POPULATION-DYNAMICS OF RHOPALOSIPHUM-PADI (HOMOPTERA, APHIDIDAE) IN CORN IN RELATION TO BARLEY YELLOW DWARF EPIDEMIOLOGY IN SOUTHWESTERN IDAHO

Population dynamics of Rhopalosiphum padi (L.) in seed and silage corn were investigated from 1981 through 1985 in southwestern Idaho. Aphid population development was divided into four phases: an immigration period, a lag period, a period of rapid increase where aphid numbers reached several million per hectare, and an emigration period characterized by an increase in alatoid nymphs and alatae. Just before peak aphid densities occurred, prereproductive development time for apterous R. padi was 9.9 +/- 0.9 (xBAR +/- SEM) in seed corn and 9.6 +/- 0.5 d in silage corn. The reproductive rate of apterae during this same period was 36.6 +/- 11.0 nymphs in seed corn and 9.8 +/- 2.9 nymphs in silage corn. Flight activity, as indicated by suction trap data, peaked in July and again from September through October or November. These two peaks corresponded with the immigration and emigration of R. padi detected in corn. Residents on corn were assayed for transmission of barley yellow dwarf virus, and the mean frequencies were 36% in 1983, 4% in 1984, and 2% in 1985. Migrant R. padi collected in a modified suction trap in 1985 showed a mean transmission frequency of 6%. Fall migrant R. padi in corn had a mean life span of almost-equal-to 14 d and a maximum life span of 41 d, which is sufficient time for fall-planted cereals to become infested with these migrants. Monthly maximum and minimum temperatures for epidemic and nonepidemic years differed most in December, January, and February, suggesting a possible temperature-disease relationship.