INDUCTION OF COTTON APHID OUTBREAKS BY INSECTICIDES IN COTTON

During 1989 and 1990, field studies were conducted to determine effect s of cypermethrin and sulprofos, which are used for control of Helioth is virescens and Helicoverpa zea in cotton, on outbreaks of cotton aph id, Aphis gossypii Glover. During 1989, cotton aphid outbreaks occurre d in both insecticide-treated and untreated plots. However, aphid dens ities were greatest in the sulprofos-treated plots and lowest in the c ypermethrin-treated plots, even though insecticide-induced aphid morta lity could not be detected in either treatment. In 1990, sulprofos-tre ated plots contained the most aphids, but unlike 1989, the untreated p lots had the fewest aphids. Rates of aphid parasitism by Lysiphlebus t estaceipes (Cresson) were low in 1989 and 1990, and aphidopathogenic f ungus infections were common only late in the season during 1990. Para sitism and fungal infections were most common in the sulprofos-treated plots, which contained the most aphids. Sulprofos-induced cotton aphi d outbreaks could not be attributed entirely to the destruction of nat ural enemies. Late-season growth and fruiting patterns of the cotton p lants indicated that sulprofos-treated plants continued active growth after the plants in the other treatments had begun to senesce. Further more, higher concentrations of the amino acid threonine and total esse ntial amino acids were detected in the plant sap of sulprofos-treated cotton plants. Sulprofos may have contributed indirectly to aphid outb reaks by altering the biochemistry of the plant in a way that better s uited the aphid's nutritional requirements, thus increasing aphid popu lation growth.