Interactions among Ants (Formica montana), Aphids (Myzius persicae), Imported Cabbageworm (Pieris rapae) eggs, and Pink Lady Beetles (Coleomagilla maculata) and their implications for an Oilseed Crop (Brassica napus)

Canola has been produced in the United States since World War II and in the Northern Plains and Pacific Northwest, is dominated by the spring variety B. napus. Canola supports and rich and abundant arthropod community (including ants, aphids, lepidopteran eggs, and ladybeetles), and interactions within this community could have important implications for yield. The purpose of this study was to determine whether aphids mediate interactions between ants and imported cabbage worm eggs, and if ants tending aphids will protect eggs from predation by pink lady beetles. We examined the interactions among these four insect groups under laboratory conditions in a partial factorial design. The interactions between the herbivores and ants were examined in one set of trials, and the effects of lady beetles on these interactions were examined in a separate set. Experimental endpoints of the experiments were ant activity levels, weight and survival of aphids, and survival of P. rapae eggs and C. maculata adults. Ants readily tended the aphids, and consumed P. rapae eggs regardless of whether aphids were present or not. Inclusion of lady beetles increased ant activity, and lady beetles were quickly killed. A similar pattern in aphid and P. rapae survival was seen regardless of whether lady beetles were present or not. Our data indicates that aphid-attending ants can still be important predators of other herbivores in the system. Also, lady beetles have a demonstrated effect on ant activity levels, possibly because the ants perceive the beetles as threats to their aphid attendees.