This August is turning out to be a bit more mild than what is typically expected in Kansas, but it is still warm and humid enough for heat-loving plant pests to wreak their normal havoc on landscape and garden plants. Spider mites are particularly common this time of year, and their feeding often goes unnoticed in the early stages. Gardeners should keep an eye out for this pest, and consider control if populations reach high enough numbers to cause substantial damage to plants.
Spider mites are tiny eight-legged creatures that are related to spiders and ticks. They feed on a wide range of plants by inserting a straw-like mouthpart called a stylet into plant tissue and sucking out the juice. They often hide on the undersides of leaves where they go unnoticed by unsuspecting gardeners.
The first sign of a spider mite infestation is a symptom referred to as stippling, which refers to the appearance of numerous small dots or specks. In each spot that a mite feeds, leaf tissue turns yellow or white, leaving the top of the leaf with hundreds of yellow or white spots in close proximity. As mite colonies grow, stippling becomes more widespread and noticeable.
If symptomatic stippling is observed, mite populations are easily confirmed by holding a white piece of paper under the symptomatic leaf tissue and shaking the leaves. If mites are present, they will appear as tiny dark specks moving on the paper. A 10x magnifying lens or reading glasses can also reveal the mites on plant material or paper.
In very heavy spider mite populations, a thin silken webbing may also be apparent on leaves and stem tissues.
When mite populations are observed, gardeners should base their decisions about what to do on the density of spider mites, plant species, whether or not the plant has or is expected to produce a crop, and other site-specific factors.
More specifically, spider mites on tomato plants in September are less of a concern than spider mites on everbearing raspberry plants in July. The tomato plants are an annual crop that is nearing the end of its season in this example, while the raspberries are a perennial crop with continual harvest and concern for long-term health of the plant.
Low levels of spider mite infestation are little cause for concern, especially on annual crops and flowers. Spider mite infestations that are heavy enough to cause severe yellowing or whitening of leaves and subsequent leaf drop should be monitored and may be cause for concern. Very high populations, especially on small herbaceous plants and when compounded with other factors, can eventually stress a plant enough to kill it.
Water stress is also a concern — plants that are stressed from too much or too little water are less able to withstand damage from spider mites, so in some cases regular watering or alleviating drainage issues can reduce the issue.
If control is warranted, the easiest option is to literally wash the mites from the infested plants. Use a high-pressure nozzle on the end of a garden hose and spray the mites from as much of the leaf tissue as possible. Actively feeding mites are unable to quickly remove their stylet from the leaf tissue and it will break off as they are washed to ground. Some mites will survive, but this method can remove enough of the mites to avoid substantial damage.
Mites also tend to be worse on dusty plants, so washing can further reduce mite populations by making a less favorable environment.
In crop or other valuable plantings where more than washing is justified, try a commercially blended horticultural soap or oil (available with other pest-control products). As a last resort, try a miticide. Remember that spider mites have many natural predators that are also killed by miticides. Spider mite populations sometimes boom after repeated miticide use because of the lack of natural predators that survive.
Spider mites are common on tomatoes, raspberries, blackberries, roses, salvia, squash and other cucurbits, beans, strawberries, maple, boxwood, spruce, burning bush, butterfly bush, and many other plant species. Two-spotted spider mites are the most common species found in the Midwest, but there are many mite species, including some that are specific to certain plants like the spruce spider mite.