Curcurbita pepo subspecies delineates striped cucumber beetle (Acalymma vittatum) preference

Simulation of early season herbivory via mechanical damage affects flower production in pumpkin (Cucurbita pepo ssp. pepo)

BACKGROUND

Damage from insect herbivores can elicit a wide range of plant responses, including reduced or compensatory growth, altered volatile profiles, or increased production of defence compounds. Specifically, herbivory can alter floral development as plants reallocate resources towards defence and regrowth functions. For pollinator-dependent species, floral quantity and quality are critical for attracting floral visitors; thus, herbivore-induced developmental effects that alter either floral abundance or attractiveness may have critical implications for plant reproductive success. Based on past work on resource trade-offs, we hypothesize that herbivore damage-induced effects are stronger in structural floral traits that require significant resource investment (e.g. flower quantity), as plants reallocate resources towards defence and regrowth, and weaker in secondary floral traits that require less structural investment (e.g. nectar rewards).

METHODS

In this study, we simulated early-season herbivore mechanical damage in the domesticated jack-o-lantern pumpkin Cucurbita pepo ssp. pepo and measured a diverse suite of floral traits over a 60-d greenhouse experiment.

KEY RESULTS

We found that mechanical damage delayed the onset of male anthesis and reduced the total quantity of flowers produced. Additionally, permutational multivariate analysis of variance (PERMANOVA) indicated that mechanical damage significantly impacts overall floral volatile profile, though not output of sesquiterpenoids, a class of compounds known to recruit specialized cucumber beetle herbivores and squash bee pollinators.

CONCLUSIONS

We show that C. pepo spp. pepo reduces investment in male flower production following mechanical damage, and that floral volatiles do exhibit shifts in production, indicative of damage-induced trait plasticity. Such reductions in male flower production could reduce the relative attractiveness of damaged plants to foraging pollinators in this globally relevant cultivated species.

Failure of a mass trapping method against the striped cucumber beetle (Coleoptera: Chrysomelidae) in organic cucurbit fields

The striped cucumber beetle (SCB) Acalymma vittatum (F.) is one of the most important pests in North American cucurbit crops. While conventional chemical control methods are usually effective in controlling SCB populations, few alternative control methods are available for organic cucurbit crops. The goal of the present study was to evaluate an optimized mass trapping system using yellow traps baited with a floral-based semiochemical. More specifically, the objectives were to determine if the trapping method could (i) significantly reduce SCB populations and (ii) maintain these populations below the economic threshold throughout the growth season within organic cucurbit crops. The method did not reduce nor maintain the SCB populations below the economic threshold of one SCB per plant. Possible hypotheses explaining the diverging results are discussed.

Vittatalactone is the Male-Produced Aggregation Pheromone of the Western Striped Cucumber Beetle, Acalymma trivittatum

We found that vittatalactone, specifically (3R,4R)-3-methyl-4-[(1S,3S,5S)-1,3,5,7-tetramethyloctyl]oxetan-2-one, is the male-produced aggregation pheromone of the western striped cucumber beetle, Acalymma trivittatum (Mannerheim), as was previously shown for the striped cucumber beetle, Acalymma vittatum (F.) (Coleoptera: Chrysomelidae). A synthetic mixture containing 9% of the authentic natural pheromone, is attractive to both sexes of both species in the field, as demonstrated by trapping using baited and unbaited sticky panels in California and earlier in Maryland. Females of both species do not produce detectable vittatalactone. This finding expands the usefulness of the synthetic vittatalactone mixture for pest management throughout the range of both A. vittatum and A. trivittatum. Development of vittatalactone time-release formulations combined with cucurbitacin feeding stimulants offer the potential for selective and environmentally-friendly cucurbit pest management tactics.

Herbarium specimens reveal herbivory patterns across the genus Cucurbita

PREMISE

Quantifying how closely related plant species differ in susceptibility to insect herbivory is important for understanding the variation in evolutionary pressures on plant functional traits. However, empirically measuring in situ variation in herbivory spanning the geographic range of a plant-insect complex is logistically difficult. Recently, new methods have been developed using herbarium specimens to investigate patterns in plant-insect symbioses across large geographic scales. Such investigations provide insights into how accelerated anthropogenic changes may impact plant-insect interactions that are of ecological or agricultural importance.

METHODS

Here, we analyze 274 pressed herbarium samples to investigate variation in herbivory damage in 13 different species of the economically important plant genus Cucurbita (Cucurbitaceae). This collection is composed of specimens of wild, undomesticated Cucurbita that were collected from across their native range, and Cucurbita cultivars collected from both within their native range and from locations where they have been introduced for agriculture in temperate North America.

RESULTS

Herbivory is common on individuals of all Cucurbita species collected throughout their geographic ranges. However, estimates of herbivory varied considerably among individuals, with mesophytic species accruing more insect damage than xerophytic species, and wild specimens having more herbivory than specimens collected from human-managed habitats.

CONCLUSIONS

Our study suggests that long-term evolutionary changes in habitat from xeric to mesic climates and wild to human-managed habitats may mediate the levels of herbivory pressure from coevolved herbivores. Future investigations into the potential factors that contribute to herbivory may inform the management of domesticated crop plants and their insect herbivores.

Divergence of defensive cucurbitacins in independent Cucurbita pepo domestication events leads to differences in specialist herbivore preference

Crop domestication and improvement often concurrently affect plant resistance to pests and production of secondary metabolites, creating challenges for isolating the ecological implications of selection for specific metabolites. Cucurbitacins are bitter triterpenoids with extreme phenotypic differences between Cucurbitaceae lineages, yet we lack integrated models of herbivore preference, cucurbitacin accumulation, and underlying genetic mechanisms. In Cucurbita pepo, we dissected the effect of cotyledon cucurbitacins on preference of a specialist insect pest (Acalymma vittatum) for multiple tissues, assessed genetic loci underlying cucurbitacin accumulation in diverse germplasm and a biparental F₂ population (from a cross between two independent domesticates), and characterized quantitative associations between gene expression and metabolites during seedling development. Acalymma vittatum affinity for cotyledons is mediated by cucurbitacins, but other traits contribute to whole-plant resistance. Cotyledon cucurbitacin accumulation was associated with population structure, and our genetic mapping identified a single locus, Bi-4, containing genes relevant to transport and regulation - not biosynthesis - that diverged between lineages. These candidate genes were expressed during seedling development, most prominently a putative secondary metabolite transporter. Taken together, these findings support the testable hypothesis that breeding for plant resistance to insects involves targeting genes for regulation and transport of defensive metabolites, in addition to core biosynthesis genes.

Field Attraction of Striped Cucumber Beetles to a Synthetic Vittatalactone Mixture

The striped cucumber beetle, Acalymma vittatum (F.) (Coleoptera: Chrysomelidae), is a key pest of cucurbits in eastern North America, rapidly colonizing young plantings and vectoring bacterial wilt of cucurbits. Its aggregation pheromone has been identified and synthesized stereospecifically, but has not been field tested to date. Here, we present field bioassays of this pheromone, using mixed vittatalactone made with a novel and cost-efficient semispecific synthesis. This mixture of eight stereoisomers of (2R,3R)-vittatalactone proved highly attractive to both sexes of striped cucumber beetle, using two different trap types and a pilot attract-and-kill combination with watermelon containing the diabroticine feeding stimulant cucurbitacin-E-glycoside, under field conditions in cucurbit vegetable plantings. Availability of mixed vittatalactones could enable highly effective and specific management of striped cucumber beetle.