Effects of Bt cotton expressing Cry1Ac and Cry2Ab and non-Bt cotton on behavior, survival and development of Trichoplusia ni (Lepidoptera: Noctuidae)

Abstract

With an assumption that the larvae of cabbage looper, Trichoplusia ni (Hübner), a secondary pest of cotton, Gossypium hirsutum (L.), could move between cotton plants in the field, we conducted a series of laboratory experiments to determine the larval movement, food choice, consumption, survival, and development on Bt (Bollgard II expressing Cry1Ac and Cry2Ab) and non-Bt cotton. On non-Bt cotton, all T. ni larvae fed and stayed on the leaves. In choice tests between a non-Bt and Bt cotton leaves, 73.3%, 86.7% and 93.3% of first instar larvae moved to non-Bt cotton leaves after 1, 8 and 48 h, respectively, indicating that larvae were able to detect and avoid Bt cotton leaves. On the non-Bt cotton leaves, 90% of larvae initiated detectable feeding damage, compared with only 16.7% on the Bt cotton leaves. The larvae feeding on non-Bt cotton leaves consumed an average of 0.226 cm 2 leaf per larva in 48 h, whereas the larvae feeding on Bt cotton leaves consumed an average of 0.018 cm 2 leaf per larva. The developmental times of each of the five larval stages or pupal stage were generally not significantly different with a few exceptions when they fed either on non-Bt leaves or a mixture of non-Bt and Bt leaves. The pupae that developed from the larvae that fed on non-Bt were 21.6–24.7% heavier than those that developed from the larvae that fed on a mixture of non-Bt and Bt cotton leaves. The total developmental time of larvae in the mixed-leaf treatment was significantly longer than that of larvae on non-Bt leaves. No T. ni larvae survived when they fed exclusively on Bt cotton leaves. Starved larvae died significantly sooner than those on Bt leaves. High percentages of larvae survived when they fed either on non-Bt leaves (92.7%) or on the mixture of non-Bt and Bt cotton leaves (91.7%) for 5 days. The recognition and migration of the first instar T. ni larvae from Bt cotton leaves to non-Bt cotton leaves imply that the merit of Bt and non Bt cotton seed mixture at planting should be further evaluated as a strategy for Bt cotton resistance management of lepidopteran pests.