The contribution of predator phenology and diversity to secondary pest suppression in alfalfa

There is growing evidence that diverse predator communities are more efficient at suppressing pest populations than a single predator species. Studies addressing predator diversity and pest suppression, however, represent diverse communities primarily with adult predator stages. In a survey of predatory insect populations in commercial alfalfa (Medicago sativa) fields we found that adult and juvenile predators occurred simultaneously during the mid to late growing season when secondary pests, in particular, the pea aphid (Acyrthosiphon pisum), were problematic. Functional differences between adult and juvenile predator stages in foraging strategy, habitat preference, and mobility may influence interactions in diverse communities. Differences between adults and juveniles could lead to positive effects on pest suppression due to resource partitioning. Alternatively, the presence of juvenile stages may increase negative interactions (e.g., intra-guild predation) decreasing the impact of diverse communities. In field cage studies, using a substitutive design, we manipulated species richness (0, 1, 3 predator species) and life stage (adult only, juvenile only, or both) of the four most common predatory insects (Nabis sp., Geocris sp., Hippodamia convergens, and Coccinella septempunctata) represented in our field survey. We then recorded the impact of these manipulations on pea aphid suppression. Overall, the presence of adult and juvenile predators decreased pea aphid populations. More importantly, monocultures performed best when both adults and juveniles occurred together at equal proportions. Pea aphid suppression, however, was greatest in diverse predator communities containing a greater proportion of adults than juveniles.