Biodegradable alginate hydrogel bait delivery system effectively controls high-density populations of Argentine ant in commercial citrus

The Argentine ant, Linepithema humile (Mayr), poses a significant economic threat to citrus production in southern California. Broad-spectrum insecticide sprays provide poor ant control and present a hazard to human and environmental health. Liquid sucrose bait infused with a low concentration of insecticide is an effective alternative treatment but current approaches require considerable economic investment in plastic dispensers and continual maintenance. To produce a baiting product for control of L. humile suitable for large-scale application, a biodegradable, broadcastable alginate hydrogel for delivery of aqueous low-dose thiamethoxam sucrose bait was developed and evaluated in replicated field trials in commercial citrus groves. Ant activity was significantly reduced in hydrogel-treated trees. Peak L. humile suppression was achieved 48 h following the final hydrogel disbursement, with an estimated 91% reduction in activity from baseline estimates and a 17-fold lower activity in treated trees in comparison with untreated trees. Significant residual activity of the hydrogel treatments was recorded, with a nearly 70% reduction from pre-treatment levels persisting at least 3 weeks after the last application. We conclude that alginate hydrogels can provide excellent control of L. humile while deploying 99.99% less insecticide into orchards than commercial barrier spray treatments.