How to control mites

Arachnida / Acari

Tetranychus urticae

Panonychus ulmi

What are Mites?

Mites are actually arachnids, distantly related to spiders and scorpions (which explains why mites have eight legs). Mites are tiny creatures; some species can only be seen with a powerful hand lens or a microscope. Usually they're diagnosed by the damage they cause, because they aren't easy to see. Mites have piercing and sucking mouthparts which they spear into plant cells and suck out the fluid contents; this can have a drastic effect on plant health.

Spider Mites are a distinct family of mites, who earned their name by their habit of leaving fine webs on host plants. Spider mites are most active in hot, dry weather, which allows them to complete their life cycle, from eggs to adults, inside a week. For this reason, mite populations can expand very rapidly when conditions suit them.

In NZ, the most challenging spider mites for gardeners are the Two Spotted Mite (Tetranychus urticae, which attacks a wide range of vegetables, fruit trees and berries) and the European Red Mite (Panonychus ulmi, a serious pest of deciduous fruit trees and roses).

Both these species can cause a great deal of damage to plants in a very short time. They can both seriously reduce yields of infested fruit and vegetables. It's often possible to tell their symptoms apart, as follows:

Two spotted mites may be just visible as pale yellow-green mites with a couple of dark spots on the back, about half a millimetre long. In autumn, their colour changes to orange. They'll usually be clustered on the underside of leaves, plus they often spin fine webbing on leaves. Stippling, mottling or bronzing of leaves is common, progressing to leaves browning off and dying.
European red mites are slightly smaller and pinkish-red in colour, but they don't leave visible webs - their damage often shows up first as tiny yellow specks on leaves, that spread out into pale bleached-looking or bronzed areas over the whole leaf.

Mites are often found on plants that are growing in dry situations (indoor plants and glasshouse-grown plants are especially vulnerable). Mites overwinter in protected places to start next years population. Two spotted mite adults conceal themselves in soil to get through winter, but European red mites overwinter in egg form, in crevices on tree bark. Delaying your mid-winter application of Yates Lime Sulfur until just before bud break is a wise strategy.

Mites can also develop resistance to insecticides quite quickly, so it's important not to over-use sprays; always stick to the dilution rates and wait the required number of days before respraying, as specified on the label. Don't be the person who trains the next generation of mites to be more resilient!

Mites really dislike moist, humid conditions so overhead watering can be useful as a control tactic, especially in a glasshouse.

Mite Symptoms

Yellow mottling or stippling, silvering or bronzing of the leaves.
Leaf edges may brown and die off, followed by entire leaves.
There may be significant distortion of flower and leaf buds.
Fine silky webbing can be seen on heavy infestations (this indicates the culprit is Two Spotted Mite. If there's no visible webbing, it may be European Red Mite).
Severe infestations can cause defoliation of plants or premature fruit drop.