2 Spot Spider Mite

It is that time of year again, when the two-spotted spider mites (Tetranychus urticae; TSSM, Figure 1), and other mite species, show up in full force and wreak havoc on fruit and vegetables. These pests are very inconspicuous and often go unnoticed until the resulting damage appears. For TSSM this includes the webbing produced on heavily infested leaves or to the more trained eye, the characteristic yellow speckles or mottled symptoms on the upper surface of the leaves (Figure 2 and 3). The mites can be found on the underside, feeding with their sucking mouthparts.

Figure 1. Eggs, larvae, nymphs and adult two- spotted spider mites. (Photo by J. Obermeyer)

Figure 2. Yellow speckles on the upper surface of cucumber leaves heavily infested by two-spotted spider mites.

The Two-spotted Spider Mite The twospotted spider mite, Tetranychus urticae is a serious pest of a variety of agricultural crops and ornamental plants. This mite has been recorded to feed on more than 180 different plant species in both greenhouse and outdoor environments. Two-spotted spider mites are one of the most widespread pests of field, nursery, orchard and greenhouse crops worldwide. They have also become increasingly damaging pests of commercial cannabis.

Figure 3. Webbing produced on heavily infested cucumber leaves by two-spotted spider mite.

Cucumber, especially in organic production, can be the most susceptible. However, this pest feeds on a wide range of plants including tomatoes, melons, peppers, strawberries, apples, pears and grapevines, as well as flowers and field crops. Regardless of your production technique, an intervention is almost always necessary to control TSSM. In high tunnels in particular, pest populations thrive under hot and dry conditions and can wipe out a crop in a couple short weeks if no control measures are taken. Here is the best advice that I can offer, from my own experiences of growing cucumbers in high tunnels for four years and consulting with some beneficial suppliers in the region:

If the infestation of TSSM is at the point where webbing is visible on some leaves, especially in high tunnels, chemical intervention is necessary before introducing beneficial predators. There are a variety of miticides that have been effective in the past, leading to control with only one application. We have used Portal® in the past, which has since been prohibited in greenhouses. Ones that are currently silent on the label, and therefore may be used in high tunnel production depending on your local interpretations, include Acramite®, Agri-Mek® and Oberon®. These products can be used in both cucumber and fruiting vegetables. An organic alternative may include various insecticidal soaps, which kill on contact so should be carefully applied to the underside of leaves where the pests are feeding.

Mite

If you are considering beneficials for control, there are a variety of predatory mites and a midge available for purchase. Phytoseiulus persimilis is a predatory mite that only feeds on TSSM and can be effective at getting an outbreak under control. They become less active at temperatures around 90-95˚F. The predatory midge Feltiella acarisuga are available for purchase as pupae in plastic trays. They take about 3 weeks to establish in the crop and provide control of TSSM as well as other mite species. The larval form of this insect feed on the mites and can resemble syrphid fly larvae, which are common and feed on aphids. There are a variety of Amblyseius species, another predatory mite, that can establish and maintain long term control. The species that is most heat-tolerant and therefore suitable for high tunnels at this time is Amblyseiuscalifornicus. They remain active at temperatures up to 110 ˚F.

Pest ID

Pest ID of Two-Spotted Spider Mites

  • Nymphs resemble full-size adults, but may have 6 or 8 legs, depending on stage, but do not have the ability to reproduce.
  • Adults are very small at only 1/60 (female) to 1/80 (male) inch in size - approximately the size of salt grains - when fully developed. Adults are greenish yellow to orange to brown with 2 darker spots - similar to saddle bags - on their body.

Symptoms

Leaf shows spider mite stippling.
  • Two-spotted spider mites damage crops by piercing leaves and feeding on the plant juices. Mites suck on the bottom sides of soybean leaves, removing plant moisture and nutrients, resulting in a yellow or whitish spotting on the top side of the leaf surface. In heavy infestations, it's common to see leaf burning and stippling.
  • Hot spots will typically be noticed first on field margins, as infested plants take on a wilted appearance. Drought-prone fields or field areas that contain lighter soils or sands are often affected first by spider mites. As populations increase, two-spotted spider mites will move across the entire field if left unchecked.

Life Cycle

  • Two-spotted spider mites have 4 stages of development: egg, larva, nymph and adult.
  • This pest overwinters as adults in field edges and roadsides bordering fields, feeding on weeds until spring.
  • After early spring mating, female spider mites lay eggs on weeds that usually hatch in 3 to 5 days.
  • Two-spotted spider mites do little feeding during the first larval stage of development.
  • Females live 20 to 30 days and typically produce 50 to 100 eggs during their lifetime.
  • The entire life cycle of this pest can be completed in 5 to 14 days. This pest has the potential for up to 10 generations per year during the growing season.
  • Populations of spider mites increase significantly during extended hot, dry conditions.

Management

2 Spot Spider Mites

  • Two-spotted spider mites are difficult to see with the naked eye so conduct a simple 'paper test' by shaking the plant over a white paper plate. This will allow a grower to see the tiny orange- to yellow-colored mites slowly moving on the paper.
  • Some universities suggest treating for this pest if 20-50% of the leaves are discolored before pod set. After pod set has begun, the suggested treatment threshold is 10-15% of the leaves discolored.
  • Natural predators, including fungi and thrips, can help keep populations in check.
  • If a spray treatment is made, the most consistent performance has been with a full rate of an organophosphate.
  • Natural predator populations can be affected by spray applications of synthetic pyrethroids.
  • Residual control of most treatments is short-lived and applications only control adults and nymphs. Scout fields 5 to10 days after spray applications to check for re-infestation.