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

Abstract

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.

Buttercup Squash Provides a Marketable Alternative to Blue Hubbard as a Trap Crop for Control of Striped Cucumber Beetles (Coleoptera: Chrysomelidae)

“…The preference patterns we found are consistent with the hypothesis that cucurbitacin drives preference; Blue Hubbard and buttercup squash are relatively high in cucurbitacin B, which is highly attractive to beetles (Chambliss and Jones 1966), while butternut squash is relatively low in cucurbitacins (Andersen and Metcalf 1987). It should be noted, however, that in one study Þeld-collected beetles consumed more tissue from cucumbers lacking cucurbitacins than from an isogenic line with high cucurbitacin levels in no-choice tests (Smyth et al 2002), despite a general preference of colony-reared beetles for the high cucurbitacin line in choice trials. This suggests that low cucurbitacin levels may confer some resistance to beetle feeding only when crops are grown in proximity to a high cucurbitacin crop, as is the case with PTC systems.…”

“…Variation in beetle attractiveness between cucurbit species has been well established in the literature (Andersen and Metcalf 1987, Brust and Rane 1995, Pair 1997, McGrath 2000, Smyth et al 2002, Boucher and Durgy 2004, and is generally held to be associated with different concentrations and ratios of the bitter cucurbitacin compounds within the plant (Chambliss andJones 1966, Metcalf et al 1980). The preference patterns we found are consistent with the hypothesis that cucurbitacin drives preference; Blue Hubbard and buttercup squash are relatively high in cucurbitacin B, which is highly attractive to beetles (Chambliss and Jones 1966), while butternut squash is relatively low in cucurbitacins (Andersen and Metcalf 1987).…”

Plant chemistry and insect sequestration

“…For some insects no sequestration costs could be proven (Isman 1977;Duffey et al 1978;Kelley et al 2002). In a few species it could be shown that insect fitness parameters (like size, weight, growth rate) were negatively correlated with concentration of plant metabolites (Brower and Glazier 1975;Cohen 1985;Camara 1997a;Smyth et al 2002). In contrast, other species even seem to gain weight through sequestration, for example, by cleavage of glucose from phenolglucosides, which is subsequently used for nutrition .…”

“…The uptake of unusually high amounts of CUCs (0.05 mg/beetle) in other Diabrotica species also has negative effects on the longevity of the beetles, but usually only short term ingestion occurs in nature . Even for a specialist, there seems to be a trade-off between sequestering sufficient amounts of CUCs to be defended against enemies (see below) and detoxification costs (Smyth et al 2002).…”

Allelochemicals in Plant–Insect Interactions

“…93 It is well known that cucurbitacins act as feeding stimulants for cucurbitaceous-feeding beetles (Figure 9). 97 The beetles ingest cucurbitacins until they have sequestered enough to make them sufficiently bitter to repel enemies, but sequestration of additional cucurbitacins entails a fitness cost. 94 Diabrotica undecimpunctata howardi, whose adults feed on leaves of many crops such as cucumber, soybean, cotton, and bean, is stimulated to feed by cucurbitacin E-glucoside.…”