Larval deltamethrin exposure causes severe cross-stage carryover toxicity and male reproductive failure in Osmia cornifrons

Solitary bee larvae rely entirely on maternally provisioned pollen, making them especially vulnerable to dietary contamination. Yet, little is known about how pesticide exposure during development influences the adult performance, health and sensitivity to subsequent pesticide exposure. Here, we investigated the carryover effects of larval exposure to deltamethrin on adult performance and reproductive traits in the orchard bee Osmia cornifrons. In a two-phase experiment, larvae were reared on pollen containing field-relevant concentrations of deltamethrin (0, 0.001, 0.1, and 10 ppm), and emerging adults were subsequently exposed to a near-LC₅₀ concentration in sucrose or maintained as controls. We quantified larval survival, emergence from cocoons, adult survival, male body mass, sex-ratio, and sperm abundance and viability. Larval deltamethrin exposure significantly reduced survival to adulthood in a dose-dependent manner, with over 50% mortality in the higher treatment (10 ppm). Female emergence declined with increasing larval deltamethrin exposure, resulting in increasingly male-biased sex ratios (and lower-weight males). Even mild developmental exposure to deltamethrin (0.001 or 0.1 ppm) caused persistent impairments in male reproductive quality, with significant reductions in sperm abundance and viability. Sperm abundance was further exacerbated by subsequent adult exposure compared with controls, indicating cross-stage carryover effects rather than tolerance. Our findings demonstrate that developmental exposure to deltamethrin can generate long-lasting physiological and reproductive deficits in O. cornifrons, compromising individual fitness and potentially leading to population declines. This study provides a realistic assessment of pesticide risks to solitary bees and highlights the need to incorporate cross-stage carryover effects into pollinator risk assessment frameworks.