Field performance of cottons expressing transgenic CryIA insecticidal proteins for resistance to Heliothis virescens and Helicoverpa zea (Lepidoptera: Noctuidae)

Abstract

Transgenic cotton plants (BTK lines), carrying a gene construction that codes for the CryIA(b) or CryIA(c) δ-endotoxins from Bacillus thuringiensis variety kurstaki were evaluated in the field for resistance to naturally occurring tobacco budworm, Heliothis virescens (F.), bollworm, Helicoverpa zea (Boddie), and cabbage looper, Trichoplusia ni (Hubner). Five BTK lines (each line was an independent insertion event) were evaluated in 1990 and 6 in 1991. Also 2 nontransgenic commercial cultivars, 'Coker 312', the explant source cultivar for the BTK lines, and an adapted cultivar, 'Stoneville 453', were evaluated. A split-plot design was used with lepidopteran control as the main plot and cotton lines as the subplots. Main plot treatments were weekly spray applications of insecticide for lepidopteran control during 8 or 9 wk of the growing season, or no insecticidal sprays for lepidopteran control. The season mean numbers of tobacco budworm and bollworm larvae on plant mainstem terminals, flower buds and capsels, and the percentage of flower buds and capsels injured were lower for all BTK lines, in sprayed and unsprayed plots, than for the 2 nontransgenic cultivars. Averaged over both years, the season mean percent injury in plots of the unsprayed Coker 312 was ≃23% of the flower buds and 12% of the capsels. In unsprayed plots of the BTK lines, the season mean percent injury was ≃2.3 of the flower buds and 1.1 of the capsels. In unsprayed plots, the number of cabbage looper larvae and the amount of cabbage looper feeding injury to leaves also was lower on the BTK lines compared with the 2 nontransgenic cultivars. Using the recommended treatment thresholds for tobacco budworm and bollworm of 8-15% injured flower buds, none of the BTK lines would have been treated, whereas injury in the 2 non-transgenic cultivars would have required multiple insecticide applications. Average over both years, the yield of unsprayed Coker 312 was 1,050 kg/ha and the average of all unsprayed BTK lines was 1,460 kg/ha. Expression of these cryIA genes in Coker 312 cotton resulted in greater yield, 408 kg/ha, 38.8%, in the presence of lepidopteran pests. The use of these insect-resistant BTK lines or equally efficacious commercial BTK cultivars in production agriculture should reduce insecticide applications for tobacco budworm, bollworm, and cabbage looper while increasing farm profit and opportunities for the use of biological control.