This summer, like most other hot, dry summers, is conducive for outbreaks of spider mites around the home landscape. Spider mites thrive in hot, dry conditions which are usually unfavorable for healthy plant development. They can attack a host of plants in the landscape, including annuals, perennials, shrubs, trees, and certain mites even attack turf grass. Often times, recent additions or transplants that have not become fully established in the landscape often exhibit spider mite symptoms sooner than established ones. These plants may appear to be suffering from drought stress, but in fact, they may be suffering a spider mite infestation.
Spider mites are actually tiny, smaller than a poppy seed, red arthropods, closely related to ticks and spiders. They are common around the home choosing to live on any plant or shrub they can find. Juniper, arborvitae, succulents and pine trees are common hosts, but just about any vegetation will do. Including your garden plants, like tomatoes.
Spider mites feed on plant cells and produce characteristic small, yellowish, speckled feeding marks. The feeding marks are usually the first sign of a mite infestation and are often confused with some fertilizer deficiencies. Fine, silken webs can be detected on heavily infested leaves and flowers with these plant parts quickly withering and turning brown. In extreme cases, plant death may occur.
Unlike other mites, spider mites can reproduce quickly. Several cycles may be complete in one season. If conditions are good, they may go through all cycles in under a month. Their cycle includes egg, nymph, two molts of the nymph and then adult. Since they donít migrate quickly, most populations will grow around each other, slowly moving outward as their population increases. The main part of their nest is usually where damage is most prevalent. Expect to find dead leaves and plant parts. Upon very close observation, one may see the mites feeding or slowly moving if you disturb them.
Infestations are easiest to control when detected early, before the mite populations have reached very high levels. First off, make sure that the plants are well watered. Then, try spraying plants with a strong stream of water from a garden hose or faucet. This can dislodge many mites from leaf surfaces. This approach is generally more effective on smaller plants (e.g., houseplants), with non-dense foliage and low mite populations. Streams of water should be directed upward against the lower leaf surfaces, making sure to get the underside of the leaf. And the technique will need to be repeated on regular intervals.
Plants not under stress have a better chance at defending themselves than ones that are under some type of stress, such as drought stress. If this does not eliminate the problem within a few days, then an insecticide treatment may be necessary.
Elimination of moderate to heavy infestations of spider mites usually requires the use of specific pesticides known as miticides. Some, but not all, insecticides will also control mites. Some kill only active mites while others also kill eggs. Always read and follow the directions accompanying the product you are using. Some miticides may harm or discolor certain types of landscape plants.
Good spray coverage is essential when applying miticides. Thoroughly wet the foliage and try to contact as many mites as possible, paying particular attention to leaf undersides where most mites are living. In most cases, two or more applications at 5-10 day intervals will be needed for satisfactory control. Spider mite eggs that have not yet hatched are unaffected by most miticides; the same may be true of larvae and nymphs that are molting. During molting, spider mites remain inactive beneath the former skin, which serves as a barrier against insecticides. The quiescent mites also do not feed, rendering products that kill by ingestion temporarily ineffective. Consequently, if only one application is made, some of the mites may survive and the infestation will persist.
Effective homeowner options include Kelthane, horticultural oils, and insecticidal/miticidal soaps. Horticultural oils and soaps can be effective alternatives to conventional miticides, but require thorough coverage so that all mite stages are contacted.
Horticultural oils can be used on landscape plants during the warmer months of the year when green foliage is present and the plants (and mites) are actively growing. Horticultural oils are applied at rates of 1.0 to 2.0 %. Dormant oils are applied in winter or early spring prior to bud break, or in the fall after the leaves have dropped and there have been several light frosts. They are useful for killing overwintering mite eggs and, therefore, can help to suppress infestations of spruce spider mites, European red mites and southern red mites which overwinter as eggs on infested plants. A dormant oil spray will not guarantee mite-free plants the following year, but will delay mite buildup the following spring. Dormant oils are specially-formulated petroleum based products of horticultural oils applied at higher rates (3.0 to 4.0%). When using any type of oil, be aware that these sprays will discolor many conifers that have a bluish, waxy coating on their needles. Certain plant species tend to be oil sensitive. (Refer to the product label for a complete list of plants which may be sensitive to these products.)
Insecticidal/miticidal soaps are also widely available to homeowners. These products are useful in the warmer months when plants are actively growing, and may also be used to control cool season mites. Thorough coverage is essential.