Two-spotted Spider Mites

Crops Affected: Saskatoon berry, raspberry, strawberry, and other crops

Life Cycle:

• Mites are not insects, but are closer related to spiders (arachnids)
• Very small (visible with naked eye, but not distinctly)
  • They resemble moving, crawling grains of sand on the leaf surface
• Mites feed by piercing and sucking
• Mites may have multiple generations per year, with overlapping generations
• TSSM have two irregular dark spots on their backs
• TSSM overwinter as adult females among fallen leaves or under the bark of hosts
  • They move up the stems of plants in the spring
• Mites feed on leaf undersides and flower buds, preferring young leaves in the crown area
• Hot, dry, sunny conditions favour mite development and cause population explosions

Symptoms:

• High populations will decrease plant vigour
• Very fine webbing may be seen on the underside of leaves
  • Webbing indicates a very heavy infestation of spider mites
• Leaves may have various symptoms including:
  • Yellow spots
  • Stippling/yellowing/mottling (irregular discolouration)
  • Browning
  • Crinkling or curling
  • Smaller in size and red or bronze coloured
  • May dry up and fall off if there are heavy infestations
• In raspberries:
  • New canes may have stunted growth
  • Some fruit drop may result from feeding
  • Fruit may be smaller and there may be reduced fruit bud production

Monitoring:

• Hot dry weather favours population explosions
• Dusty conditions may add to large mite population problems
• Drier areas of the field will have more damage
• It is important to start scouting early
  • Look for stippled/yellowed leaves
  • Check the undersides of leaves for webbing or for the presence of mites
• Breathe on leaflets or tap leaflets over a white sheet of paper to allow counting
• General economic threshold = 20 percent of leaves infested
  • At this point, control measures should be undertaken

Management:

• Use chemical control products that are registered for mites, as not all insecticides will kill mites (they are not actually insects)
• The introduction of predatory mite species, as well as the encouragement of natural enemies, can reduce populations
• Ensure that plants have adequate water, so as to minimize plant stress
• Eliminate weeds that might serve as alternate hosts
• Cultivate in the fall to disturb overwintering sites and reduce survival