Survivorship and seasonal phenology of the soybean aphid on its overwintering host in Indiana

Since its detection in 2000, the soybean aphid Aphis glycines Matsumura (Hemiptera: Aphididae), has been a serious pest in cultivated soybean Glycine max, (L) Merr. The soybean aphid has a heterocious holocyclic life cycle that alternates between hosts for sexual reproduction. The annual alternation involves a primary host (buckthorn) used for sexual reproduction and the secondary host (soybeans) colonized by parthenogenetic morphs. Eggs of the soybean aphid are deposited on the margins of the buds of the primary hosts where they overwinter from October to late March or when warmer and optimal conditions for spring migration are reached. Prior to this study the rates of survival and sources of mortality on these overwintering hosts are not well known. Studies were conducted from October, 2006 to March, 2009 in two Indiana locations to monitor the abundance and survival of the soybean aphid eggs on its overwintering host, Rhamnus cathartica. Study sites were located in the northeastern part of Indiana (Rome City), and a second site in west central Indiana (West Lafayette). To determine effects of predation on this host, the survival of caged and exposed soybean aphid eggs was observed and recorded throughout fall and spring. Mortality rates were high in both locations throughout the experiment in caged and exposed treatments. Exposed eggs experienced greater mortality (85%) than eggs protected with cages (55%). This difference in soybean aphid overwintering mortality was attributed to biotic and abiotic factors. The numbers of predators observed during fall observations suggest that natural enemies contributed significantly to overwintering aphid egg mortality. During the fall of 2008, predators arrived at R. cathartica with soybean aphid migrants, lowering the aphid population to less than detectable levels. In the absence of natural enemy activity during the winter months, mortality of eggs continued to increase reaching an overall mortality as high as 100% in 2007. The study suggests that both abiotic and biotic causes of mortality influence the overwintering populations of the soybean aphid in Indiana