Twospotted Spider Mite

The adult female twospotted spider mite is oval-shaped and measures approximately 1/50 inch (0.4 mm). The adult male is smaller than the female and has an elliptical shape; its body is tapered at the end. Twospotted spider mites are usually translucent or greenish-yellow in color, but can also be brown or orange-red. Just prior to diapause, adult females change to bright orange. Twospotted spider mites have dark spots on each side of their body, which is due to the accumulation of body waste products.

Adults overwinter in soil cracks beneath plant debris or on living plant tissue. Females lay eggs on the underside of leaves and depending on temperature, eggs hatch in 4 to 5 days. The complete life cycle takes approximately 14 to 16 days and there are several generations per season.

Feeding damage appears on the upper surface of leaves as silvery or dry spots. Heavy infestations cause leaves to turn bronze in color and drop. Economic damage can occur from a reduction in oil yield and quality, and a shorter duration of productivity of mint fields.

Infestations of twospotted spider mite can occur in all peppermint growing regions of California. Commonly, populations increase quickly during the hot and dry periods between June and August. Although most damage occurs during this period, economic populations have been observed during late April and early May. Control options include reducing plant stress through optimum irrigation and fertilization, biological control using predatory mite releases, and miticides.

The spider mite predator, Neoseiulus fallacis, can occur in all mint-growing areas of California. Adults have a pear-shaped body and measure approximately 1/50 inch long (0.4 mm). They are shiny with long legs and tan to light orange. Immatures are semi-transparent and cream colored.

Predatory mites can be distinguished from spider mites by their more active searching behavior. N. fallacis can also be purchased from commercial insectaries. Releases should take place when twospotted spider populations are well below the threshold of five mites per leaf. For optimum results, N. fallacis should be released at 2000 per acre when approximately 20% of sampled leaves have at least 1 spider mite. Preserve N. fallacis by using a compatible miticide.

Using biological control is an excellent tool to manage miticide resistance, because biological control organisms are indiscriminate in their feeding behavior.

Clarified extract of neem oil and predatory mites can effectively control twospotted spider mites. The efficacy of neem oil is slower than conventional miticides, because it is an insect growth regulator and does not kill by contact. When using neem oil, apply at a lower threshold of approximately three mites per leaf.

Minimize water stress and control dust along dirt roads, especially during hot weather.

There are two different monitoring methods: enumerative and presence/absence sampling.

1. Twospotted spider mites tend to be distributed unevenly throughout peppermint fields. Therefore sample several different locations when monitoring.
2. Sample a minimum of 7 locations for every 30 acres. Sample more locations, depending on the field's history.
3. The sampled location should measure approximately 15 feet by 15 feet.
4. Sampling methods and treatments:
   • Enumerative. Count all nymphs and adults on the underside of about 23 randomly selected leaves at every location. Treat if five mites per leaf or more are found.
   • Presence/absence. If a leaf has two spider mites or more, note the leaf as infested, otherwise it is considered not infested. Apply a miticide if 15 of the 23 leaves sampled (~65%) are infested. This indicates a density of approximately five mites per leaf.