| | 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
Symptoms:
- 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
Monitoring:
- 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
Management:
- 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
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Western Flower Thrips nymph |
Photo by: Whitney Cranshaw, Colorado State University, Bugwood.org |
Symptoms:
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)
Monitoring:
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)
Management:
- 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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