Stopping Spider Mites
- Spider mites suck plant juices in leaves, their damage causes yellow-white dots to appear amongst the green leaf tissue and is called stippling. Heavy feeding can lead to complete death for the leaf and the “browning” of the tree.
- Spider mites have fast life cycles and can produce over 20 generations in a single year
- Spider mites are best controlled with biorational insecticides like insecticidal soap or horticultural oil. These kill mites but leave the beneficial organisms, which eat mites, alone.
Spider mites are small, plant feeding, arachnids that are part of their own family (Tetranychidae). They feed on hundreds of different species of plants by using their chelicera (the “fangs” that arachnids have) to suck juices from leaves. Generally they feed on the underside of the leaf and are famous for their ability to spin webs like their cousins, the spiders. Spider mite problems can occur on almost any plant but they tend to favor plants near roads or plants that are on the margins of fields.
Spider mites go through 5 stages, egg, larva, protonymph, deutonymph, and adult. As adults they are sexually mature and can mate to produce more pesky mites. The total time it takes to progress through these stages can vary depending on the weather, but it can be done in about 5 days. Most spider mites reproduce better when the weather is warm, so we see the highest populations in July and August. Some, like spruce spider mites, do better with cooler temperature of April and September.
Damage and monitoring
Spider mites feed by draining chlorophyll and other liquids from a plant’s leaves. As they do this, they also drain the color from the feeding site. This causes the leaf to develop little white and yellow dots amongst all the green. At first, damage is hardly noticeable, but as populations of mites build rapidly the damage soon becomes overwhelming. Outdoor plants that have experiences spider mites in the past should be monitored every other week and indoor plants should be examined once a week. Take a blank, white piece of paper and hold it under the leaves of the plant. Shake the leaved and look to see if there is any movement on the page. Mites are small but detectable with the naked eye.
Predators, parasitoids, and disease usually keep spider mites in check. However, during hot summers or after an insecticide application spider mite problems can explode. If you are monitoring for them, a fine summer, horticultural oil will be an option when mites or damage are first detected. If lots of damage has already occurred, try an insecticidal soap or neem oil. Do not apply oils and soaps when temperatures are over 90°. A final option would be sulfur dusts, but these can be lung irritants so are best saved for extreme cases. Do not use carbaryl or pyrethroid insecticides as they can actually help the mites to reproduce more quickly.
Stippling damage on bean leaves. The small white and yellow dots eventually congeal to form large dead spots on the leaf. Photo by Jim Kalisch; UNL Entomology