Food selection and fitness optimisation in insects

Abstract: To optimise fitness is a continuous process which assists species survival in changing environments. Choosing an optimal diet is an essential part of the optimisation process and a key element of optimal foraging theory. The relationship between food choice and fitness optimisation has been tested on three ground beetles and several collembolan species by using food items naturally occurring in the same habitat as the animals. The ground beetles Harpalus rufipes and Amara similata are considered being seed-eaters but can include insects in their diet in the field. An addition of insect species to a mixture of seeds did not improve beetle fitness, though a seed mixture was of a higher quality to the beetles than seeds of single plant species. However the beetles did not show a high degree of selectivity between the specific seed species and their choice was not always in agreement with the quality of the seeds. The collembolans, on the other hand, showed a high degree of selectivity and Folsomia fimetaria, Protaphorura armata and Heteromurus nitidus preferred fungi that optimised their growth, survival and fecundity. Folsomia fimetaria could even select the optimal food when a fungal species was grown in different soil substrates. The high degree of selectivity corresponding to food quality that was seen in the collembolans might be due to a production of fungal odour that can be detected by the collembolans. Plants producing proteinase inhibitors experience reduced attacks by herbivores thus the effect on predators feeding on the herbivores was studied. The herbivorous caterpillar Helicoverpa armigera was fed an artificial diet containing a proteinase inhibitor and the ground beetle Harpalus affinis was fed with H. armigera. H. affinis reduced its feeding when the caterpillars had been fed the proteinase inhibitor containing diet. This indicates that changes in food quality at the plant trophic level cascaded to the predator trophic level. In spite of the many laboratory studies we still lack knowledge of selection of food in the field for this insects. A PCR-based method to detect DNA of ingested food species has been developed. Diversity of fungi found in the gut of collembolans was low compared to that of whole animals. This might be due to the dissection method where the head with mouthparts and the uppermost part of the gut was discharged, which presumably contains most undigested fungal material. This method will make it possible to study food selective behaviour of cryptic soil organisms in the future.