Spider mites

Photo: John A. Weidhass, Virginia Polytechnic Institute and State University, Bugwood.org

Updated: August 3, 2021

Appearance

  • Eggs:  Minute, spherical, translucent white to yellow/orange; laid singly on undersides, and occasionally on top of, leaves. Leaf surfaces are covered with silk strands.
  • Larvae: First instar (growth stage) larvae are tiny, pale, with nearly spherical bodies and 3 pairs of legs.
  • Nymphs:  These next two instars resemble the larval stage, but are usually red or green, with 4 pairs of legs.
  • Adults: Tiny, eight-legged arthropods (related to spiders) about the size of a punctuation period. Body shape is oval to elongate-oval. Yellowish-green or reddish-orange in color and some species have 2 to 4 dark spots on the back. Species identification based on body color is unreliable.
  • Adult spider mites
    Photo: Dr. Mike Raupp

    cluster of spider mite adults
  • Closeup of spider mite adults
    Photo: Dr. Mike Raupp

    Closeup of adults Photo: Dr. Mike Raupp

Life cycle/habits

  • Adults overwinter in plant debris or under bark and produce multiple generations yearly; life cycles can be as short as 8 days.
  • European red mite and two-spotted spider mite are the most common species in Maryland vegetable gardens.
  • Mites suck chlorophyll from leaf tissues, creating fine white spots referred to as stipples.
  • Feeding occurs primarily on leaf undersides and is visible on the upper surfaces as stippling.
  • Mites also spin fine webbing, hence their name.
  • Populations develop and increase rapidly in hot, dry conditions.

Host plants

  • Primarily infest bean, tomato, and cucurbits (cucumber, squash, melons).
  • Also found on numerous other vegetables such as beet, carrot, corn, eggplant, parsley, pea, pepper, and sweet potato, as well as strawberry, blackberry, and other fruits. (Tree fruits and ornamentals are usually infested with two-spotted spider mite.)

Signs/symptoms

  • Spider mites' puncture-and-suck mode of feeding produces yellow or white stippling and leads to reddish or pale discoloration of leaves. 
  • Plants lose vigor and become unthrifty. 
  • Heavy mite infestations stunt plants and fruits, and premature leaf drop may occur.
  • There is visible webbing on leaves when mite populations are high.
  • It is very difficult to see mites without a hand lens (magnifying glass).
severe damage (stippling) on bean leaves caused by spider mites
Spider mite damaged bean leaves

Monitoring

  • Look for fading leaves, especially older ones.
  • Inspect top of leaves for fine stippling and underneath for fine webbing.
  • Heavy feeding causes yellowing, browning, and even leaf death. 
  • Mites are barely visible to the naked eye. To view, tap a leaf over white paper and inspect for crawling dots. A magnifying glass may be needed.

Prevention/control

  • Mites like it hot, dry, and dusty. Hose off plants to cool and clean them. Hosing leaf undersides dislodge the mites.
  • Reduce plant stress through proper watering and fertilizing practices – avoid excessive nitrogen fertilization, which increases mite populations.
  • Horticultural oil and insecticidal soap are most effective on mite eggs. Soap with pyrethrum can be used. Use when damage is first observed. Spray in early morning when it's cool and plants have rehydrated overnight. Caution: Spraying badly damaged leaves can cause further injury.
  • Pull up and dispose of badly infested plants. Clean up overwintering sites, especially bean plant debris.
  • Encourage predatory mites by avoiding toxic sprays. You can purchase predatory mites and release them into your garden to control pest mites.
  • Mites overwinter in nearby weeds and migrate to the garden, so keep weeds suppressed.
  • Floating row cover can be used to exclude mites.