Oviposition and length of adult life in Caryedon gonagra (F.) (Col., Bruchidae)

Abstract

Oviposition and the length of adult life in the Bruchid beetle Caryedon gonagra (F.), an important pest of unshelled groundnuts (Arachis) in West Africa, were studied in continuous darkness in incubators at 30 and 40°C. at relative humidities of 50, 70 and 90 per cent., and at 45°C. and 70 per cent., and also under conditions of diurnal alternation of periods of light and darkness in a constant-temperature room at about 27·5°C. and 75 per cent. R.H. Newly emerged beetles were placed, singly (unmated or after a single mating) or in pairs, in tubes with 1–5 unshelled groundnuts. In addition, the influence of the absence of groundnuts was investigated.Length of life of the adults increased with decreasing temperature, increasing relative humidity, the absence of nuts (for females only) and the absence of mating (for females, and, in absence of nuts, for males). Adults survived for 3–4 days at 45°C. and 70 per cent. R.H. Optimum conditions are probably about 27·5—30°C. and 70—90 per cent. R.H., under which the adult length of life was about 21 days.The adults were sexually mature on emergence from the cocoons, and mating took place within 24 hours of emergence. Under the conditions of the experiments, the preoviposition period was between 24 and 48 hours. At 27·5 and 30°C., the mean length of the oviposition period (9—13 days) was only slightly influenced by relative humidity, being slightly shorter at the lowest R.H. (50%); an increase in temperature from 30 to 40°C. caused a marked reduction in the period.About 80 per cent, of the eggs were laid in crevices in the shell of the nut, where they are difficult to find. In the absence of nuts, the females laid readily between the cork and the glass and at the bottom of the tube.At 40°C., oviposition was inhibited at 50 and 70 per cent. R.H., but at 90 per cent. R.H. many eggs (but significantly fewer than at lower temperatures) were laid; the largest mean numbers (106–115) were laid at 27·5–30°C. and 70–90 per cent. R.H.; absence of nuts did not influence the numbers of eggs laid, but caused some irregularity in the oviposition pattern. More than one copulation appeared to be necessary for the female to lay a full complement of eggs.Under conditions of alternating periods of light and darkness, there was a daily rhythm of oviposition, with a strong correlation between numbers of eggs laid and the periods of darkness.The results are compared with those recorded in the literature.