These pests can form colonies of thousands and attack a wide range of plants – they can cover a lawn in fine silk netting and can swarm in numbers severe enough to kill a tree. Would you be able to ID a spider mite?
Common name: Spider mites
Scientific name(s): Tetranychus and Oligonychus species.
Common species: Notable pest species include the two-spotted or red spider mite Tetranychus urticae, grass-webbing mites Oligonychus sp. and the European red mite Panonychus ulmi.
Description: Unlike many mite species, spider mites are visible to the naked eye, although only just – most common species are below 0.5 mm in size. Depending on species and age, they may be pale green, brown, red, straw-coloured, or a number of other possibilities.
Larvae have six legs, but older nymphs and adults have eight, and in ideal conditions such as hot dry weather they can grow from egg to adult in five days. Some spider mites only use silk when laying their eggs, but many spin extensive sheets of very fine silk on and underneath the leaves of their host plants. These silk tents may protect tens of thousands of mites.
Geographic distribution: Some pest species may be found worldwide, as the mites can be blown very long distances on strands of their own silk. Others are native to Australia, but not in every state or territory.
Habitat: On and around the plants they are feeding on, usually on the underside of leaves. A suspected mite infestation can be confirmed by breaking off a leaf, and shaking it into a brightly lit white container. A hand lens is recommended to identify if any mites are present.
Pest status: Spider mites are serious pests of many crops, ornamental plants and fruit trees, with initial damage revealed as leaves with bronzed or yellow spots, eventually turning entirely yellow. Leaves may grow deformed or be lost entirely, but by the time the host plant is that ill, the webbing constructed by the mites should be obvious.
Colonies of the grass-webbing mites form increasingly large, roughly circular, silk-covered yellow spots on the lawn.
Spider mites prefer hot, dry weather and and their rapid reproduction rates can mean large spider mite infestations can develop by the end of a hot, dry summer. Numbers die off in the cooler, winter months.
Treatment: Plants may be more vulnerable to infestation if drought-stressed, heat-stressed, or insufficiently fertilised. So keeping plants healthy and well-watered is the best preventative measure. Predatory mites that feed on spider mites are naturally occurring and other natural predators may also limit spider mite numbers. Neem oil, other insecticidal oils and insecticidal soaps may be effective, as can misting the underside of the leaves of infested plants. A number of acaricides are available on the market, but many spider mites are showing high rates of resistance to pyrethroids and carbaryl and the use of these pesticides will kill off natural predators.
Daniel Heald, technician and entomologist