Screening for resistance to Aphis gossypii Glover, the cotton aphid, began in hopes of developing superior-yielding cottons resistant to the aphid. It was anticipated that genotypes would range in resistance from susceptible to resistant. Original screening studies were conducted in a greenhouse to test general resistance. Sixteen cotton genotypes were tested in a completely randomized design. In general, resistance was varied, proving that genetic variability for resistance existed (P=.0026). Several top, bottom and middle performers were noted, and the top and bottom performers were tested against each other in choice and no-choice tests. These tests confirmed host plant resistance. The top and bottom performers currently are used as standards in ongoing screening tests. Additional screening studies were conducted in a greenhouse using a randomized block design. Six genotypes were tested per trial with two of these genotypes being the standards. Different treatments of pyrethroid-treated and infested, infested and non-infested plants without pyrethroid treatments were used in studying the effects and interactions of cotton aphid numbers with different cotton genotypes. Cotton plants were infested with cotton aphids and treatment was applied at the first true leaf stage. Aphid counts and plant health measurements were taken each week for five weeks. Thus far, ten cotton genotypes have been studied for resistance to the cotton aphid in this manner.

Throughout all screening tests, the superior cotton genotype for resistance has been Gossypium arboreum (P=.0026). This genotype displays excellent resistance to cotton aphids and does not show a great deal of damage associated with cotton aphid infestations. Another genotype, CA 3084 which is an experimental line, has consistently performed second in aphid counts and plant health studies while not being statistically different from other genotypes (P=.1834). CA 3084 also has shown possible, but not significant tolerance to high aphid numbers. 'Acala 1517-75', ' Paymaster HS-26' and 'Paymaster 145' have shown moderate levels of tolerance to the cotton aphid (P=0427). One consistently poor performer in both aphid counts and plant health has been 'Stoneville 213'.

In conclusion, the cotton genotype Gossypium arboreum clearly demonstrated host plant resistance to the cotton aphid. Therefore, G. arboreum will be entered into the Texas A&M University Cotton Improvement Program at Lubbock.