Mortality of <i>Musca domestica</i> (Diptera: Muscidae) in response to exposure to a fungal entomopathogen in combination with eicosanoid biosynthesis inhibitors

The house fly, Musca domestica L., is a vector of pathogens. When sanitation measures fail to control fly populations, pesticides are often used. Fungal biopesticides have been suggested as another tool in fly control. Our study investigated whether inhibitors of eicosanoid biosynthesis (EBI), and presumably of immunity, combined with the fungal biopesticide BotaniGard ES is a feasible option for a new biopesticide formulation. The EBIs tested were dexamethasone, naproxen, and esculetin. An insecticide-susceptible and an insecticide-resistant house fly strain were tested. Compared to a control sugar cube with no EBI and no BotaniGard ES, both strains spent a shorter duration at sugar cubes with fungus alone and with fungus combined with naproxen; flies from the susceptible strain spent a shorter duration at cubes with fungus combined with dexamethasone; and flies from the resistant strain spent a shorter duration at cubes with fungus combined with esculetin. Both strains, whether exposed to fungus, fungus with dexamethasone, or fungus with naproxen, had higher mortality risk compared to a control. With or without injection of an EBI, immersion in a fungal solution increased mortality. However, with CAR21 flies, mortality was greater with the naproxen injection than with dexamethasone- or esculetin injection with the fungal immersion. Due to no greater mortality for both strains for all EBI and BotaniGard ES combinations compared to BotaniGard ES alone, it is not recommended that dexamethasone, esculetin, or naproxen be used in conjunction with BotaniGard ES for house fly control.