Foraging generalist predators are frequently confronted with a diversity of prey types that differ in profitability. Optimal foraging models predict that predators should select and exploit the most profitable prey types and reject unprofitable ones. The goal of this research was to evaluate the mechanisms underlying prey selection, prey exploitation and predation efficacy in the generalist predator Coleomegilla maculata lengi Timb. (Col., Coccinellidae).

The influence of prey species, prey size and predator age on predation efficacy and prey consumption by C. maculata was determined using non-choice laboratory tests. The three lepidopterous species occurring in cruciferous crops, namely, Artogeia rapae (L.), Plutella xylostella (L.) and Trichoplusia ni (Hubner), were used as prey. Results showed that, within a given prey instar, C. maculata preyed more on P. xylostella than on the other two species and that predation efficacy increased with increasing predator age. These experiments also revealed that coccinellid larvae had a higher prey weight consumption rate on intermediate-sized prey compared to smaller or larger prey even though they killed a higher number of small prey. It is suggested that this higher efficacy on prey of intermediate size could be a consequence of higher costs associated with the consumption of prey at both ends of the size spectrum.

To evaluate behavioral parameters of prey selection in C. maculata larvae, an image analysis system, initially developed for parasitoids searching for hosts, was modified. This system was programmed to locate the eggs used as prey and to detect and follow the movements of the C. maculata larval head. This user-friendly system can detect and follow the movements with less than 0.1% of error.

Using this tracking system, the discriminatory capacity and the prey selection behavior of C. maculata larvae were evaluated in choice situations in which prey differed in quality (unparasitized, parasitized, young or old T. ni eggs). Coccinellid larvae did not exhibit any preference between parasitized and unparasitized eggs of the same physiological age. However, when physiological age differed, coccinellid larvae always preferred the younger eggs despite parasitism. Longer handling time and detrimental effects on immature developmental time and survival was always observed on the less preferred egg type.