Spider Mites in the Landscape and Nursery
There are many types of mites, but spider mites, eriophyid mites (many being gall-inducing mites) and predatory mites are the most important groups when plant care is involved. In this article, spider mites and their natural predators will mostly be addressed.
Much of what we know about mites has been discovered just within the past 60 years for two reasons: mites are so small that they go unnoticed and they were rarely a problem for plants until the advent of chemical pesticides, which disrupted the natural check and balance system between spider mites and their predators, thus allowing for outbreaks. Mites are not insects, of course, and are more closely related to spiders and ticks. Adult mites have four pairs of legs and their mouth is comprised of a pair of fangs (chelicerae) that puncture the tissue of the host plant, creating a tiny wound that exudes sap. The mite then drinks the sap. Each individual puncture results in a tiny yellow spot. Repeated puncturing by a multitude of spider mites can quickly deplete foliage of their chlorophyll, giving the leaf a bleached appearance. However, when examined closely, thousands of individual 'piercing-sucking' wounds can be identified.
Mites, in general, are usually unseen and forgotten until the plant damage that some of them are responsible for creating suddenly becomes all too obvious. Oftentimes, this damage is the direct result of human mismanagement of other arthropods; this phenomenon is known as a secondary pest outbreak and is caused by the use of chemical pesticides. In fact, spider mites were rarely a problem in agriculture and horticulture until the invention and widespread use of chemical insecticides after World War II. In most cases, spider mites were kept at low numbers on plants and in large-scale plantings due to natural controls, specifically predatory mites.
Within the animal world, there are two basic strategies for survival of offspring: K-strategy and r-strategy. K-strategy animals produce very few offspring per individual and invest large amounts of care into those offspring. Humans are a great example of K-strategy. Those animals, such as insects and spider mites, that use r-strategy produce vast numbers of offspring and invest almost nothing in their survival other than depositing the eggs on or near a suitable plant host. Also, such species usually go through their generational time quite quickly and often have multiple generations per year.
Such r-strategy species are generally those that we need to pay extra attention to in agriculture and horticulture, given their ability to reproduce quickly, pass along genes for pesticide resistance quickly, and become major pests that are difficult to control. Many of the chemical insecticides of the past (and some in the present) have selected for spider mite resistance, as well as acted to annihilate the predatory mites, which are not well designed to develop resistance to pesticides. Given that some spider mite species can go from the egg stage to reproductive adult within 5 days under optimal weather conditions, the prevalence of resistance to pesticides and the sheer numbers of plant-damaging mites can increase almost overnight. Therefore, many spider mite species are the quintessential r-strategy plant pests in the landscape and nursery.
In addition to spider mites being well-adapted and quick to develop pesticide resistance, often within a growing season, humans also worsen the problem with the very same pesticides by destroying the predatory mite populations, thus eliminating the natural controls on the spider mite species and allowing them even more opportunity to explode in population numbers.
In the world of spider mites (family: Tetranychidae), there are two basic types that relate to their activity times: cool-season mites and warm-season mites. An example of a cool-season species is the spruce spider mite here in New England. This mite becomes active in the spring from over-wintering eggs that were deposited on the host plant the previous fall. These mites feed, develop and reproduce into June but then go dormant as the annual hot and dry conditions of summer begin. Then, as temperatures cool in late summer/early autumn, they re-appear and begin feeding and reproducing again. If the fall remains atypically warm, these mites can become problematic in September-October, a time when few are thinking about insect and mite problems, so such problems are often not noticed until the damage has reached unacceptable visible levels.
Warm-season mites, such as the two-spotted spider mite, also become active in the spring but continue to develop and reproduce at increasing rates as daily temperatures become warmer, reaching peak reproductive and developmental capabilities during the hottest time of the summer. When temperatures are in the 90º–100º F range, such mite species can go from egg to reproductive adult within 5 days, with the potential for each female to produce more than 100 eggs.
Many of the extremely important predatory mite species responsible for keeping spider mite numbers below damaging levels do not function well in such high temperature environments and often leave the plant for leaf litter or other cooler and more protected areas and go dormant. Some may stay but their foraging and feeding activity is greatly reduced, thus providing another opportunity for warm-season spider mite species to increase in numbers virtually unimpeded. Lastly, host plants for spider mites often do not perform well during the hot and dry periods that favor spider mites. Drought especially puts pressure on plants to fend off insects and mites. Water is involved with the natural production of defensive compounds that plants make, often in the roots, to defend themselves from invertebrate herbivores such as spider mites. At the onset of drought, plants usually have a sudden increase in the production and translocation of these defensive compounds, but as drought continues, this production and movement drops off considerably, thus leav-ing the plant more susceptible to damage from mites and insects. Drought stressed plants generally experience greatly reduced growth, thus prohibiting them from producing new foliage to 'outgrow' damage created by spider mites.
Management of Spider Mites
Plant-care professionals need to be aware of those spider mite species that are inherent to specific plants, as well as recognizing the potential presence of mite population build-up: fine yellow stippling, the presence of fine silk, and the mites themselves, usually on the foliage undersides. One can monitor for spider mites by gently shaking host plant foliage over a piece of paper and examining closely (with a hand lens) what falls onto the paper. In general, spider mites are oval and predatory mite species have more of a teardrop shape, but not always.
A general rule of thumb for a naturally managed spider mite population is a ratio of 1 predatory mite to every 10 spider mites. However, this varies given the time of year (warm-season mites are very prolifi c during hot and dry times and predators are not actively feeding). Also, if large numbers of spider mites are present, despite the presence of predators, it may indicate a situation where the natural controls are now incapable of reducing the pest species below damaging levels.
Insecticides and miticides are readily available but the risk of pesticide resistance remains strong with such compounds. Some of the newer miticides are growth regulators and disrupt some normal biological process of the mites, such as reducing the ability to molt properly, sterilizing female mites, killing the embryo within the egg, or forcing the molting process when the mite is not yet ready; all methods being lethal to the mite. Some growth regulators are so specific that they kill spider mites but not predatory mites. Some are only available as restricted-use compounds. The properties of each of these mite growth regulators varies widely and labels should be thoroughly read, understood and used properly.
Horticultural oil sprays are very effective for many mite species but need to be targeted to where the mites are feeding in order to be effective, usually the foliage undersides. Repeated applications may be necessary for warm-season mites. Overall, the secret to sound management is to avoid mite build-up in the first place and the development of pesticide resistance. Understanding how mites become a problem and avoiding the human factors that contribute so strongly to problems with spider mites goes a long way in the management of these tiny yet potentially serious pests.
Written by: Robert Childs