Insect of the Month - Different Thrips species

  Hort Snacks - October 2018
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 Onion Thrips | Eastern/Western Flower Thrips

Onion Thrips

Thrips tabaci

Crops Affected:
Onions & related bulb vegetables (Allium spp.), beans, cole crops, cucumbers, peas, peppers, squash, tomatoes; can overwinter in a number of other host crops (winter wheat, alfalfa, etc)

Life Cycle:

  • Thrips will feed on multiple host species (polyphagous)
  • Thrips are pale coloured (yellow to brown), thin cigar-shaped insects, with finely haired, narrow wings
    • Nymphs are pale yellow and resemble adults without wings
  • Adults overwinter on plant and plant parts (weeds, standing winter crops, bulbs in storage, field refuse) in fields or surrounding areas
  • Adult females lay eggs under the surface of host leaf tissues as the weather warms
    • This can be directly in the host crop of concern or on other adjacent host crops prior to migrating into the other host later
  • Populations develop from the field border inward, following prevailing wind patterns
  • Nymphs hatch from the eggs
    • Initially, nymphs tend to say near the base of the plant where leaves are close together
    • Gradually they move out over the leaf surfaces, feeding by rasping tissues and sucking plant juices
    • Nymphs pupate in the soil
  • The duration of all developmental stages are dependent on temperature
  • Thrips will move between crops when host crops are harvested
  • Thrips can move long distances by staying airborne on winds
    • They are weak flyers
  • Thrips feed by piercing the leaf tissues with their rasping mouthparts and sucking up plant juices
    • Damage manifests as silver streaking on leaves, which develops into white patches as streaks join together
    • Tips of leaves may die back or the entire crop may appear “blasted” in hot, dry conditions
    • Bulbs may be smaller or plants may die off
  • Cool, wet weather and rain can reduce damage and population levels
  • Remove the leaves from a number of sample plants and wash or shake to get numbers of adults
  • Yellow, white or blue sticky traps can also be used to trap
  • Trap or collect samples weekly in hot, dry weather, particularly in mid to late-summer and/or when nearby host crops have been harvested (resulting in migratory introductions)
    • Economic thresholds vary from 3-5 adult or nymph thrips per plant for a 20 plant sample on a large scale planting
  • Avoid growing host crops near to other harbour host crops (e.g. onions near alfalfa, winter wheat) to prevent migration after harvest
  • Heavy overhead irrigation can reduce populations of thrips (similar to rainfall)
  • Bury in-field overwintering sites, including plant debris and culls, at the end of season
    • Headlands and grasslands adjacent to fields can be cultivated to bury overwintering thrips
  • Some natural predators will attack thrips, but will not likely significantly manage larger populations of thrips
  • Apply registered chemical controls at appropriate stages (see product labels for details)

Eastern/Western Flower Thrips

Frankliniella tritici, F. occidentalis

Crops Affected:
strawberries, raspberries (and other caneberries), cucumber, tomato, peppers (field and greenhouse), weeds, tree fruit, ornamentals

Life Cycle:

  • Thrips are tiny, cigar-shaped insects
    • Adults are brown to yellow, with fringed wings
    • Nymphs are wingless, pale yellow and speedy
      • Nymphs pupate in the soil
  • Eggs are laid inside plant tissues
  • Thrips tend to hide in protected places in the plant (flower clusters, plant crevices, etc.)
  • Thrips often migrate in on southerly winds during the growing season and are not known to overwinter outdoors
  • Thrips may enter greenhouses via vents or doorways or may move on infested plants, soil, equipment, tools and clothing
  • Thrips may have more than one generation per growing season
    • The rate of development of thrips is strongly tied to temperature and humidity
      • The rate is faster in warmer and drier conditions
  • Thrips can be a vector for the transmission of Tomato Spotted Wilt Virus in tomato and pepper crops in some areas
Western Flower Thrips nymph
Photo by: Whitney Cranshaw, Colorado State University,
Berry fruit
  • Feeding by rasping/sucking mouthparts damages the affected areas
  • Berry tissues may crack and split (under fruit calyx) and become scarred and discoloured
    • Symptom is referred to as “bronzing”
    • Feeding damage on 1-2cm berries
  • Berries tend to be reduced in size, seedy, cracked and somewhat brown in colour
Other Crops
  • Feeding by nymphs and adults results in silver-white streaks, specks or striations on plant tissues (leaves, blossoms and fruit)
    • Brown fecal matter (frass) may also be visible
  • Photosynthetic ability and yields is reduced
  • Fruit of some crops may be deformed (e.g. cucumber fruit)
Berry Crops
  • Count the number of thrips at various locations in the field, at bloom or in small fruit stages (1cm)
    • Collect 10 flower buds from each location or 50 random fruit
    • Put flower buds or collected berries in plastic bags and use heat to kill thrips, making counting easier
    • More than 10 thrips per blossom or 0.5 thrips per berry necessitates treatment
Other Crops
  • Blue (or yellow) sticky traps can be positioned throughout the crop
    • Want immature thrips populations of less than 10 per leaf (check a minimum of 25 leaves per 2000m2)
  • Repeated introduction of biological control agents are necessary to keep thrips populations in check
  • Cover vents with fine mesh to prevent introduction into greenhouse
  • Clean up plants and disinfect greenhouse between crops
    • Heat greenhouse for several days after crop is removed
  • Chemical controls can be used, but tend to be difficult to apply due to where thrips feed
    • The sensitivity of predatory insects and the tendency of thrips to develop chemical resistance are other issues
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on September 24, 2018.