Plant care guide
Thrips on Alocasia: Damage Signs and Treating Moss or LECA
Updated
Why does my Alocasia have thrips?
Thrips on an alocasia leave pale stippling that turns silvery or bronze, with tiny black tar-like specks of excrement, often near the midrib and on new leaves. Their non-feeding pupal stages can drop into the growing medium, so moss, LECA or soil can carry the next generation even after the leaves are clean.
What to do first
Isolate the plant, confirm thrips with a white-paper shake and a hand lens, rinse both leaf surfaces, and cut off the worst leaves. Follow with an insecticidal soap or horticultural oil labeled for indoor plants, repeated. Replace the old medium, and keep checking new leaves and sticky traps for weeks.
Thrips on an alocasia are discouraging, because they hide and come back. A single plant caught early can be cleared, though. It takes repeated rounds and attention to the pot, not one spray.
University of Maryland Extension lists the aroid family, including Alocasia, among the indoor plants thrips commonly attack (UMD: thrips). NC State Extension’s Alocasia page flags spider mites as the more usual alocasia pest. So confirm what you have before you treat.
Signs on an alocasia
- Stippling. UMD describes pale flecks that turn silvery, or yellow and then bronze, before drying out. Heavy feeding merges into thin, silvery patches.
- Black specks. The RHS says silvered areas are usually marked with many tiny black excrement spots (RHS: thrips). UMD calls them tar-like and glued to the leaf.
- Crinkled new leaves. UMD says leaves fed on while still expanding can stay stunted, crinkled or curled.
- Where to look. UMD points to young leaves, leaf margins, along the main vein, and where leaf stalks join the stem.
Confirm before you treat
Thrips are 2 mm or less, slender and shaped like a grain of rice. UC IPM suggests shaking foliage over white paper to dislodge them (UC IPM: thrips). Clemson Extension suggests blowing gently on leaves, which makes thrips run (Clemson HGIC 2252). Use a hand lens.
Spider mites also cause pale stippling. Clemson says thrips damage looks similar, and that mites leave a silky web in heavier infestations. Black excrement specks point to thrips. See the alocasia spider mite guide for that comparison.
Why the medium matters
Thrips develop through two feeding larval stages, then two non-feeding stages: prepupa and pupa. UC IPM says these pale stages of most species drop to the soil or leaf litter, or lodge in plant crevices. The RHS says they take place in the soil and/or sheltered places on the plant. UMD adds that juveniles pupate on leaves, in the soil or in leaf litter.
That is why leaves can look clean while new adults keep appearing. Our sources do not discuss sphagnum moss or LECA specifically. Treat any loose medium as a possible pupation site.
UMD says repotting indoors might remove pupae in the soil, if you remove all the old mix and replace it. It also warns that repotting does nothing for thrips still on the plant. UMD notes that mixes high in organic matter, like compost, may favor thrips.
What to do
- Isolate the plant. Clemson advises keeping a suspect plant away from other houseplants until the pest is controlled, which may take several weeks or more.
- Rinse it. UMD suggests a strong jet of plain water on both sides of each leaf. Clemson also lists rinsing leaves first.
- Cut the worst leaves. UMD and UC IPM both advise trimming badly damaged or infested parts. Damaged tissue does not recover.
- Replace the medium. Remove the old soil, moss or LECA and repot in fresh medium. We have not found extension guidance on cleaning used LECA, so fresh medium is the safer choice.
- Spray with a low-toxicity product. UMD names horticultural oil and insecticidal soap. Clemson says soap works only on contact and stops working once dry. UC IPM says sprays must cover the shoot tips and other parts where thrips feed. Use a product labeled for indoor plants, and UMD warns against household soap or detergent. Clemson suggests testing on a small part of the plant first.
- Repeat. UMD says repeat applications are needed because missed thrips reproduce quickly. Follow the label interval.
- Monitor. UC IPM suggests bright yellow sticky traps for adults. The RHS uses blue sticky traps to trap and monitor thrips under glass.
UC IPM says spinosad is generally more effective than soaps and oils. It also says imidacloprid commonly fails against thrips. Check that any product you buy is labeled for houseplants indoors.
How long to keep checking
UMD says egg-to-adult takes about a month near 16 °C (60 °F), and two weeks or less near 29 °C (85 °F). Adults may live a month or more, and females can reproduce without mating. Keep checking new leaves and traps for at least a full cycle after you last see thrips.
UMD also says a heavily infested plant is often best discarded. Clemson agrees for plants that are severely damaged and not valuable: discard the plant and its soil.
Keep them out
Clemson advises isolating new plants for six weeks and checking them about once a week, especially leaf undersides. UC IPM notes overwatering and excess nitrogen fertilizer can raise thrips numbers. UMD says high humidity may discourage most thrips indoors.
For more on alocasia care, see the alocasia hub, alocasia humidity requirements and alocasia brown spots. Our houseplant thrips article covers other hosts.
References
How this guide was put together: University of Maryland Extension, UC Statewide IPM Program, RHS, Clemson Cooperative Extension and NC State Extension guidance.