BIOTIC AND ABIOTIC REGULATION OF APHIS-GOSSYPII GLOVER IN WEST TEXAS DRYLAND COTTON

Population development of the cotton aphid, Aphis gossypii Glover, was studied in cotton planted in late April, late May, and late June for seven consecutive years, 1988-1994. 'Paymaster 145' cotton was grown d ryland each year at the Texas Agricultural Experiment Station at Chill icothe, TX. Abiotic factors that were monitored included maximum daily temperature, both ambient and under the leaf surface, number of days during late July-early August with minimum ambient temperatures less t han or equal to 20 degrees C, solar radiation, daylength, light intens ity and wavelength. Biotic variables included percentage leaf moisture and nitrogen, square/boll ratios, predators (lady beetles, lacewing a nd syrphid fly larvae), parasites, and the pathogen, Neozygites fresen ii (Nowakowski). Multiple regression and correlation analyses were use d to determine the most influential variables affecting population dev elopment including timing of population increase, average and peak num bers of aphids per leaf, timing of peak population numbers, and rate o f population decline. Suppression of cotton aphid populations during J une and July, followed by population increase during August, was corre lated with abiotic factors including high temperatures and solar radia tion. Rate of population decline was related to biotic factors includi ng predator numbers, plant nutrition, and peak aphid density. Timing o f peak populations in August was related to high temperatures, number of nights with low temperatures less than or equal to 20 degrees C and to plant nutrition. Average population density during August was regu lated by abiotic and biotic factors and their interactions; however, a phid numbers in the late April and late May plantings were mostly infl uenced by high temperatures while numbers in the late June planting we re mostly influenced by plant nutrition. Peak numbers of aphids were c orrelated with solar radiation, daylength, and percentage leaf nitroge n. The influence of biotic and abiotic factors on population regulatio n is discussed for each planting date and for all planting dates combi ned.