Effect of population density of Cylas formicarius (Fabricius) (Coleoptera: Brentidae) on the progeny populations

Adults of the sweet potato weevil, Cylas formicarius, were allowed to lay eggs on a 150 g tuberous root of sweet potato for 3 days under conditions of 27°C, 60% RH, and 14L:10D. The number of progeny increased with the increase of parental density, and reached the saturation level. The number of progeny per parental female showed a peak at 30–100 pairs of parents. The density response mode of this insect characterizes it as the Allee type. At very low density, the mortality of eggs and larvae was high, which might have been due to the unfavorable conditions of the tuberous root, the quality of which had not yet been improved by the frano-inducing factor secreted during feeding by the larval and adult weevil. Comparatively more eggs were laid on the tuberous root conditioned by adult-feeding or oviposition. Size of adult progeny became smaller with the increase of parental density. The duration of their development was shortened, and variation of the time period until their emergence from the tuberous root became smaller with the increase of parental density. Optimal parental density for mass-rearing was found to be 65 pairs per tuberous root of 150 g for 3-days oviposition.