Acquisition, transfer and metabolism of [¹⁴C] imidacloprid among workers of the subterranean termite, Reticulitermes flavipes (Isoptera: Rhinotermitidae)

Abstract:
Imidacloprid, the active ingredient in Premise®, is a slow-acting, non-repellent termiticide used to control termites in residential and commercial applications. Workers acquire imidacloprid from contact with treated soil, and then transfer the AI among individuals throughout the colony. However, the mechanisms of acquisition, distribution, and metabolism of this AI in individuals, as well as the transfer of this AI among termites within the colony, are not clear. We found that workers of the subterranean termite Reticulitermes flavipes continuously exposed to 50 ppm [14C] imidacloprid-treated sand (w/w) for 48 hr, had 5 to 10-fold greater amounts of imidacloprid (IMI) internally (about 1,750 pg) than on the cuticle (about 250 pg). Untreated workers exposed to treated workers in various ratios groomed the intoxicated workers and ingested toxic amounts of AI. Soldiers were not observed to groom nestmates and did not acquire significant or toxic amounts of imidacloprid from workers topically applied with AI. Topically applied imidacloprid appears stable on the cuticle surface under the conditions of the experiment, but is metabolized internally to possibly 11 compounds that are more polar than the parent molecule. Six of these have been identified (from least to more polar): olefin-IMI (major), 5-OH IMI, 4,5-di-OH IMI, desnitro olefin IMI, desnitro IMI, and 5-chloronicotinic acid (tentative). In termites exposed to metabolite-treated sand, we found that both the olefin-IMI and 5-OH IMI exhibited significant activity in workers, but were about 10-fold less toxic than imidacloprid. The other metabolites were non-active at the highest dose tested (100 ppm) by this method of exposure. Termites excreted these metabolites onto the substrate. Those termites that were removed from the AI-treated sand and allowed to recover eventually had undetectable levels of the olefin-IMI, 5-OH IMI and 4, 5-di-OH IMI in their bodies, whereas only the desnitro IMI compounds and 5-CNA remained.