AbstractBACKGROUNDThe corn leafhopper, Dalbulus maidis (Hemiptera: Cicadellidae), spreads maize stunt pathogens and requires timely and effective crop protection. We determined the interaction between maize phenology and the vector feeding/infection period by stunt pathogens with the residual efficacy of neonicotinoid insecticidal seed treatments. Greenhouse‐ and field‐grown maize plants, seed‐treated with clothianidin or imidacloprid insecticides, were infested during seven growth stages with corn leafhoppers reared under controlled conditions on maize plants displaying infection symptoms by both spiroplasma (corn stunt spiroplasma, Spiroplasma kunkelii) and phytoplasma (maize bushy phytoplasma) pathogens.RESULTSIn the greenhouse and field settings, seed treatment reduced the stunt disease symptoms and corn yield loss during the VE–V4 maize growth stages and caused no phytotoxicity. The neonicotinoid seed treatment reduced 20–60% of the yield losses from the corn stunt disease until the V4 growth stage. Infestation by infective corn leafhoppers in the V12 maize growth stage caused a 25–30% yield loss irrespective of seed treatment, yet no stunt disease symptom was evident. Nonetheless, corn yield losses and visual stunt symptoms as rated by a nine‐category ordinal scale were strongly correlated (r = 0.79, P < 0.01).CONCLUSIONThese results reinforce that maize plants are more susceptible to leafhopper stunt disease during the VE–V4 growth stages (emergence to the fourth‐leaf stage). Seed treatment helps reduce the damage in the early growth stages (VE–V2), although supplemental control measures depending on leafhopper population density may be needed from VE–V12 to protect yield losses from the maize stunt condition. © 2021 Society of Chemical Industry.