The Groundnut Bruchid, Caryedon gonagra (F.)

Abstract

SummaryInformation on the world distribution and host-plants of the Groundnut Bruchid, Caryedon gonagra (F.), has been summarised from the literature. As a result of misidentification of the species that attacks groundnuts and of differing views about its generic assignment, this information has been given under several different names. The records brought together here are believed all to represent C. gonagra, and in a number of cases this has been confirmed by examination of the material concerned by Mr. B. J. Southgate.C. gonagra is widely distributed in the Old World tropics and sub-tropics, but it is absent from Australasia, and it has only a restricted distribution in the New World tropics. Almost all its host-plants belong to the Leguminosae, the principal one being the tamarind, Tamarindus indica, in the pod of which the beetle has been found to develop more quickly than in the groundnut.Data on the life-history and habits, and on the duration of pre-adult development and adult life, are summarised from the literature.Experiments are described to determine the optimum conditions for multiplication of C. gonagra, using groundnuts as food. When adult females were allowed to lay large numbers of eggs on shelled and unshelled nuts, only a small proportion of the resulting larvae completed their development, the number of adults that emerged averaging about 2·4 per kernel in shelled nuts and 3·2 in unshelled ones. The developmental period at 70 per cent. relative humidity was 42 days at 30°C. and 91–98 at 25°C.; the former figure is less than any recorded in the literature at higher or lower temperatures, and all subsequent experiments were therefore made at 30°C. and 70 per cent. relative humidity.Adults confined for three weeks, starting when four days old, at the rate of 25 pairs on 160 g. of a mixture of broken and intact kernels gave rise to some 314 F1 adults; this total was virtually unchanged when 50 pairs were used, but lower densities, though giving rise to greater yields per parent adult, gave much lower yields per weight of food. Emergence of adults began six weeks after the date of egg-laying and reached its peak from one to three weeks later.The yield from ten females on intact kernels was greater than from the same number on the mixture, but differed little whether the parent adults were left on for one week or three weeks, indicating that the majority of the eggs are laid in the first 11 days of adult life. The yield from unshelled nuts was nearly twice as great as that from shelled nuts, and also significantly larger than that from shelled nuts with ground shell added. Possible causes of this are discussed.The sexes can be distinguished in the adult stage by differences in the terminal abdominal segments.