Strawberries are a favorite fruit of many families in South Carolina. Every year, many home gardeners grow strawberries in their gardens or in containers. While several pests attack strawberry plants, the two-spotted spider mite (Tetranychus urticae Koch) is the pest that causes the greatest damage and is the most difficult to control. Two-spotted spider mites (TSSM) feed and live on the underside of leaves, making control of this pest very difficult. They reproduce quickly in warm temperatures. Because of these characteristics, good control of TSSM can be very difficult for home gardeners.
Commercial miticides that are effective in TSSM control are not available for homeowner use. Products that are available to homeowners may not be very effective.
An adult TSSM has a pale green body with two dark spots on its back, hence the name “two-spotted”. TSSM are very small in size (about 1/60 of an inch) making them difficult to see with the naked eye. The female TSSM is larger than the male.
TSSM with a pale green color are “active” (feeding and laying eggs). During winter, females change color to a reddish orange. TSSM with a reddish orange color are “dormant” and are not feeding or laying eggs
Effects of Temperature on Spider Mites
Spider mites prefer hot and dry conditions. They are most active between 48 °F and 111 °F; however, they prefer temperatures between 55 °F and 98 °F. TSSM feeding and reproduction depends on body temperature, and the body temperature of the mite depends on air temperature.
Females can lay up to 100 eggs in an average lifetime of 68 days. Depending on day and night time temperatures, TSSM eggs will hatch between 3-19 days. At temperatures below 48 °F, a spider mite egg will hatch in about 19 days. At temperatures above 70 °F, spider mite eggs will hatch in 3-7 days.
TSSM goes through four stages of development from egg to adult. Spider mite eggs are round and clear. They are laid on the undersides of leaves. The first set of eggs is generally laid on the lowest leaves on the plant. When a spider mite egg hatches, it produces a young spider mite that starts to feed immediately.
Both adults and young feed on the underside of the plant leaf. TSSM feed by sucking plant juice. Symptoms of plants affected by TSSM are:
- Stunted growth
- Yellow mottling of leaves
- Small and misshaped fruits
- Bronzing of leaf underside (with severe feeding)
Biological control of TSSM offers homeowners the best option and safest way to control this pest. Biological control is safe for the homeowner, pets and the environment. Most of the miticides used in commercial strawberry production are not available for use by homeowners.
Predatory Mites: Two species of predatory mites that are effective in controlling TSSM on strawberries are Phytoseiulus persimilis and Neoseiulus californicus.
Releasing these predatory mite species will give the homeowner season-long control of TSSM. Other crops in the garden will also benefit from the release of the predatory mites. Releasing predatory mites will also encourage and promote the population build-up of other beneficial insects in the home garden that feed on TSSM.
Timing the release of predatory mites is very important for successful control of TSSM. Before ordering the predatory mites, the homeowner must inspect the strawberry plants for TSSM or their eggs. There must be TSSM for the predatory mites to eat once they are released. If they don’t have a food source, the predatory mites will either die or migrate to other plants.
Check the undersides of strawberry leaflets for TSSM. Count the number of adults, crawlers (newly hatched immatures) and eggs present. If you find two or more adults with eggs per leaflet, order the predatory mites. The predatory mites will typically arrive one week after placing an order. Predatory mites should be released within 4 hours after receiving them. Predatory mites can be kept up to 24 hours in a refrigerator, but must be released within this time.
If the population of the TSSM is very high, homeowners can apply a 1% horticultural oil spray (Bonide All Seasons Spray Oil Concentrate, Ferti-lome Horticultural Oil Spray Concentrate, Monterey Horticultural Oil Concentrate, or Southern Ag Parafine Horticultural Oil) or an insecticidal soap spray (Bonide Insecticidal Soap Concentrate, Natural Guard Insecticidal Soap Concentrate, Garden Safe Insecticidal Soap Concentrate, Safer Brand Insect Killing Soap Concentrate, or Espoma Earth-tone Insecticidal Soap Concentrate) to help reduce the TSSM population until the predatory mites arrive. Follow label directions for use.
Sources of Predatory Mites: Some suppliers of predatory mites are provided at the following web sites:
Other Beneficial Insects: Several other beneficial insects are predators on TSSM. Examples of these predators are shown below.