Seasonal Occurrence of the Pea Aphid and a Braconid Parasite, Aphidius smithi, in Kentucky123

Abstract

The seasonal occurrence and abundance of the pea aphid, Acyrthosiphon pisum (Harris), and its parasite, Aphidius smithi Sharma & Subba Rao, were studied in alfalfa and red clover fields in Kentucky from December 1967 to January 1970. Populations of pea aphids indicated a 2-peaked curve of abundance, with a vernal peak occurring in May and June and an autumnal peak in September and October. Vernal population peaks were reached at 975 aphids per 100 sweeps in 1968 and 1395 aphids/100 sweeps in 1969. Autumnal peaks were 15,606 aphids/100 sweeps in 1968 and 11,822 aphids/100 sweeps in 1969. Summer lows in pea aphid populations were 129 aphids/100 sweeps in July 1968 and 13 aphids/100 sweeps in August 1969. Low numbers were typical during winter months. Insecticidal treatments for control of the alfalfa weevil reduced the numbers of pea aphids but did not change population trends. The degree of parasitization fluctuated each year and resulted in several peaks in the population of wasps, Parasite populations exhibited a high degree of density-dependency, with populations responding rapidly to an increase in host density and declining as host density declined. The peak percentage of parasitism was 46% in 1968 and 69% in 1969. Greater numbers of wasps were observed in autumn, but the percentage of parasitism was slightly lower than in spring and early summer. Insecticidal treatments appeared to have little effect on parasite populations. The parasites collected were 61.6% females. The female: male ratio was high during June, September, October, November, and December.