Interactions of Coccinella novemnotata (Coleoptera: Coccinellidae) and Coccinella septempunctata (Coleoptera: Coccinellidae) While Foraging for Aphids

The importation and establishment of Coccinella septempunctata L. (Coleoptera: Coccinellidae) in North America purportedly caused the displacement and local extirpation of the native ninespotted lady beetle, Coccinella novemnotata Herbst (Coleoptera: Coccinellidae), across most of its former range, and several reports have shown that C. septempunctata maintains competitive advantages over C. novemnotata. We investigated the role of aphid density on the retention time of these two species on fava bean plants, and the effect of con- versus heterospecific pairings of adult beetles on the foraging behavior of C. novemnotata. We found that aphid density did not affect C. novemnotata's retention time, but did affect the retention time of C. septempunctata, which left plants without aphids significantly faster than C. novemnotata. Additionally, C. septempunctata females left plants significantly faster than their male counterparts, whereas we observed no difference between the two sexes for C. novemnotata. Our test of pairs of beetles showed that C. novemnotata were together on plants more frequently than conspecific pairs of C. septempunctata and heterospecific pairs of beetles, and that all beetles were more likely to be found together on the aphid-infested plant versus the non-infested plant regardless of the pairs' composition. These results show that C. novemnotata spend more time foraging for aphids when aphids are scarce compared with C. septempunctata, and that C. novemnotata is less tolerant of occupying plants inhabited by C. septempunctata versus a conspecific beetle, adding additional mechanisms whereby C. septempunctata could outcompete C. novemnotata in the field.