Lewis Spider Mite

Lewis mite has emerged as an occasional pest of strawberries, primarily in Oxnard, Salinas, and other coastal production regions. Lewis spider mite, or Lewis mite, infests the undersides of strawberry leaves, where they may form colonies and produce light webbing when abundant. Lewis spider mites are very small (about 0.01–0.014 inch in length) and are barely visible to the naked eye. The Lewis spider mite undergoes one larval and two nymphal stages before becoming an adult. Mobile mites are green to a yellowish. Under a hand lens, multiple dark blotches can be seen on each side of the adult mite's body. Eyes appear as two red eyespots. Lewis spider mite eggs are laid on the undersides of leaves and are spherical, clear, and colorless when laid but become pearly white as hatch approaches. Lewis spider mite colonies often first appear near leaf edges or veins. Eggs emerge about 3 days after they are laid, and the lifecycle from egg to adult emergence occurs in about 14 days, at 77°F. Development time takes longer at lower temperatures.

Lewis spider mite has a wide host range, including ornamentals such as poinsettia and roses. It can also be found on weeds, such as castor bean, in and around the field. With the onset of warm weather, these mites migrate to the foliage of the plant and begin to lay eggs. In the mild winter coastal growing regions of California, it is unusual for a large percentage of mites to become dormant; instead they continue to grow and lay eggs, although at a slower pace during the winter months than in summer. The Lewis spider mite undergoes one larval and two nymphal stages before becoming an adult. The life cycle, under ideal conditions of hot and dry weather, can take place in 10 days.

Spider mites, including the Lewis spider mite, feed on plant juices and cause a yellow stippling on the leaf surface. As the number of mites grows and feeding progresses, leaves turn yellowish brown before drying up and falling off. Feeding by Lewis spider mites on fruiting strawberries reduces plant vigor and fruit yield and size.

The key to successful management of Lewis spider mites is to monitor their numbers and to initiate insecticide applications in a timely manner. Once mite numbers are high, damage has already occurred, and the mite numbers are difficult to control.

The most effective biological control agent of Lewis spider mite are the predatory mites Neoseiulus californicus, N. fallacis, and Amblyseius andersoni, which are introduced species. N. californicus does best in temperatures of 50º to 91ºF, so it is compatible with both spider mite and strawberry plant development. Although they occur naturally in California strawberry production areas, they may be purchased and released in fields for additional control.

It is best to release predatory mites during cool mornings and avoid releasing them during high dry winds. Release the predatory mites early in the season, before spider mite populations build up. Apply predatory mites at the rate of 10,000 to 20,000 per acre in conventional fields and 40,000 or more per acre in organic fields.

Phytoseiulus persimilis is not an effective predator of Lewis spider mite.

Roguing (removing) heavily infested plants can reduce mite numbers. Controlling dust by watering or oiling surrounding roads and planting dust control barriers helps to reduce Lewis spider mite numbers.

Use cultural and biological controls including the release of predatory mites, and narrow range oil sprays, such as Organic JMS Stylet oil, on organic strawberries.

Learn to differentiate between Lewis and twospotted spider mites, since the control strategies for the two mites are different. No precise treatment thresholds have been established for Lewis spider mites in strawberries. Monitor to keep track of increasing pest mite numbers as well as predatory mite numbers to determine the risk that populations will continue to grow. A ratio of 1 predator to 10 Lewis spider mites is considered favorable for biological control. Good under-leaf coverage is essential when applying acaricides. This is especially important with non-residual products such as the spray oils, since mites that escape contact with the spray will survive.