Indole synergizes the striped cucumber beetle (Coleoptera: Chrysomelidae) response to aggregation pheromone without attracting pollinators

Deploying semiochemicals in pest management is challenging for pollinator-dependent crops because floral-based attractants can divert bees and other pollinators. Cucurbit crops, for example, are highly dependent on pollination and their primary pests, Acalymma vittatum (F.) (striped cucumber beetle, StCB) and Acalymma trivitatum (Mannerheim) (western striped cucumber beetle, WStCB), are strongly attracted to the scent of cucurbit blossoms. To identify a more pollinator-friendly semiochemical lure for StCBs, we conducted a 2-year field experiment across 4 US states (IN, MD, VA, CA) that evaluated pest responses to: (i) a cucurbit floral mimic (TIC), (ii) indole, a single component of TIC; and (iii) vittatalactone, an aggregation pheromone of StCB. These compounds were tested both individually and in combination to test for non-additivity. We also assessed responses from 2 other co-occurring pests—Diabrotica undecimpunctata howardi Barber (spotted cucumber beetle, SpCB), Diabrotica virgifera virgifera LeConte (western cornrootworm, WCR)—and pollinators (Apis mellifera L., honey bees; Eucera and Lasioglossum). We found that vittatalactone, but not TIC, was a strong and consistent attractant of Acalymma species (StCB, WStCB), whereas the reverse pattern was observed for Diabrotica species (SpCB, WCR). As expected, TIC—but not indole or vittatalactone—attracted pollinators. Pest trap catch was almost always highest when vittatalactone was combined with either indole or TIC, and this increase was synergistic in 33% of cases. These data suggest that combining indole with vittatalactone creates a potent pest attractant that does not attract bees, making it a pollinator-friendly lure.