Comparison of single versus multiple species of generalist predators for biological control

An open question in biological control concerns the value of using multiple as opposed to a single species of natural enemies. To find out how the mechanisms by which multiple biocontrol agents might interact, I conducted laboratory experiments quantifying the behavior and effectiveness of predator species used either singly or in combination to control bean aphids, Aphis fabae Scopoli. The predators were larvae of a lacewing, Chrysoperla plorabunda (Fitch), and the sevenspotted lady beetle, Coccinella septempunctata L. The experiments consisted of 4 treatments tested in a 2 X 2 factorial design crossing 0 or 1 first-instar C. plorabunda with 0 or 1 first-instar C. septempunctata per fava bean plant, Vicia faba L. I also tested 2 additional treatments of 2 larvae of a single species, for a total of 6 treatments. The predators slowed the growth of aphid populations; no evidence of interaction between the predator species could be seen in terms of their effects on the sizes of the final aphid populations. This result may be explained by the finding that larvae of C. plorabunda and C. septempunctata differed significantly in where they were most frequently found on the plant. No clear evidence for intraguild predation was found. Thus, the majority of the data indicated that C. plorabunda and C. septempunctata did not have significant interactions.