Are Aphidophagous Ladybirds (Coccinellidae) Prudent Predators?

Aphidophagous ladybirds tend to lay eggs close to their prey and their voracious larvae forage in a way that results in them remaining in or close to aphid colonies. Although ladybird beetles have frequently been used to control aphids, they are rarely effective in reducing aphid abundance. Aphids have very high rates of increase and are very sensitive to changes in the quality of their host plants. On average, aphid colonies rarely last for more than a month. That is, they are a very unstable food resource for predators. In addition, the developmental time of ladybirds is about a month. As a consequence ladybirds face two risks. If they lay their eggs too late in the development of an aphid colony, the larvae will not mature before prey becomes scarce. If they lay too many eggs their larvae will severely restrict the rate of increase of the aphids and cause an earlier collapse of the colony. In order to survive the larvae will then have to resort to cannibalism. Theory indicates that the best strategy for optimizing fitness is for ladybirds to lay a few eggs at the beginning of the development of aphids colonies. Both experimental and field data indicate that is what ladybirds do, i.e., they behave as if they are ‘prudent predators’. This accounts for why aphidophagous ladybirds have been very ineffective biological control agents.