Thrips are a small insect that is common outdoors and in greenhouses. It often gets into homes through plants bought from nurseries or the garden trade.
Thrips damage plants by sucking their juices and scraping at fruits, flowers and leaves. Plant leaves may turn pale, splotchy, and silvery. Injured plants are twisted, discolored and scarred. Thrip damage rarely leads to the death of the plant, but it does weaken it and reduce vigour, and as new growth is a prime target for thrips a bad infestation can lead to plants that are unsightly and damaged looking.
Adults are very small (less than 1/25 inch) straw-colored or black slender insects with two pairs of feathery wings. Without the use of a hand lens, they resemble tiny dark threads and have a distinctly cigar shaped body.
Thrip damage to mature foliage looks like a silver discolouration and is often accompanied by black spots on the leaf. The black spots is actually the thrip 'frass' (their droppings).
New growth on a plant is often damaged while still developing, and new leaves that emerge twisted, misshapen or with dead areas is a classic sign of thrip activity on the plant.
Unlike other pests like spider mites, thrips rarely kill a plant outright unless left to completely get out of hand. But the damage is unsightly and should be treated to keep your ornamental plants looking their best.
The life cycle of a thrip consists of five stages - egg, larval, prepupal, pupal and adult. Female adult western flower thrips live up to 30 days and lay 2-10 eggs per day. At 20°C, development from egg to adult takes approximately 19 days.
Adults can be winged and so can spread to other plants nearby or in the same room or greenhouse.
When a thrip reaches the prepupal stage it will drop to the soil and pupate, before emerging as an adult. This means it is extremely difficult to ever completely get rid of thrips once they have a hold in a collection - their lifecycle and lifespan is long, they lay eggs constantly as adults, they pupate in the soil so can escape treatments of the leaves.
It is extremely difficult to entirely get rid of thrip once they have established. However it is possible to control their numbers and minimise any damage to a collection.
Always quarantine new plants away from your collection for at least two weeks, and thoroughly inspect the plant for damage both in the shop before purchase, after getting it home and before moving it into a room with other plants. Look for the characteristic silvery damage and for other pests like spider mites, mealybugs or scale.
After inspection, rinse the foliage of the plant with firm blasts of water to dislodge any hidden insects or eggs.
Diatomaceous earth (DE) is a naturally occurring sedimentary rock that crumbles into a fine white powder. Diatomaceous earth consists of fossilised remains of diatoms, a type of hard-shelled single celled organism. When small insects come into contact with DE, the particles act like tiny knives, piecing their soft bodies and causing them to die. Sprinkling DE on the soil of your plants will interrupt the lifecycle of the thrip by killing the pupating stage (instar). Used in addition to some of the other methods on this page it can be highly effective at controlling thrip numbers.
Diatomaceous earth is completely harmless to humans and animals and only affects small insects.
A horticultural oil, such a Neem or Peppermint oil can be used to control thrip. It can be bought in solution or mixed in a 1:100 ratio to water with some dish soap to break down the soil so it mixes with the water.
Important: Some plants are sensitive to oils and to soap - so google the plant first or try the solution on a small part of a leaf, wait 24 hours and check for discolouration or browning.
Spray the plant thoroughly. Oils work on contact with thrip or their eggs or larvae. Get it into all crotches and folds in the plant. Leave for 10 minutes and then rinse the plant down with water.
It's important to repeat this every 3 days for two weeks to effectively control thrip. When the numbers are controlled you can consider replacing the top cm to inch of soil in the plant pot to try and remove any pupating thrips.
Spinosad is an insecticide that is effective in the control of thrips and is relatively non-toxic. However it is not available in some countries and is a controlled substance in many others. Always follow the directions on the container.
Releasing beneficial insects into a collection serves two purposes - firstly they often attack stages of the pest that are hard to see with human eyes such as the pupal or larvae stages of thrip. They also work around the clock and with bad infestations can rapidly reduce pest numbers.
Lacewing larvae are natural predators of thrip larvae and other soft bodied pests such as whitefly eggs, aphids, scale insect, mealybug and young caterpillars. They are able to eat over 300 pests each, so they are ideal for clearing heavy infestations. If you dislike insects predatory mites are extremely difficult to see so may be better - the lacewing larvae is a few mm long and so visible with the human eye.
Amblyseius mites (Amblyseius cucumeris, Amblyseius swirskii) are tiny mites that feed on the juices of a number of thrip larval stages. They are typically sold in slow release foil pouches that contain a mixture of live mites, mite eggs and feed for the mites or in containers which can be shaken out onto the foliage and around the base of the plant. They live on the leaves and stems of the plant so do not attack the pupal stage of the thrip. Studies have shown that Amblyseius swirskii is the more effective thrip predator.
Macrocheles robustulus (Mighty mite) or Hypoaspis miles are mites that live in the soil and feed on the pupal stage of the thrip.
These mites are effectively invisible to the naked eye and can be effective ways of controlling thrips.