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 dryland each year at the Texas Agricultural Experiment Station at Chillicothe, TX. Biotic variables included percentage leaf moisture and nitrogen, square/boll ratios, predators (lady beetles, lacewing and syrphid fly larvae), parasites, and the pathogen, Neozygites fresenii (Nowakowski). Multiple regression and correlation analyses were used to determine the most influential variables affecting population development including timing of population increase, average and peak numbers of aphids per leaf, timing of peak population numbers, and rate of population decline. Suppression of cotton aphid populations during June and July, followed by population increase during August, was correlated with abiotic factors including high temperatures and solar radiation. Rate of population decline was related to biotic factors including predator numbers, plant nutrition, and peak aphid density. Timing of peak populations in August was related to high temperatures, number of nights with low temperatures less than or equal to 20 degree C and to plant nutrition. Average population density during August was regulated by abiotic and biotic factors and their interactions; however, aphid numbers in the late April and late May plantings were mostly influenced by high temperatures while numbers in the late June planting were mostly influenced by plant nutrition. Peak numbers of aphids were correlated with solar radiation, daylength, and percentage leaf nitrogen. The influence of biotic and abiotic factors on population regulation is discussed for each planting date and for all planting dates combined.