Challenging the Preference-Performance Hypothesis in an above-belowground insect.

The relationship between female oviposition preference and offspring performance has been a question of special interest in the study of host plant selection by phytophagous insects. The Preference-Performance Hypothesis (PPH) is one of the main hypotheses proposed to explain this relationship, stating that females should preferentially lay eggs on plants providing the best larval development. The PPH has been extensively tested on aboveground insects but its application to species with belowground larvae is still mostly unknown. In this study, the PPH was quantitatively tested in an above-belowground insect, the cabbage root fly Delia radicum. Female oviposition preference and larval performance were estimated on three brassicaceous species (Brassica oleracea, Brassica rapa, and Sinapis alba) as well as between four cultivars of B. rapa and four cultivars of S. alba. Larval performance was estimated through their survival and through three life-history traits (LHT) of emerging adults. The PPH was supported at the intraspecific scale but only in B. rapa and for some, but not all, of the life-history traits. No support for the PPH was found in S. alba as well as at the interspecific scale. This study pleads for the integration of insects with both above- and belowground life stages in the preference-performance debate. Moreover, it raises the importance of measuring several variables to estimate larval performance and to test the PPH quantitatively, both at the plant intraspecific and interspecific scales, before drawing general conclusions.