Effects of age, sex, and dietary history on response to cucurbitacin in Acalymma vittatum

The chrysomelid Acalymma vittatum is stenophagous, subsisting almost entirely on plants in the Cucurbitaceae, which generally contain cucurbitacins. Cucurbitacins are extremely bitter tetracyclic triterpenoids that are toxic to most organisms. As do other diabroticite beetles, A. vittatum sequester cucurbitacins, which have been shown to act as phagostimulants and arrestants. Our results reveal, however, that for A. vittatum the response to cucurbitacin diminishes with continued sequestration. Colony-reared A. vittatum were fed only roots (as larvae) and foliage of either `Marketmore 76' (which contains a normal amount of cucurbitacin, `bitter') or `Marketmore 80' (a near isogenic line that contains no cucurbitacin, `non-bitter') cucumber. Over 1200 individual beetles from the day of adult emergence to 15 days following emergence were placed in choice and no-choice arenas containing potted cotyledons of the two cucumber varieties for 24 h. In choice tests, overall preference for the bitter cucumber cultivar was maintained, but degree of preference changed with age and became significantly less for beetles reared on bitter diets. Furthermore, in no-choice tests, age, sex, dietary history, and interactions among these variables all significantly affected the feeding response to cucurbitacin. For A. vittatum reared without cucurbitacin, total consumption of the bitter cultivar increased over time. For beetles reared with cucurbitacin, total foliage consumption of the bitter cultivar declined, within nine days, to equal that of the non-bitter cultivar. Feral A. vittatum, unexpectedly, consumed more of the non-bitter than the bitter cultivar in no-choice tests. Ecological and applied implications of this variation in response to cucurbitacin are discussed.