Effect of Application Technology and Bacillus thuringiensis Subspecies on Management of B. thuringiensis subsp. kurstaki-resistant Diamondback Moth (Lepidoptera: Plutellidae)

Abstract

Field and laboratory tests were done to determine effects of application technology, plant age, Bacillus thuringiensis (Berliner) subspecies, and rate of application on mortality of 2 populations of diamondback moth, Plutella xylostella (L.). One population was susceptible (Geneva 88) and the other (Loxa b) was resistant to Bacillus tlwringiensis subsp. kurstaki. In the field, a knapsack, a drop nozzle, and an electrostatic sprayer were used to apply Javelin WG (6.4% [AI] (B. thuringiensis subsp. kurstaki) and XenTari WG (3.2% [AI] (B. thllringiensis subsp. aizawai.) Laboratory bioassays were done with sprayed leaves. Method of application significantly affected efficacy of B. thuringiensis. Compared with the other two sprayers, the electrostatic technique showed significantly lower variation between plant sections. Efficacy of XenTari increased 2-fold when applied with the electrostatic sprayer. Mortality of Geneva 88 with either formulation was >90% even at the lowest rates tested, but Javelin was significantly more effective compared with XenTari. Mortality of Loxa b with Javelin was <60% even at double the recommended field rate. Susceptibility of Loxa b to XenTari remained constant across generations, whereas resistance of Loxa b to Javelin de-creased from 624-fold at F2 to 41-fold at F6 in absence of selection. Javelin and XenTari did not show cross-resistance although they share some of the S-endotoxins. Studies with individual toxins are needed for a better understanding of the use of B. thuringiensis subspp. kurstaki and aizawai against resistant P. xylostella.