THE POPULATION STRUCTURE OF DACTYNOTUS AMBROSIAE (THOMAS) (HOMOPTERA: APHIDIDAE)

The aphid Dactynotus ambrosiae (Thomas, 1878) was recorded in both North and South America, most of the time on Compositae plants. In West Lafayette, Indiana, it occurs from May to November, mainly on Solidago missouriensis, as well as on Solidago spp., Ambrosia sp., and Vernonia sp. Incidently, it was also recorded on Convolvulus sepium (Convolvulaceae) and Polygonum convolvulus (Polygonaceae). The active aphid population developing on Solidago spp. was followed in 1975 and 1976, and from May to July in 1977, when the fluctuation of population was observed and related to climatic and biotic factors. The aphid development presented a positive reaction when daily mean temperatures were around 22 to 24(DEGREES)C with intense fluctuations of daily maximum and minimum temperatures. The length of the life cycle was extended from 8 to 12 days to 13 to 17 days when the daily minimum temperature dropped from 13 to 7(DEGREES)C or less, and the maximum dropped from 33 to 24(DEGREES)C or less. Precipitation presented a negative correlation with the aphid population recorded in 1975. The mechanical effect of washing the population off the plants by heavy rainfalls was, in general, followed by a quick recovery. Chrysopa sp., and unidentified species of Syrphidae, Coccinela novemnotata, Cycloneda saguinea, Hippodamia convergens, and Podabrus rugulosus are the most important biological agents in controlling the aphid population. Two Formica species were recorded in association with the aphids. The utilization of scintillating nail polish to mark the aphids showed that one female may survive for 19 days and 12 days was the average longevity for about 50% of the adults. The average proportion of winged females was found to be 12.81%, changing from less than 1% in early May to 48% during the crowding peaks of the population. Age frequency tests, conducted at intervals of eighteen days in 1975, 1976 and 1977 showed intense changes in population structure. Partial life tables were developed for the aphid species, under field conditions. They presented convex survivorship curves with an average mortality lower than 40% at the end of the second nymph stage. The following average indexes were found: effectiveness of colony maintenance = 10.27%; generation mortality = 89.73%; actual index of population trend = 0.60; fecundity = 6.38 nymphs per female per day; constant mortality rate = 84.33%