The two-spotted mite or spider mite Tetranychus urticae
is a tiny creature (about the size of a full stop)
that damages plants by feeding on the chlorophyll in leaves. The mite is yellow-green with two large, dark spots
on its back. In autumn it turns reddish-orange, hence its other common name, red spider.
The first sign that your plants are under attack is usually a white spotting on the surface of the leaves. In
heavy infestations, the mites remove nearly all the chlorophyll and the leaves turn yellow and drop off. Similar
symptoms - but with the addition of black marks on the underside of leaves - are more likely to be thrips.
Two-spotted mite feeds on a wide range of plants, particularly cucumbers, tomatoes, capsicums, beans, roses,
orchids, strawberries, berry fruits, apples and peaches.
Suggested Organic Strategies:
- Common organic practices such as making compost, mulching the soil and avoiding chemical insecticides
help to encourage predatory mites, a major predator of two-spotted mite. A healthy garden will have a
resident population of predatory mites to keep pest mites under control.
- Try a high pressure hosing in the early morning, 3 days in a row.
- An unlikely pest control device is a hand-held vacuum cleaner! After vacuuming, tip the contents
immediately into a plastic bag and place in the freezer for a few hours.
- Keep your plants healthy by feeding, mulching and watering.
Organic Strategies for Two-Spotted or Spider Mite Control
© Frances Michaels
With their common use of compost and mulch, organic gardeners have a head start in the control of this
Mites secrete a very fine, silk-like webbing which protects them from enemies and chemical sprays. When
large populations have been present for a few weeks, webbing may cover the whole plant. Females leave plants
via threads of webbing blown on the wind, or they may drop off a plant and crawl to a new plant. Female
two-spotted mites over-winter in the soil under leaf litter, tree bark, or cracks in greenhouse walls,
emerging in late spring to lay eggs. At a temperature of 30°C their life cycle can be completed in 8 days,
giving 12 to 20 generations over summer. Mite numbers build fastest in hot, dry weather with low humidity.
Organic gardeners have an advantage regarding spider mite control. Common organic practices such as making
compost, mulching the soil and avoiding chemical insecticides help to encourage predatory mites, a major
killer of two-spotted mite. A healthy garden will have a resident population of predatory mites to keep pest
mites under control. Predatory mites are abundant in the top layers of soil, in humus and animal manures.
Two types pf predatory mites are available commercially from
- Phytoseiulus persimilis
and Typhlodromus occidentalis
. They are best introduced when pest numbers are low.
Phytoseiulus persimilis are available from: Bioworks (NSW); Biomites (Qld); Manchil IPM Services (WA);
and as part of the OCP Backyard Buddies range.
Typhlodromus occidentalis are available from Biological Services (SA).
Gardeners who commonly spray insecticides to control caterpillars and other pests risk killing off the natural
enemies present, as predatory mites are very susceptible to pesticides. Research has also shown that certain
insecticides stimulate mite reproduction, so the gardener may have controlled one pest only to find that the
population of a much more serious pest, the two-spotted mite, has exploded. In this case, the organically
acceptable caterpillar control, Bt (
), would have been a much better
choice as it would have not affected the beneficial predator at all.
Physical and Cultural Controls
There are some easy things you can do that will have a big impact on spider mite numbers, keeping in mind
that spider mites prefer high temperatures, low humidity and dusty conditions.
Least Toxic Chemical Controls
- Try a high pressure hosing in the early morning - three days in a row.
- Hosing plants mid-season to remove dust on the leaves helps too.
- An unlikely pest control device is a hand held vacuum cleaner! After vacuuming, tip the contents
immediately into a plastic bag and place in the freezer for a few hours. You can control whiteflies doing
this as well.
- Pruning affected plants and removing infested leaves will reduce pest numbers.
- Try to control weeds that harbour the pest such as plantains, black nightshade and marshmallow.
- Keep your plants healthy by feeding, mulching and watering.
Two-spotted mites are not insects and are highly resistant to many chemical insecticides. There are several
organic controls that work well, as their mode of control differs from that of a chemical insecticide. Mites
are mostly found under the leaves, so spray and coat this area particularly well. Due to the very fast
development time of this pest, especially during warm weather when eggs are laid continuously, it is important
to carry out repeat spraying if recommended.
- Potassium soap sprays such as
Natrasoap or Nature's Way Vegie
and Herb Spray are a good choice of control for the home gardener. Soap sprays work by blocking the mite's
breathing pores and dissolving its outer covering, resulting in dehydration. They are considered very safe
for the environment.
- A horticultural oil such as Eco-Oil
can be very effective in smothering the mites and mite eggs. Avoid using it at temperatures higher than 35°C
as it may cause leaf burn. A follow-up spray 3-5 days after the initial spray application is usually needed.
Eco-Oil is certified organic, made
from botanical oils and has an enhanced formulation which helps to attract beneficial insects to the garden.
- Neem is a botanical insecticide made from extracts of the neem tree.
Eco-Neem is a registered organic spray that
controls a wide range of insects and mites including spider mite. It works in multiple ways with the two main
actions being suppression of insect appetite (they starve to death) and restricting growth (unable to moult
successfully). It is approved in Australia for use on ornamental plants only.
- Wettable sulphur or dusting sulphur can only be used in cool weather, or it will cause leaf burn. You
should avoid using an oil product for at least a month after applying sulphur because in combination it can
cause phytotoxic problems for plants.
Research has shown that a 2% rosemary oil solution can kill mites, without harming the beneficial predatory
mite. Make sure to always dilute the oil prior to spraying it. To make 1 litre of spray, use 980 ml of lukewarm
water with 10-20 ml (2 - 4 teaspoons) of rosemary oil. Adding a teaspoon of castile soap (soap made of olive
oil) to the solution will help it stick. Always test a small area of leaves before spraying the whole plant
and never spray in very hot weather. A range of other plant extracts, including garlic extract, clove oil,
mint oil, eucalyptus oil and cinnamon oil have also been found to be effective.
Treating House Plants
Pest problems on indoor plants often increase rapidly due to the absence of 'good bugs' inside our homes
(conditions are simply unsuitable for them to survive). Always treat all susceptible house plants at the
same time. Trim, bag and remove heavily infested leaves and discard severely infested plants. Take the plants
outside and spray with an organically acceptable oil or soap spray. Re-apply the treatment at one or two week
intervals as long as the pest persists. If plants are able to be easily lifted, a regular rinsing under the
shower will help prevent mite problems.