Benefits of mixed diets for predatory ladybirds: meta-analysis of laboratory experiments

The development of cost-effective methods for laboratory and mass rearing of biological control agents necessarily includes the selection of the optimal diet. Experiments with some predatory coccinellids revealed so-called ‘additive effects’: feeding on mixed foods ensures lower mortality, faster larval development and adult maturation, higher fecundity, etc., than feeding on each of the components. These effects were demonstrated for eight of the 11 studied species (although not for all used combinations of foods). Meta-analysis shows that significant additive effect was found in 29.3% of 140 cases resulted from experiments with 11 species of predatory Coccinellidae performed in 21 published studies. However, if (1) one of the two mixed foods was artificial or factitious or if (2) one of the two mixed foods (that of higher quality) was limitedly provided, or if (3) the beneficial effect of mixed foods was estimated by fecundity, rate of maturation, or other indicators of reproductive activity, the probability of detection of a beneficial additive effect increased up to about 50%. These results could be used for the planning of further studies on the selection of optimal diets for laboratory and mass rearing of predatory ladybirds. In particular, a combination of a limited supply of an expensive high quality food with unlimited cheap low quality food could be the most promising and cost-effective approach.