Spider mites
Almost invisible without a hand lens. Pale yellow to red specks on the underside of the lance-shaped leaves and clustered around the base of the flower spike. Hot dry summer weather, which snapdragons already hate, triggers a population boom in days.
Tiny pale yellow dots across the upper leaf surface, then bronze patches that spread along the central vein. Fine webbing strung between the opposite leaves and around the flower spike in heavy infestations. Heat-stressed snapdragons defoliate fast under mite pressure and stop producing new buds.
Shower the foliage every 3 days for 2 weeks
Aim a hose nozzle 12 inches from the plant and spray the underside of every leaf and around the flower spike for 30 seconds. Mites can't reattach quickly when knocked off, and the cooling rinse helps the heat-stressed plant. Snapdragons tolerate a hard rinse well. Repeat every 3 days for 2 weeks.
Insecticidal soap at dusk, every 5 days for 3 rounds
Use ready-to-use insecticidal soap (Safer Brand, ~$10) at dusk to avoid leaf burn in the heat.
Spray the underside of every lance-shaped leaf and the base of each flower spike where mites congregate.
Repeat every 5 days for 3 rounds. That covers the full egg-to-adult cycle in summer heat.
Move the plant to afternoon shade and mulch the root zone
Snapdragons are cool-season annuals and decline in summer heat. A heat-stressed plant cannot fight off mites. Move container snapdragons to morning sun and afternoon shade. Mulch in-ground plantings 2 inches deep to keep the root zone cool. The plant rebounds and the mite population crashes when the host stops being stressed.
