Predicting the dosage of fumigant needed to eradicate insect pests from stored products.
The following is virtually the authors' summary of this account of laboratory investigations in Britain. Samples of wheat weighing 10 g and containing up to 100 pre-imaginal stages of Sìtophilus granarius (L.) either 30 or 29-31 days old were exposed in a metal chamber to a concentration of 10 mg methyl bromide per litre for periods of 1-6 hours [cf. RAE A 54 600]. Probit regression lines were fitted to the results for a series of generations of the same stock of weevils, and differences found in the LD50.s were attributed to the difficulty of obtaining populations of pre-imaginal weevils in similar developmental stages. The LD99 estimated by extrapolating these lines was compared for three generations with the values obtained by exposing 320-g samples containing about 2, 500 weevils. The probit regression lines for the large samples were curved upwards, so that high doses of fumigant gave larger kills than those predicted by the extrapolated straight lines. A dose of 52.5 mg per hour per litre killed all the 6, 000 weevils exposed to it, and 19 survived out of more than 40, 000 exposed to 50.1 mg per hour per litre. The survival period and fertility of the surviving individuals were impaired, so that, for instance, after exposure to 34.7 mg per hour per litre, the weevils yielded a generation only 5% the size of that yielded by untreated ones. This next generation was fully fertile. When 97% of a population was killed by fumigation at 41.7 mg per hour per litre, the LD50 of the next generation was raised by about 25%. The observed curvature of the probit lines is consistent with that which would result from the mixture of two or more homogeneous populations with different susceptibilities regardless of the slopes of the probit lines of these populations.