EFFECT OF APPLICATION TECHNOLOGY AND BACILLUS-THURINGIENSIS SUBSPECIES ON MANAGEMENT OF BACILLUS-THURINGIENSIS SUBSP KURSTAKI-RESISTANT DIAMONDBACK MOTH (LEPIDOPTERA, PLUTELLIDAE)

Field and laboratory tests were done to determine effects of applicati on 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 (Genev a 88) and the other (Loxa b) was resistant to Bacillus thuringiensis s ubsp. kurstaki. In the field, a knapsack, a drop nozzle, and an electr ostatic sprayer were used to apply Javelin WG 6.4% [AI] (B. thuringien sis subsp. kurstaki) and XenTari WG 3.2% [AI] (B, thuringiensis subsp. aizawai). Laboratory bioassays were done with sprayed leaves. Method of application significantly affected efficacy of B. thuringiensis. Co mpared with the other two sprayers, the electrostatic technique showed significantly lower variation between plant sections. Efficacy of Xen Tari increased e-fold when applied with the electrostatic sprayer. Mor tality of Geneva 88 with either formulation was >90% even at the lowes t 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 remai ned constant across generations, whereas resistance of Loxa b to Javel in decreased from 624-fold at F-2 to 41-fold at F-6 in absence of sele ction. Javelin and XenTari did not show cross-resistance although they share some of the 8-endotoxins. Studies with individual toxins are ne eded for a better understanding of the use of B. thuringiensis subspp. kurstaki and aizawai against resistant P. xylostella.