Sex- and maturity-dependent antennal detection of host plant volatiles in the cabbage root fly, Delia radicum.

Adult insect behaviour in response to plant-emitted volatile compounds varies between the sexes and as a function of maturity. These differences in behavioural responses can be due to modulation in the peripheral or central nervous system. In the cabbage root fly, Delia radicum, behavioural effects of certain host plant volatiles on mature female behaviour have been evaluated, and a large number of compounds emitted by brassicaceous host plants have been identified. We recorded here dose-dependent electroantennogram responses to all tested compounds and investigated if the antennal detection of individual volatile compounds emitted by intact and damaged host plants differs between male and female, as well as immature and mature flies. Our results showed dose-dependent responses in mature and immature males and females. Mean response amplitudes varied significantly between sexes for three compounds, and between maturity states for six compounds. For some additional compounds significant differences occurred only for high stimulus doses (interaction between dose and sex and/or dose and maturity status). Multivariate analysis revealed a significant global effect of maturity on electroantennogram response amplitudes and for one experimental session also a significant global effect of the sex. Interestingly, allyl isothiocyanate, a compound stimulating oviposition behaviour, elicited stronger responses in mature than in immature flies, whereas ethylacetophenone, an attractive flower volatile, elicited stronger responses in immature than in mature flies, which correlates with the behavioural role of these compounds. Several host-derived compounds elicited stronger responses in females than in males and, at least at high doses, stronger responses in mature than in immature flies, indicating differential antennal sensitivity to behaviourally active compounds. Six compounds did not cause any significant differences in responses between the different groups of flies. Our results thus confirm peripheral plasticity in plant volatile detection in the cabbage root fly and provide a basis for future behavioural investigations on the function of individual plant compounds.

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.