Hydrogen Cyanide as an Immediate Alternative to Methyl Bromide for Structural Fumigations

Flour mill fumigations are currently carried out with methyl bromide (MB). By 2005, this gas will be phased out in developed countries because it has been listed under the Montreal Protocol as a substance that depletes the ozone layer. Hydrogen cyanide (HCN) was used as the fumigant of choice for mills in the first part of the last century, and was then replaced by MB. Consequently, it should now be re-evaluated as a potential alternative to MB. However, knowledge about HCN's efficacy as a fumigant is outdated and does not conform to the notion of concentration time (Ct) product. Therefore, the objective of this study was to determine the biological efficacy of HCN in terms of Ct product for the different stages of three of the most prevalent species infesting mills and food manufacturing premises, the confused flour beetle Tribolium confusum, the red flour beetle Tribolium castaneum, and the Indianmeal moth, Plodia interpunctella In addition, the granary weevil Sitophilus granarius was studied in the adult stage only, since this is the only stage present in mills. Two series of trials were carried out: one consisted of, dose-trials to define the relationship between Ct product and insect mortality without presence of a rearing medium; the other consisted of trials in which inserts were placed at different depths in flour to measure HCN efficacy in penetrating residual heaps of flour. Our findings on the different stages of each species (eggs, young larvae, old larvae, pupae and adults) show that the Ct product required to obtain a LD90 ranged from 1 to 4 g.h!m-3, according to developmental stage, this being very low when compared to MB. Nevertheless, in contrast to MB, large differences were found between the species as shown in the high tolerance of granary weevils to HCN. The poor penetration of HCN into flour is due to significant sorption. Consequently, to be effective against T.!confusum adults buried in flour, a Ct product as high as 60 and 100 g.h!m-3 is required to kill this species at depths of 10 and 15 cm, though it does not kill S.!granarius. In practice, the Ct product needed to kill all stages of these major mill and food factory pests should be around 10 g!h!m-3, though to obtain HCN penetration and kill insects at a depth of about 10 cm in flour heaps, the prevailing Ct products should be around 60 g.h!m-3. This implies that the corresponding initial dose should be 5 g m-3, taking into account the presence of minor leaks.