As we roll into April, the combination of an early spring and drought-like conditions presents a risk of everybody’s second least favorite arachnid: two-spotted spider mites. As any greenhouse or high tunnel farmer can attest, spider mites absolutely love the hot, dry conditions that we have seen over the past month, and the NOAA Climate Prediction Center’s one-month forecast calls for a high probability of continued predominantly warm and dry conditions for the next 30 days. Prevention is always the best policy. To create a hostile environment for spider mites, be sure that plants are not under-watered, as stressed plants make good hosts. Keep dust to a minimum by spraying water on bare patches of earth, make use of mulches, or place cover crops or grasses around the plant to ensure a healthy and contained soil. Keep beds clean and free of weeds. Most importantly, should you have a problem with a different pest, avoid the use of non-discriminating contact insecticides. These insecticides kill beneficial and predatory bugs as well. Pyrethrin-based insecticides and carbaryl are known to increase the risk for spider mite infestations.
Scout regularly. The mites themselves are very tiny and difficult to see without a magnifying glass. Instead, check leaf tops for light-colored stippling, as if someone has repeatedly tapped the leaf with a very fine pointed colored pencil. Check the underside of leaves for the white, fine webbing from which the mites get their name. Should you find any affected leaves, a good method for finding mites is to place a white sheet of paper under the branch and tap the branch forcefully so that the mites fall onto the paper. Here, they are far easier to inspect. You are looking for a very small pale white to yellow-brown/gold bug, usually with two brown spots on its abdomen. If you see signs of mites but can’t find any, there’s a chance that they’ve already moved on and spraying would be a waste of resources. For more information on scouting, including photographs, visit http://edis.ifas.ufl.edu/in307 .
If spider mites are spotted and properly identified, the next step is to douse the plants in chemicals and toss a lit match for good measure, right? Not necessarily. Fact is your plants can handle a small population of spider mites without any adverse effects. This is true of many pest species, and how you react largely depends on your circumstances. In a well-designed integrated pest management system, the grower has a defined population threshold based on where the economic loss from pest damage exceeds the cost of taking control measures. An ornamental greenhouse nursery two weeks away from shipment date will be able to tolerate a much lower population of spider mites than a homeowner two weeks away from the rainy season when populations naturally drop.
Commercial, conventional growers have access to chemical miticides that are unavailable to homeowners. However, these may not always be the best options for owners of U-Pick farms, as the conventional miticides tend to have seven to fourteen day windows where the produce cannot be picked. For U-Pick, organic, and home growers, the best options often come in the form of insecticidal soaps and oils. These products allow produce to be safely picked the next day and work by covering and suffocating the mites, so thorough coverage is absolutely essential to ensure maximum control, especially on the bottom of the leaves.
However, there is a problem with soaps and oils in Florida: they can cause plant damage if applied when temperatures exceed 90 degrees. In these circumstances, purchasing and dispersing predatory mites are another proven control method. You will want to make sure that infected plants are spaced close enough that the predatory mites are able to spread throughout the orchard. If this is not possible, split the dispersion between plants according to the directions on the label. Soaps/oils and predatory mites can be used together by first spraying the soaps to reduce the pest population and then applying the predatory mites a few days after. The combination of prevention, scouting, proper identification, and targeted control measures are highlights of integrated pest management systems and key tools in a successful and sustainable agricultural venture.
Extension programs are open to all persons without regard to race, color, sex, age, disability, religion, or national origin.
Matt Smith, M.S. is a Sustainable Agriculture and Food Systems Agent
UF/IFAS Extension Sumter, Pasco, Hernando County
7620 State Road 471, Suite 2 Bushnell, FL 33513