Hot, dry and dusty are pretty typical outdoor conditions for this time of year. The last rains were months ago; late summer and early fall often features toasty daytime temperatures with low relative humidity. Most people, plants and animals don’t prefer it, but it’s ideal for pesky spider mites. More than 200 different plant species, ornamental and edible, evergreen and deciduous, can be significantly damaged by these pests. Roses, azaleas, dahlias, raspberries and strawberries, fruit trees, tomatoes, sugar peas and beans to name a few. They even love houseplants!
Spider mites are tiny arachnids, related to spiders, ticks and scorpions. Also called web-spinning mites, the two-spotted spider mite, the Pacific spider mite and several other species are among the most ubiquitous of all pests in the garden. With eight legs and an oval body 1/20 of an inch-long, they look like moving dots to the naked eye. Living in colonies that may include hundreds of individuals, they reside mostly on the undersides of leaves, collectively spinning fine webbing for protection from natural enemies and weather.
As the nearly invisible mites suck juices from plant cells, tiny, yellow or white dot-like patterns appear on the leaves. When many feeding spots occur near each other, the upper leaf surface takes on a bronze or rusty color, and ultimately turns yellow and drops off. If you shake leaves that appear to be infested over a piece of white paper and see tiny dark specs moving on the paper – you have spider mites.
Populations can build up to tremendous numbers in a very short time; a generation can be completed in less than a week if the conditions are right – hot weather, low humidity and dust and dirt on leaf surfaces. Low numbers may result in some cosmetic damage; prolonged heavy infestations slow plant growth and cause leaves and fruit to drop prematurely. Severe infestations can kill plants, especially if they are suffering from water stress. Regular use of insecticides in the garden can destroy natural enemies and encourage spider mite outbreaks.
Prevention is the best protection against an invasion of spider mites. Start by keeping plants healthy, well-watered and free of dust on leaves. Provide adequate nutrients to plants but go easy on nitrogen fertilizer that can stimulate growth and induce mite outbreaks. Periodic hosing of plants with a forceful jet of water, especially the undersides of leaves, can physically remove and kill many mites, as well as remove the dust that collects on foliage and interferes with mite predators. Knocked to soil level, these pests aren’t very successful at getting back on the plant. As with any application of water to foliage, do it early in the day so leaves can dry.
Natural enemies in the home garden are the most important weapons against spider mites. These include general predators like lacewings, big-eyed bugs and minute pirate bugs, and some specialized predators – the shiny black, oval-shaped spider mite destroyer lady beetle and Western predatory mites.
Avoid using insecticides in the garden. They do not control mites (as they are arachnids, not insects), they may kill off natural enemies and can stimulate mite reproduction, especially when applied during hot weather, causing dramatic spider mite outbreaks within a few days.
If you have an infestation now, prune out damaged portions of plants or those with significant webbing. As the seasons change, temperatures drop and rains return, high mite populations generally decline rapidly and take a break until next summer. Implement preventive measures early next year and you can keep spider mites at a tolerable level.
By Nanette Londeree, Master Rosarian