Thrips on Plants

Symptoms

Thrips feeding scars fruit with scabby, silvery to dark brown discoloration. The same kind of discoloration can also show up on leaves and petals. It can also leave dark specks of excrement on fruit or leaves. Shoot tips and leaves often become distorted, curled, or galled. Severely affected shoot tips and leaves can die back completely. On the leaf surface, you may see stippling or discolored flecking. The whole leaf can take on an overall silvery look. Feeding that kills petal tissue before a bud opens causes a color break. That color break shows up as a pale or dark discoloring of the petal. On rose petals specifically, that feeding injury shows up as dark streaks and spots. Fruit scars from thrips often look dark at the center, surrounded by a lighter halo. Flowers and foliage can also show general streaking and distortion. In some cases, affected buds fail to open at all.

Causes

Overwatering your plants can increase the number of thrips you see. Applying nitrogen fertilizer can increase thrips populations in the same way overwatering does. Shearing a plant stimulates a flush of new growth. That new growth is especially easy for thrips to feed on. Weeds growing near your plants can act as alternate hosts for pest thrips. In other words, those weeds give pest thrips another place to live close to your garden. Sun-loving plants that end up growing in too much shade can become stressed. A plant under that kind of stress becomes more susceptible to thrips damage than it would be otherwise. Overwatering, nitrogen feeding, shearing, nearby weeds, and too much shade are the causes named here. Those are five separate causes that can lead to a thrips outbreak. Both the weeds nearby and too much shade can make a plant more prone to thrips.

Diagnosis

Small dark flecks of excrement on leaves or fruit are a clear sign of feeding. Stippling, flecking, or a silvery sheen on the leaf surface is another sign of feeding. Scabby, silvery to dark brown discoloration on fruit, leaves, or petals also points to thrips. Distorted, curled, or galled shoot tips are a sign too, and new growth is especially susceptible to this damage. Rose petals may show dark streaks and spots from thrips feeding. Petals killed by feeding before a bud opens show a pale or dark color break. In some plants, buds never open at all. Thrips-damaged fruit often has a dark scar with a paler ring around it. Streaking and distortion across flowers and foliage point to thrips as well. A sun-loving plant growing in too much shade is more susceptible to thrips damage. Nearby weeds that act as alternate hosts for pest thrips can also keep the problem going.

Treatments

Prune off plant parts that are declining, injured, or infested. Cut just above branch crotches and nodes instead of shearing off terminals. On Indian laurel fig, prune off galled, rolled terminals in winter. This can reduce thrips damage on that plant the next summer. Time avocado pruning to January to cut down on fruit scarring. On citrus trees, prune the interior to help predatory mites build up in the canopy. Avoid overwatering and skip the nitrogen fertilizer. Keep plants well irrigated instead. Control nearby weeds that act as alternate hosts for thrips. Remove and dispose of old, spent flowers, since they can harbor thrips. Use resistant plant varieties where they are available. Cover small plants with row covers or cages to keep out thrips and other flying insects. A layer of reflective mulch on the soil can repel flying thrips. Prune off colonies of greenhouse thrips when you find them. Avoid persistent pesticides so you protect the parasites and predators that attack thrips. Natural enemies such as green lacewings, minute pirate bugs, mites, and parasitic wasps all help keep plant-feeding thrips in check. On citrus, Euseius mites are important predators of citrus thrips. Contact insecticides with azadirachtin can also help control thrips. Horticultural oil, insecticidal soap, or pyrethrins give temporary control, especially for greenhouse thrips. Spinosad is a foliar insecticide that moves a short distance into sprayed tissue. It can keep working for about a week or more. Avoid spraying spinosad on flowering plants. A trunk spray or injection of a systemic neonicotinoid insecticide gives fairly fast control. Dinotefuran is one option professional applicators use for good control of thrips. A soil application or trunk spray is a better choice than injecting or implanting the tree. Hold off on systemic insecticides until the plant finishes its seasonal flowering, to protect pollinators.

Prevention

Keeping plants well irrigated, rather than overwatered, helps keep thrips numbers down. Skip extra nitrogen fertilizer too, since it is linked to higher thrips numbers. Control weeds near your plants, since they can act as alternate hosts for thrips. Remove and dispose of old, spent flowers, which can harbor thrips. Choose resistant plant varieties where you can find them. Row covers or cages over small plants keep thrips and other flying insects away. Covering bare soil with reflective mulch is another way to repel flying thrips. Prune off declining, injured, or infested plant parts as you spot them. Make cuts above a branch crotch or node rather than shearing off the terminals. Shearing stimulates new growth that is especially susceptible to thrips.

Plants most often affected

Thrips feed on a wide range of garden and landscape plants. Common hosts include roses, citrus, avocado, blueberry, grape, strawberry, apple, nectarine, and raspberry. Impatiens, petunia, azalea, photinia, rhododendron, myoporum, Indian laurel fig, toyon, orchid, and African violet are also on the list. Bean, garlic, onion, and pepper round it out.

• Aji
• Arabian Coffee
• Aztec Marigold
• Belladonna
• Christplant
• Common Lilac
• Common Milkweed
• Daffodil
• Dracaena masoniana
• Eastern Teaberry
• Eggplant
• Field Pumpkin
• Fragrant Dracaena
• French Hydrangea
• Garden Cosmos
• Garden Cucumber
• Garden Onion
• Kidney Bean
• Lantana
• Lilac Chastetree
• Monstera
• Muscadine
• Nightblooming Cactus
• Octopus Tree
• Pothos
• Purpletop Vervain
• Rugosa Rose
• Sacred Thorn-apple
• Silver Cock's Comb
• Snake Plant
• Stinging Nettle
• Strawberry Tree
• Sweet Bay
• Tuberose
• Weeping Fig