Depression of the minimum concentration of methyl bromide effective against diapausing warehouse moth larvae at 15°c by prior exposure at a moderate concentration

Abstract

Warehouse moth (Ephestia elutella) larvae in diapause were exposed at 15°C to methyl bromide at 8 mg litre−1 for 14.5 h and then immediately exposed at a lower concentration. The exposure at 8 mg litre−1 killed 44–69% of the larvae treated. Subsequent concentrations down to 1.1 mg litre−1 obeyed Haber's rule (concentration × time= k, a constant for mortality), but a higher concentration‐time product (ct) was required for over 90% kill at 0.8 mg litre−1. Only concentrations down to 1.9 mg litre−1 obey Haber's rule if there is no prior exposure at a higher concentration. Although minimum effective concentrations are lower at 15°C than at 25°C, exposure at a higher concentration depresses the subsequent level to a similar extent at each temperature. The contribution to the efficacy of a treatment, of low concentrations persisting at the end of fumigation, is thus likely to be even greater at moderate to low temperatures than at 25°C. The implications for the development of resistance to methyl bromide are discussed.