Intraguild predation between two lady beetle predators is more sensitive to density than species of extraguild prey

Oenopia conglobata(Linnaeus) andMenochilus sexmaculatusFabricius (Coleoptera: Coccinellidae) are important predators of the pistachio psyllid,Agonoscena pistaciaeBurckhardt and Lauterer (Hemiptera: Psyllidae), a key pest of pistachio. The abundance ofM. sexmaculatushas recently increased in pistachio orchards, while that ofO. conglobatahas declined. We designed laboratory experiments to (1) evaluate the potential for intraguild predation (IGP) between these two species, (2) detect any possible asymmetries in IGP interactions, and (3) characterize the sensitivity of IGP to varying densities of two extraguild (EG) prey,A. pistaciaeandAphis gossypiiGlover (Hemiptera: Aphididae). We measured rates of IGP by fourth instar larvae and female adults (both starved for 12 h) on vulnerable immature life stages of the other species (eggs, first, and second instar larvae). IG and EG prey consumption were tallied after 12 h. All factors influenced rates of IGP, but species of IG predator, and life stage and density of EG prey, contributed the most variance. IGP decreased parabolically with increasing prey densities. IGP on eggs was higher than on first instars, which was higher than on second instars, suggesting palatability declined in later life stages. The largerM. sexmaculatusexhibited higher voracity than the smallerO. conglobatain both predation and IGP. Both species consumed moreA. pistaciaethanA. gossypiiin all treatments, which may reflect either lower handling time or lower food value per prey.