Callosobruchus Maculatus - Behavior

Behavior

Like several other species of beetle (Broscus cephalotes and other ground beetles), C. maculatus have fully functional wings. Both the dispersal and the sedentary morph can fly. However, both morphs, especially the sedentary, tend to walk instead of fly when moving, unless threatened.

As mentioned in the life cycle section, the adults of C. maculatus do not need to feed or drink. However, if sugar water or yeast is available, they will take advantage of it. Females are more likely to exhibit this behavior and thus utilize more resources than the males. Scientists tested the hypothesis that access to nutritional resources would affect the frequency of second matings. After their first mating, females were isolated for 24 hours with sugar, water, or nothing available. They were then presented with another male. Those that had access to sugar are significantly less likely to remate, though a high percentage do. Being able to feed makes the female less receptive to advances from males for mating. C. maculatus males present a nuptial gift as part of the mating process. The nuptial gift consists of nutritional content and is integrated into ejaculate content. If the female has access to nutrition elsewhere, then she is less likely to want or need anything from the males and can thus afford to be choosier in mate selection.

Females without this additional nutrient source are significantly less picky. Most of the time females do not appear to apply any criteria toward choosing a mate. Most species avoid mating with closely related relatives to avoid the negative effects of inbreeding. However, C. maculatus does not avoid mating with siblings. Researchers have tested this by placing virgin females with virgin males (either siblings or unrelated). Females are just as likely to mate with their brothers as with unrelated males of the same age. In remating trials, females still do not avoid mating with brothers, even if their previous mate was also a brother. The species does suffer from inbreeding depression, but it does not seem to take behavioral action to avoid it.

Once the female has mated and is ready to lay the eggs, she must determine egg location and distribution. Females prefer to lay their eggs on the smooth “cheek” of the bean. They avoid the wrinkled tops of the bean, and they avoid legumes that do not have smooth surfaces. C. maculatus females also have some unknown method to determine the mass of each bean they encounter. When the beetles are presented with a combination of bean sizes (a mix of small and large), they distribute their eggs so that each larva has access to roughly the same amount of nutrients. Their method is not based on surface area, but rather on the mass of the bean and the number of eggs already present.

Despite the mother’s efforts, there will likely be more than one egg on each bean. The larva within the bean form a pupal cell and passages through the bean. The pupal cell is formed as the larva presses feces into the wall of the cell. As the larva eats its way through the bean, it will encounter other larvae. Rather than fight, both retreat and create walls of feces. If the wall is removed, the two larvae will fight to the death. The reasons or mechanisms behind this reaction are currently unknown.