Prevalence of Predators Associated with Acyrthosiphon pisum (Homoptera: Aphididae) and Hypera postica Gyllenhal (Coleoptera: Curculionidae) during Growth of the First Crop of Alfalfa

Abstract

In 1991 and 1992, Coleomegilla maculata Timberlake adults (Coleoptera: Coccinellidae) were the most abundant predators prior to first cutting in alfalfa (Medicago sativa L.) from 16 April to 7 June at four locations in Iowa. Relative populations of C. maculata adults peaked in 1992 at more than one per sweep (38 cm diameter sweepnet) in Polk County on 15 May. In 1991 and 1992, densities of C. maculata adults were highly correlated (P < 0.1) with varying spatial densities of Hypera postica Gyllenhal (Coleoptera: Curculionidae) larvae (0.557 < r < 0.919) and Acyrthosiphon pisum Harris (Homoptera: Aphididae) (0.379 < r < 0.911). Other coccinellid species, including Hippodamia convergens Guerin, H. tredecimpunctata Say, H. parenthesis Say, Coccinella septempunctata L., Cycloneda munda Say, and Adalia bipunctata L., represented 4.8 and 20.5% of the coccinellids collected in 1991 and 1992, respectively. In 1992, when prey density was from 3 to 7 times higher than in 1991, populations of these coccinellid adults were highly correlated with the density of H. postica larvae (0.324 < r < 0.904) and A. pisum (0.573 < r < 0.805) at each location. Hemipteran predators, including unidentified nabid species and Podisis maculiventrus Say, were relatively rare in alfalfa during this 2-year study. A consistently high correlation between populations of C. maculata adults and H. postica larvae may reflect the ability of this predator to aggregate to an increasing weevil density, which helps to maintain C. maculata populations in alfalfa fields when A. pisum populations are low.