I had the opportunity to look at some raspberries recently that had a spider mite infestation. Spider mites are somewhat of a generalist in terms of what they feed on so I wanted to give a bit of information on them and possible ways of combating their infestations.Spider mites are common pest problems on many plants around yards and gardens. Injury is caused as they feed, bruising the cells with their small, whiplike mouthparts and ingesting the sap. Damaged areas typically appear marked with many small, light flecks, giving the plant a somewhat speckled appearance.
Dry conditions greatly favor all spider mites, an important reason why they are so important in the more arid areas of the country. They feed more under dry conditions, as the lower humidity allows them to evaporate excess water they excrete. At the same time, most of their natural enemies require more humid conditions and are stressed by arid conditions. Furthermore, plants stressed by drought can produce changes in their chemistry that make them more nutritious to spider mites.One reason that spider mites become problems in yards and gardens is the use of insecticides that destroy their natural enemies. For example, carbaryl (Sevin) devastates most spider mite natural enemies and can greatly contribute to spider mite outbreaks. Other chemicals can aggravate some spider mite problems, despite being advertised frequently as effective for mite control. Soil applications of the systemic insecticide imidacloprid have also contributed to some spider mite outbreaks.
Various insects and predatory mites feed on spider mites and provide a high level of natural control. One group of small, dark-colored lady beetles known as the “spider mite destroyers” are specialized predators of spider mites. Minute pirate bugs, big-eyed bugs and predatory thrips can be important natural enemies.Adequate watering of plants during dry conditions can limit the importance of drought stress on spider mite outbreaks. Periodic hosing of plants with a forceful jet of water can physically remove and kill many mites, as well as remove the dust that collects on foliage and interferes with mite predators. Disruption of the webbing also may delay egg laying until new webbing is produced. Sometimes, small changes where mite-susceptible plants are located or how they are watered can greatly influence their susceptibility to spider mite damage.
Chemical control of spider mites generally involves pesticides that are specifically developed for spider mite control, called miticides. Few insecticides are effective for spider mites and many even aggravate problems. Furthermore, strains of spider mites resistant to pesticides frequently develop, making control difficult. As most miticides do not affect eggs, a repeat application at an approximately 10- to 14-day interval is usually needed for control. Best bets for chemical control seem to center around horticultural oils.