RELAY INTERCROPPING TO ENHANCE ABUNDANCE OF INSECT PREDATORS OF COTTON APHID (APHIS-GOSSYPII GLOVER) IN TEXAS COTTON

Relay intercropping winter and spring strip crops with cotton was used to conserve and enhance the numbers of predators of the cotton aphid (Aphis gossypii Glover) in 1992, 1993 and 1994 in northern Texas, USA. Numbers of aphids and aphid predators in cotton isolated from other c rops were compared with numbers where canola, wheat, sorghum (strip in tercrops) and cotton were planted adjacent to each other and overlappe d temporally. Aphid and predator populations were also monitored in th e intercrop. The intercrops acted as a reservoir for predators during the non-cotton season; these intercrops 'relayed' aphid predators from canola and wheat in the winter to sorghum in the spring and finally t o cotton in the summer. Averaged over the three years of the study, pr edator numbers in the intercrops were highest in sorghum and lowest in wheat. In ail years, canola had higher predator numbers than wheat, s uggesting that it would be a better winter intercrop than wheat for en hancing predator numbers. Average aphid abundance was lower in relay i ntercropped cotton than in isolated cotted each year. The three-year a verage in relay intercropped cotton remained fewer than 193 aphids per leaf on any one sample date, but reached 490 aphids per leaf in isola ted cotton. Seasonal abundance of aphid populations was bimodal in rel ay intercropped cotton, with peaks in early August and early September , while populations peaked only once in late August in isolated cotton . Average predator numbers were higher in relay intercropped cotton th an in isolated cotton, and predators appeared in higher numbers earlie r in the summer in relay intercropped cotton than in isolated cotton. Results from this study suggest that where cotton is grown without ins ecticides, relay intercropping aids the early arrival and continuous p opulation increase of predators in cotton, thereby reducing numbers an d postponing the initial population increase of the cotton aphid.