Spectral sensitivity of click beetles (Coleoptera: Elateridae) and their responses to light stimuli in laboratory and field experiments

With increasingly fewer insecticides registered to control the larvae of pest click beetles (Coleoptera: Elateridae), integrative beetle management, including pheromone- and light-based trapping of adult beetles, must be explored as an alternative strategy. Here, we analyzed the spectral sensitivity and color preference of 9 elaterids across 6 genera in electrophysiological recordings and in behavioral bioassays. In electroretinogram recordings (ERGs), dark-adapted beetles were exposed to narrow wavebands of light in 10-nm increments from 330 to 650 nm. All beetles proved most sensitive to green (515-538 nm) and ultraviolet (UV) light (~360 nm). In 4-choice bioassay arenas with 3 light emitting diodes (LEDs; green [525 nm], blue [470 nm], red [655 nm]) and a dark control as test stimuli, beetles discriminated between test stimuli, being preferentially attracted to green and blue LEDs. In field experiments, Vernon pitfall traps fitted with a green, blue or white LED captured significantly more male and female Agriotes lineatus and A. obscurus than dark control traps. When traps were baited with green or blue LEDs at light intensities that differed by 10-fold, the traps baited with higher light intensity lures captured numerically more beetles but trap catch data in accordance with light intensity did not differ statistically. Light-based trapping may be a viable tool for monitoring elaterid species known not to have pheromones.

Wireworm (Coleoptera: Elateridae) intoxication symptoms indicate the postharvest presence of broflanilide residues in soil collected from potato fields

Wireworms are primary pests of potatoes in Canada with relatively few effective control options. Recently, a new meta-diamide insecticide, broflanilide, was registered in Canada and the United States as an in-furrow spray applied at planting that provides protection of tubers from feeding damage and dramatically reduces wireworm populations. As part of our routine screening of wireworm response to novel insecticides, we exposed wireworms (predominantly Agriotes obscurus; N=2320) to field soil collected from plots to which either bifenthrin (Capture 2EC) or broflanilide (Cimegra) had been applied at registered rates 124-145 and 314-335 days previously in 2018, 2019, and 2021. Wireworm behavioral responses were assessed using a previously developed numerical scale, and indicated residues were present in sufficient quantity in all broflanilide and most bifenthrin-treated plots to induce morbidity. Transfer of affected wireworms to untreated soil indicated morbidity was generally reversible after exposure to bifenthrin, but not after exposure to broflanilide. There was an inverse relation between wireworm size and the degree of morbidity induced by exposure to broflanilide, but not bifenthrin. Analyses of soil residues indicated readily quantifiable levels of broflanilide still present in undisturbed (not harvested) field soil 314-335 days after application. Insecticide residues in soil samples from disturbed (harvested) sections of the potato plots were lower, as was the degree of morbidity of wireworms exposed to this soil. The use of wireworms as bioindicators of insecticide residues, and the implications of insecticide persistence for wireworm management are discussed.