HERBIVORY TOLERANCE OF COTTON EXPRESSING INSECTICIDAL PROTEINS FROM BACILLUS-THURINGIENSIS - RESPONSES TO DAMAGE CAUSED BY HELICOVERPA SPP,AND TO MANUAL BUD REMOVAL

Transgenic cotton varieties expressing Cry IA(c) insecticidal proteins from Bacillus thuringiensis ('Bt cotton') remain vulnerable to non-le pidopteran insects. In addition, they are susceptible to lepidopteran pests when the efficacy of Bt toxins falls because of ontogenetic and/ or environmental factors. Hence the importance of knowing to what exte nt Bt cotton is able to tolerate damage. The degree of tolerance of Bt cotton to actual and simulated insect damage was assessed in three fi eld experiments. Exp. 1 compared the effects of Helicoverpa spp. (Lepi doptera: Noctuidae) on the growth, development and yield of Bt cotton with those of its near isogenic non-Bt counterpart in two genetic back grounds (Siokra V15, Sicala V2) under two regimes of chemical control of insects (S1: nine insecticide applications during the growing cycle , S2: six insecticide applications). Exp. 2 compared insecticide-prote cted Bt crops with crops manually damaged to simulate (a) early-season loss of vegetative buds, (b) loss of flowerbuds, and (c) loss of both vegetative and reproductive buds. Also using manual damage, Exp. 3 ev aluated the effect of timing of flowerbud loss on the yield and maturi ty time of insecticide-protected Bt crops. In Exp. 1, well-protected B t crops (S1) yielded 24% more than their less-protected counterparts ( S2). The less protected crops had, however, substantially more immatur e fruit at the end of the season highlighting a considerable potential for recovery. Poor soil conditions, interacting with season length ac counted for the difference between potential and actual compensation i n crops that were exposed to almost continuous damage by Helicoverpa s pp. Under more favourable growing conditions, maturity was delayed but yield of damaged Bt crops was unaffected by discrete episodes of simu lated herbivory (Exps. 2 and 3). The introduction of B. thuringiensis genes into cotton does not seem to have reduced the considerable capac ity of the crop to tolerate insect damage, and this attribute should b e considered in the development of pest management strategies for Bt c rops. (C) 1998 Elsevier Science B.V.