Rapid evolution and the cost of resistance 48 to Bacillus thuringiensis in greenhouse populations

53 The microbial insecticide, Bacillus thuringiensis (Bt), has become the mainstay of non-chemical control of 54 Lepidopteran pests, either as sprays or through the incorporation of Bt toxins into transgenic crops. Given 55 the wide use of Bt, it is striking that currently only one pest species, Plutella xylostella, has been reported 56 to have developed significant resistance to Bt outside the laboratory. By contrast, we report here the 57 frequent and rapid development of resistance to Bacillus thuringiensis kurstaki (Dipel, Abbott) in popu58 lations of cabbage loopers, Trichoplusia ni, in commercial greenhouses. Resistance to Bt appears to be 59 costly and there is a rapid decline of resistance in populations collected from greenhouses and maintained 60 in the laboratory without selection. Resistance management in vegetable greenhouses will require sporadic 61 use of Bt-based sprays or alternative use of sprays with other Bt toxins. 3