Stage specific consumption and utilization of aphids, conspecific and heterospecific eggs by two species of Coccinella (Coleoptera: Coccinellidae)

The nutritive quality of aphids, conspecific and heterospecific eggs as food for ladybirds is a controversial issue. If ladybirds find aphids more palatable than eggs then they are more likely to efficiently exploit and be biocontrol agents of aphids. If eggs of conspecific or heterospecific ladybirds are more palatable than aphids these biocontrol agents may either impede aphid pest suppression or become a threat to other species of ladybirds. Thus, prior to formulating predatory guilds of ladybirds the nutritive quality of their eggs compared to that of the target aphid needs to be determined. In this regards, the present study evaluates the nutritive quality of an ad libitum supply of: (i) pea aphid, Acyrthosiphon pisum (Harris), (ii) conspecific eggs and (iii) heterospecific eggs as food for Coccinella septempunctata (L.) and Coccinella transversalis Fabricius (Coleoptera: Coccinellidae), by determining the stage specific predation and developmental attributes of these two ladybirds reared on these diets. The conversion efficiencies and growth rates of young (first, second and third) larvae of both species of ladybirds were higher than those of fourth instar larvae and the consumption rates of fourth instar were higher than those of young larvae when fed on the three diets. When fed aphids the pre-imaginal development was faster, the consumption rates, conversion efficiencies and growth rates higher, adults bigger and mortality lower than when fed on conspecific eggs. The larvae of C. septempunctata consumed and completed their development when fed eggs of C. transversalis but the first instar larvae of C. transversalis did not consume the eggs of C. septempunctata and died of starvation. The dominance of C. septempunctata over C. transversalis in agricultural fields may be due to it consuming and utilizing aphids more efficiently and as a consequence growing faster and suffering a lower mortality along with its ability to consume and complete its development on eggs of C. transversalis. Thus, both these predators may not continue to coexist as predators of the pea aphid in agricultural fields since C. septempunctata may become a greater threat to C. transversalis in the future.