INTRA VERSUS INTERSPECIFIC INTERACTIONS OF LADYBEETLES (COLEOPTERA, COCCINELLIDAE) ATTACKING APHIDS

The nature and relative strengths of intra versus interspecific interactions among foraging lady-beetle larvae were studied experimentally by measuring short-term growth rates of predators and reductions in population sizes of prey in laboratory microcosms. In these microcosms, ladybeetle larvae foraged singly or as conspecific or heterospecific pairs, for pea aphids on bean plants over a two-day period. Similarly sized third instar larvae of Hippodamia convergens and H. tredecimpunctata, H. convergens and H. sinuata, and H. convergens and Coccinella septempunctata, were tested in experiments designed to ensure that paired larvae experienced moderate competition. Interspecific competition in these experiments did not differ significantly from intraspecific competition, in that an individual's weight gain did not depend on whether its competitor was heterospecific or conspecific. Furthermore, aphid populations were reduced equally by heterospecific and conspecific pairs. These results suggest that there is little or no difference between intra and interspecific interactions among larvae of these ladybeetles when two similarly sized individuals co-occur on a host plant. Thus, the species diversity per se of assemblages of ladybeetle larvae may have little influence over the short term on the reduction of aphid populations by ladybeetle predation.