The toxicity of mixtures of methyl bromide and methyl chloroform to stored product insects

Work on the efficacy of a suitable replacement for the chronically toxic liquid fumigants currently used in the U.K. is described. The toxicity of a 5% mixture of methyl bromide (MB) in methyl chloroform (MC) to a range of pest species was mostly attributable to the dosage of methyl bromide present. The order of tolerance of species was Trogoderma granarium > Tribolium confusum > T. castaneum > Ephestia elutella > Cryptolestes ferrugineus > Sitophilus oryzae > Rhyzopertha dominica > Sitophilus granarius > Oryzaephilus surinamensis, and in each case diapausing larvae, pupae or older developing larvae were the most tolerant stages. With 1–2% MB in MC some joint action of the two compounds was evident. At 25°C there was an indication that the performance of MB against diapausing larvae of T. granarium was enhanced in the presence of MC. In contrast, the older stages of S. oryzae survived higher dosages of MB when MC was present, indicating a degree of antagonism between the two compounds. Little evidence of either potentiation or antagonism was observed with the 5% mixture at 15 or 20°C. Despite an apparent lack of joint action of MB and MC in the 5% mixture, at 25°C lower than usual concentration levels of MB were fully effective against the different test species in the presence of MC. The results show that the efficacy of mixtures containing at least 5% MB in MC proposed for the treatment of bulk commodities can be assessed on the basis of the dosages of methyl bromide achieved throughout the bulk.