. bipunctata lacks specific adaptations for avoiding direct encounters with H. axyridis.