Description and life history
Twospotted spider mites are small animals, closely related to spiders and ticks, that get their name from their spider-like ability to spin webs. Spider mites feed on plants by piercing the leaf tissue with their mouthparts and sucking out the cell contents. Twospotted spider mites have a wide host-range, feeding and reproducing on more than 180 plant species and are important pests of many field, forage, ornamental, and horticultural crops.

Twospotted spider mites are a key pest of hop, occurring at damaging levels in nearly every hop field every year. Twospotted spider mite populations can increase rapidly, especially in hot, dry weather. In the hop growing regions of southern Idaho and Washington, where these conditions prevail during the growing season, spider mite populations frequently reach outbreak levels. Twospotted spider mite populations reach outbreak levels less frequently in the cooler, moister hop growing areas of northern Idaho and Oregon. However, if not properly managed, Twospotted spider mites are capable of completely destroying hops grown in any of the hop-producing regions of the U.S.

Twospotted spider mites overwinter as diapausing (seasonally dormant) adult females on dead vegetation, hop crowns, cracks and crevices in hop poles, and other protected areas in hop fields and adjacent areas. Male Twospotted do not overwinter. Females emerge from overwintering sites as the weather warms in early spring and immediately begin feeding. When weather is warm and food is plentiful females can begin laying eggs within two days of emergence. Eggs can give rise to either male or female spider mites: female spider mites emerge from fertilized eggs, while males emerge from unfertilized eggs. First instar, or larvae, emerge from eggs in 2-5 days. Spider mites develop through two additional molts, the second instar, or protonymph, and the third instar or deutonymph, before becoming adult mites. Development from egg to adult takes 1 to 3 weeks depending on temperature. Five to eight overlapping mite generations develop on hop during the spring and summer months. In late August and September, under the influence of shortening day length, orange-colored, diapause-form spider mites appear and begin moving down the hop plants to overwintering sites.

Adult female twospotted spider mites can usually be recognized by the combination of their small size, eight legs, yellow to yellow-green color, and paired lateral dark spots. (back to top)

Adult female twospotted spider mites are small (about 1/60 of an inch (0.4 mm) long), oval shaped with 4 pairs of legs. They are yellow to to yellow green in color and have large black spot on each side of the abdomen. present in overwintering females. Males are about ½ the size of female spider mites, yellow in color, and lack the paired lateral spots. Male spider mites do not overwinter. Overwintering females are orange to brick-red in color, but turn green as they begin feeding in the early spring. Overwintering females initially lack the characteristic black spots, which develop later in the spring.


Adult female spider mite and spider mite egg.
Eggs are clear to pearly-white spheres about 1/150 of an inch (0.18 mm) in diameter, and usually laid on the underside of leaves.

Larvae are about the same size as eggs and have only three pairs of legs. They are initially translucent, but become yellow-green to green as they begin feeding. The dark spots on the abdomen may not be particularly pronounced. Male and female larvae cannot be accurately distinguished.

Protonymphs and Deutonymphs are progressively larger then the larvae. They are more deeply green and the characteristic black spots on the abdomen are more pronounced. Male deutonymphs can be distinguished from females by their smaller size and more pointed abdomen. (back to top)

Damage to Hop
Spider mites damage hops by feeding directly on hop leaves and cones. Leaf feeding by spider mites results in silvering and browning of hop leaves, and reduced plant vigor. Severe infestations can cause complete defoliation and are accompanied by heavy production of webbing that can hinder chemical and biological control efforts. Most economic damage, however, is associated with spider mites on hop cones. Spider mites not only contaminate the cones by their presence, but spider mite feeding on cones results in dry, brittle, discolored (red) cones. Spider mite damaged cones tend to shatter so that both quality and quantity of yield is reduced. When infestations are severe, total crop loss can occur. (back to top)

Two Spotted Spider Mite Management
Low numbers of mites on hop foliage may be tolerated if weather is mild and sufficient biological control agents (largely predatory mites and coccinelid beetles) are present. However, spider mite populations can build very rapidly, and when mite numbers reach 1 to 10 per leaf, depending on the crop growth, control measures are necessary to prevent populations from reaching damaging levels. Economically effective management strategies based on the use of biological control or host-plant resistance are not currently available (see research page for new developments). Management of Twospotted spider mites on hop is therefore, dependent on the application of appropriately timed foliar miticides. Scouting to determine the need for appropriate timing of control measures is critical. Fields should be treated only in the pest population is at or exceeds the economic threshold. Treating a field that does not have damaging pest populations is a waste of money, contributes to the development of resistance, and may destroy existing natural enemy populations. Not treating a field that has damaging levels of a pest causes loss of yield and can increase control costs.

Scouting. Begin checking in late-May by removing leaves from hop plants and examining the underside of the leaves for the presence of spider mites using a 10X to 20X hand lens. Early in the season samples can be taken at the 3 ft. to 6ft. level (ca. 1m to 3m). Later in the season (after about mid-June), samples from near the wire are needed to determine the presence of mites in the hop canopy. Several leaves from each of 10 to 30 plants should be sampled depending on field size and the amount of time available.

Threshold. Treat when you have an average of 1-2 adult female spider mites per leaf through June and into early July. Treat when you have 5 to 10 mite per leaf after mid-July. (back to top)

Treatment options. Chemical treatment options are listed in the Pacific Northwest Insect Control Handbook. Because this information changes frequently be sure to check the latest edition of this book, which is available from the University of Idaho Resources for Idaho web page. Be sure to read, understand, and follow label directions. Whenever possible use compounds that are less, or non-toxic to natural enemies in hop fields.