Photo: WikipediaUnlike your neighborhood Spider-Man, there is nothing friendly about spider mites as these common pests can be quite destructive and feed on a variety of landscaping plants.
Classed as an arachnid along with spiders and ticks, the many species of spider mites are tiny, and hard to spot with the unaided eye. They come in many colors including green, yellow, red, brown, and some change in appearance depending on the season.
The most problematic spider mite species is the two-spotted spider mite (Tetranychus urticae) due to its wide range of hosts including trees, shrubs, flowers, weeds, fruits, greenhouse, and field crops.
The two-spotted spider mites become active in the warm summer months and can develop rapidly during this time period, becoming mature in as little as a week after hatching. Adult females can lay a dozen eggs daily for a couple of weeks, allowing the mite population to skyrocket quickly.
This can be exacerbated by dry weather, which causes them to feed more as the lower humidity lets them evaporate the excess water they excrete. Arid conditions also serve as a form of protection from their natural enemies, which require more humid conditions to thrive.
Symptoms indicative of a mite infestation include stippled or distorted leaves from where the spider mites use their needle-like mouthparts to pierce the leaves and suck the fluids from individual plant cells.
Spider mites prefer to feed on the underside of the leaf, so one way to examine vegetation for mites is to hold a sheet of white paper under a branch and tap the leaves sharply. The mites will fall onto the paper and appear as dark specs the size of this period.
Another sign is large amounts of webbing that will cover leaves and branches as the spider mites use it to protect themselves and their eggs from enemies, as well as help them spread to other plants.
If the population has reached a high level, the leaves of the plant will become bronze or brown and eventually drop as feeding continues. Spider mites left unchecked can stunt or even kill plants.
Some of the factors that can contribute to a spider mite outbreak include excessive use of nitrogen fertilizer, which makes the foliage more succulent, application of non-selective insecticides that kill the predatory bugs that feed on the mites, and drought conditions that the mites thrive in.
There are several different methods when it comes to controlling spider mites.
Biological
Because a number of insects feed on spider mites, if you are wanting to choose this route of control it is important to reduce your usage of pesticide sprays. Some of the most important predators are the western predatory mite (Galendromus occidentalis) and the Phytoseiulus mite species, so the use of miticides will kill these beneficial bugs along with the targeted spider mites.
Other insects that feed on spider mites include six spotted thrips, lacewing larvae, big-eyed bugs, minute pirate bugs and, of course, the spider mite destroyer lady beetle.
Rather than buying and releasing a population of predatory bugs, the best method for establishing predatory insects is to create favorable conditions for them by avoiding dry conditions and not using the insecticide carbaryl.
Cultural
As previously stated, dusty and dry conditions often lead to outbreaks, so providing adequate irrigation is important and can also protect trees and shrubs from spider mite damage, as those that are already water-stressed are less tolerant.
Periodic hosing of plants, particularly on the underside of the leaves, can help physically remove and kill many mites as well as remove dust that interferes with mite predators. Disruption of the webbing can also delay egg laying until new webs are produced. Spraying water on plants is generally more effective on smaller plants with low mite populations.
Twospotted spider mites overwinter in the nearby soil and weeds close to the host plant, so routinely mowing around the host plants can help reduce the mite populations.
Chemical
Good spray coverage is crucial when applying miticides as it will take several applications to effectively wipe out the population. Not all formulations affect spider mite eggs, allowing them to hatch and start up the cycle once more. Molting spider mites are also unaffected by miticides. Two or more applications at five to 10 day intervals can help provide sufficient control.
It is important to alternate miticide varieties as spider mites can often develop a tolerance making them harder to control. Insecticidal soaps and horticultural oils are another option, but they do require excellent coverage since they must come in contact with the mites to kill them.
