Organic Strategies for Two-Spotted or Spider Mite Control © Frances Michaels

Mites secrete a very fine, silk-like webbing which protects the mites from enemies and contact with chemical sprays. When large populations have been present for a few weeks webbing may cover the whole plant. Females leave plants on 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, they emerge in late spring and 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.

Biological Controls
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 predator 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 Phytoseiulus persimilis and Typhlodromus occidentalis. They are best introduced when pest numbers are low. More information and commercial producers of 'good bugs' can be found at: http://www.goodbugs.org.au/

Gardeners who spray a pesticide such as carbaryl to control caterpillars will unfortunately kill all the predatory mites present as they are very susceptible to pesticides. So the gardener may have controlled a few caterpillars only to find that the population of a much more serious pest, the two-spotted mite may then explode. In this case, the organically acceptable caterpillar control, Bt ( Dipel), would have been a much better choice as it would have not affected the beneficial predator at all.

Other predators of two-spotted mite include hoverflies, a tiny black ladybird Stethorus spp. also called the 'spider mite destroyer'; green and brown lacewings, damsel bugs, predatory thrips, spiders and tiny parasitic wasps.

Physical and Cultural Controls
There are some easy things you can do that will have a big impact on spider mite numbers. 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. You can control whiteflies doing this as well. Pruning back 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.

Least Toxic Chemical Controls
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. Remember to coat the undersides of the leaves well with spray as this where the mites primarily are. Eco-Oil can be very effective to smother the mites and mite eggs. Avoid using it at temperatures higher than 35°C as it may cause leaf burn. One or two follow-up sprays should be applied at 5 - 7 day intervals to kill new mites.

Insecticidal soap sprays such as Natrasoap are a good choice of control for the home gardener. They should be applied at least 3 times every 7 - 10 days. Wettable sulphur or dusting sulphur can only be used in cool weather as otherwise it will cause leaf burn. You should avoid using an oil product for at least a month after applying sulphur as in combination it can cause phytotoxic problems for plants.


Suggested Products:
Eco-Oil
Natrasoap
Pyrethrum