Spider mites
Almost invisible without a hand lens. Yellow-green to red specks moving along the underside of the glossy oval leaves and along the twining tendrils that grip the trellis. Hot dry summer weather on a sunny patio drives explosive booms on container Mandevilla, especially on the upper canopy where heat radiates off the trellis.
Pale yellow tiny pale dots that spreads to bronze or rust-colored patches on the upper leaf surface. Fine webbing strung between the trellis bars and across the climbing tendrils in heavy infestations. Mandevilla can drop the lower half of its foliage in 2 to 3 weeks once mite numbers explode, leaving the trellis with bare lower stems.
Hose down the full trellis every 4 days for 2 weeks
Stand the container where you can hose it freely and direct cool water at the underside of every leaf and along the trellis bars for a full minute. Mites can't reattach quickly when knocked off, and water sitting on the climbing structure raises local humidity. Mandevilla tolerates a hard rinse well. Repeat every 4 days for 2 weeks during heat waves.
Sulfur-based miticide on the upper canopy, 2 rounds 7 days apart
Mix wettable sulfur (Bonide Sulfur Plant Fungicide, ~$12) at 2 tablespoons per gallon of water, or use a ready-to-use miticide labeled for ornamentals.
Spray the upper canopy and trellis tips at dusk in temperatures below 85 degrees. Sulfur burns leaves applied in midday summer heat.
Repeat once 7 days later to knock down hatching eggs the first round missed.
Shade the trellis through the worst afternoon hours
Spider mites breed fastest on heat-stressed Mandevilla. Slide a sheer shade cloth over the south or west face of the trellis between 1pm and 5pm during heat waves, or rotate the container so the trellis faces morning sun. A cooler upper canopy resists mite damage and bounces back faster from rinses.
