MANAGING DIAMONDBACK MOTH (LEPIDOPTERA, PLUTELLIDAE) RESISTANCE TO FOLIAR APPLICATIONS OF BACILLUS-THURINGIENSIS - TESTING STRATEGIES IN-FIELD CAGES

Three resistance management strategies for field-sprayed commercial fo rmulations of Bacillus thuringiensis Berliner subspecies were tested i n field cages during the dry and rainy seasons of 1995 in Honduras. A local field population of Plutella xylostella (L.) with a 21-fold resi stance to B. thuringiensis subsp. kurstaki (Javelin), but no resistanc e to B. thuringiensis subsp. aizawai (Xentari), was selected for 5-6 g enerations with 16 field applications of a high (1.12 kg/ha) or low (0 .3 kg/ha) dose of Javelin, a high or low dose of Javelin in the presen ce or absence of a refuge (25%), and Xentari (1.12 kg/ha). Resistance to Javelin increased approximate to 1.9-4.4 times, but was significant only with the 1.12 kg/ha rate of Javelin irrespective of the presence or absence of a refuge. Field selection with Javelin at 0.3 kg/ha or Xentari did not cause a significant increase in resistance to B. thuri ngiensis subsp. kurstaki, nor did P. xylostella selected with Xentari evolve resistance to B. thuringiensis subsp. aizawai. During the same period, the LC50 of Javelin in P. xylostella left unselected did not d ecrease. Although the rate of resistance increase was lower for lower doses of Javelin, a smaller proportion of marketable cabbage was produ ced in comparison with higher doses of Javelin or Xentari. Our data su ggest that the deliberate inclusion of a refuge may reduce the proport ion of marketable produce, any may affect use of this resistance manag ement strategy in both sprayed B. thuringiensis and transgenic crops e xpressing B. thuringiensis toxins.