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 formulations of Bacillus thuringiensis Berliner subspecies were tested in field cages during the dry and rainy seasons of 1995 in Honduras. A local field population of Plutella xylostella (L.) with a 21-fold resistance to B. thuringiensis subsp. kurstaki (Javelin), but no resistance to B. thuringiensis subsp. aizawai (Xentari), was selected for 5-6 generations 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 presence or absence of a refuge (25%), and Xentari (1.12 kg/ha). Resistance to Javelin increased ≈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. thuringiensis 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 decrease. Although the rate of resistance increase was lower for lower doses of Javelin, a smaller proportion of marketable cabbage was produced in comparison with higher doses of Javelin or Xentari. Our data suggest that the deliberate inclusion of a refuge may reduce the proportion of marketable produce, and may affect use of this resistance management strategy in both sprayed B. thuringiensis and transgenic crops expressing B. thuringiensis toxins.