How to recognise it
Infested leaves initially have a faint speckling on the upper surface due to the mite feeding on the cell sap (smaller than the similar spots caused by thrips), soon turning a dull bronze colour before, in severe cases, withering and dying. This is caused by the mites feeding on the underside of the leaves generally along the veins (which are usually visible with a magnifying glass alongside their eggs, used egg shells and moulted adult skins) and injecting the leaf with poisonous secretions which kill the surrounding leaf mesophyll cells.
A fine silky webbing may be present; these strands act as ropes for the mites to use when moving down the plant.
Adult female mites are 0.5mm long, males slightly smaller, with four pairs of relatively short legs. They are yellowish in summer with darker green markings, turning red from autumn when they stop feeding and hibernate. They can be distinguished by two black spots on their backs.
The female lays around 100 tiny spherical eggs on the underside of leaves, which hatch to reveal six-legged larvae. Within as little as 3 days (or up to 30 days in cooler temperatures) eggs hatch and the resulting six legged larvae feed for a few days before moulting and transforming into the first stage of nymph (protonymph), which resemble the adults. The protonymph also feeds for a few days before transforming into the second nymphal stage (deutomymph) and this in turn transforms into an adult. The lifecycle from egg to adult is shortest in higher temperatures (around 62 days at 10˚C and as little as 6 days at 35˚), so in tropical glasshouses they can spread particularly quickly.
Breeding continues through spring, summer and early autumn. When daylight hours decrease in the autumn the females (now red in colour) stop producing eggs and hibernate within glasshouses and outdoors in cracks in woodwork, straw/leaf litter, under loose bark, in canes, in soil and in compost. They remain dormant until the warmer weather returns in March and April.
Why it’s a problem
In large numbers and left unchecked they can kill leaves and eventually entire plants.
On flowering crops (such as chrysanthemums) the silky strands can make the crop unsellable.
Where you are likely to find it
Most common in glasshouses (it thrives in hot temperatures), but may also be found outdoors in hot and dry weather (between June and September) on plants such as beans, raspberries, strawberries, roses, apple trees and plum trees. Spreads from plant to plant when plants are touching. Wind currents can also allow the mites to ’swing’ from plant to plant on the silky threads.
How to deter it
Ensure good levels of humidity within glasshouses, eg by spraying susceptible plants with a fine mist of water twice a day.
Carefully check under the leaves of new plants for evidence of infestation, using a magnifying lens if necessary.
Maintaining good growing conditions promotes healthy growth which will help plants survive an attack.
How to get rid of it
Can be controlled biologically using the predatory mite Phytoseiulus persimilis which eats all the lifecycle stages of the mite. Alternative biological controls are Feltiella acarisuga (a predatory cecidomyiiid) and Amblyseius californicus.
Spray affected plants with bifenthrin or insecticidal soap (spray apples in late May and repeat if necessary 3 weeks later). Alternatively plant oils/extracts, abamectim, acetamiprid or imidacloprid are alternative chemical controls.
Foliar feeding with urea (also called foliar lattice) can help to control the pest.
Professional horticulturalists can benefit from winter fumigation of glasshouses with chemicals such as formalin or burning sulphur to kill off the hibernating females. A insecticide containing abamectin can also be used.
Is it good for anything?!
Other useful information
This mite is very similar to Tetranychus cinnabarinus and some populations of hybrids between the two species have been found.