The effect of larval competition on adult weight and its mechanism in Callosobruchus maculatus (F.) (Coleoptera: Bruchidae)

Larvae of the seed beetle Callosobruchus maculatus develop inside the seeds of their host plants, and are incapable of moving between seeds. Many studies reported a reduction of adult weight as larval density increases due to resource shortage. However, I observed a different trend that larval competition led to an increase of adult weight when the host size was small. The phenomenon disappeared when host size was large, indicating that host size was a crucial factor for the trend. In this study, I tried to answer how adult weight increases under larval competition. Since egg-to-adult development was not prolonged under larval competition, increased feeding period via longer development is not the cause of the greater adult weight. Furthermore, the proportion of large individuals of the observed distribution at high larval density was significantly higher than that of a simulated distribution where small individuals were eliminated by an imaginary selection. Thus, there must be other causes leading to greater adult weight besides selection on body size. I further hypothesized that cannibalism increases adult weight under larval competition. By dissecting beans with 3 larvae at regular intervals, I found interference behavior and the evidence of cannibalism in larvae of C. maculatus, the finding potentially supporting the cannibalism hypothesis because conspesifics could be extra food source when bean resource was limited. Nevertheless, dominant larvae could also gain more weight by raising total food consumption in response to competition. In this study, the total consumption at high larval density was significantly higher than that at low larval density, indicating the possibility of increased efficiency. Further investigation is required. Finally, a previous study demonstrated that South India strain of C. maculatus could emit feeding vibration at larval stage to affect conspesifics weight. But, no indication in my experiment showed feeding vibration of 4C6-4 strain affected adult weight.