Thrips (Thysanoptera) are small, slender insects with ravelled wings, from which comes the scientific name Thysanoptera: a combination of the Greek thysanos (ravel) and pteron (wing).
Other common names for thrips are storm flies and storm bugs. Thrips feed on sap from the cells of various plants (and on some animals) by pricking holes in the cell and sucking out the contents. Thanks to this, many species of thrips are regarded by farmers as pests because they damage plants. Other species of thrips feed on the fluids of other insects or mites and are regarded as farmers’ friends, while the rest of the species in the family feed on the fluids of moulds. So far we know of around 5000 different species.
Thrips are very small (<1 mm in diameter) and cannot fly very well, although by using thermals and wind they can be found in large numbers at very high locations. Under optimal conditions thrips can reproduce and multiply very quickly to the point that they’re seen as more than just annoying but an infestation to be dealt with.
This creature has mouthparts that stab and suck which it uses to pierce and suck dry cells, thanks to which the cell dies. Unlike lice they do not release any poisonous fluids into the cell but they do introduce air. After a short while the plant will begin to take on a gray, dried out appearance because its cells are being sucked dry. These insects can move location easily and as a result are capable of causing considerable damage in a short space of time.
The thrips lays its eggs in leaf tissue, and they hatch after 8 days. The young larvae are whitish in colour and live together in groups. At a later stage of the lifecycle they spread out over the whole leaf. After 14 days they begin their pupation.
The most common species of thrips are the Striped Greenhouse thrips (Parthenothrips dracanae) and the Tobacco thrips (Thrips tabaci). The Californian thrips is especially difficult to combat thanks to its good resistance to various insecticides.
Californian thrips are mostly found in greenhouses, but during the summer they can also cause problems outside. It is unclear whether they can survive European winters outside though. The Californian thrips generally pupates underground or in sheltered places, but sometimes also on leaves and in flowers. The lifecycle lasts 2 to 3 weeks when temperatures are between 20-30°C. Californian thrips is a carrier of TSWV (Tomato Spotted Wilt Virus) affecting various plants, including chrysanthemums and amaryllis.
Characteristics of Californian thrips
- about 1.3-1.4 mm in size
- adult thrips are yellow to orange in colour
- adult thrips are mostly found in the uppermost sections of the plant
- their antennae have eight segments (you can check with a magnifying lens)
- flower heads and growth shoots can grow seriously deformed
- if there are buds or flowers present, the thrips can also transport pollen to them
- pupation generally takes place underground
- does not enter diapause (hibernation in adverse conditions)
- can bring viral infections with them
For a start it is most important that you raise moisture levels up to around 75-80% because then at least the infestation will spread more slowly; thrips does not like high moisture levels. This on its own will not be enough to rid yourself of the critters. But washing the plants with a solution of gentle liquid soap (20 g per litre) and 10cc of methylated spirit Is a simple remedy. If this doesn’t give the desired result the following techniques might help:
If you want to try a biological method you can work with a predatory bug. You can buy these from specialist companies. They feed on insects including thrips and mites, and these are the most effective:
- Amblyseius cucumeris: a light brown, very mobile hunting mite that prefers to work in high air moisture and a temperature around 25 degrees. The cycle of egg to adult takes (depending on the temperature) 8-11 days. This predatory mite lives for 18-21 days and takes the thrips while they’re in their larval stage and sucks them dry. This is also why it’s worth releasing some of these predators as a preventative measure in every infestation and/or with each new planting – in fact, it’s a recommendation that you do.
- Amblyseius degenerans: a predatory mite that’s a bit bigger than its 'sister' Cucumeris. The lifecycle is comparable with that of the Cucumeris. Degenerans is more mobile, less sensitive to temperature and thrives better in lower humidity. Degenerans is more common than Cucumeris in the buds and catches thrips in the same way as Cucumeris. The higher mobility, lower sensitivity to temperature and air moisture content and ability to hide itself in the flower/bud makes Degenerans somewhat more successful / effective than Cucumeris, especially in low humidity conditions. The drawback is that Degenerans is harder to raise and therefore more expensive and not always available.
- Orius laevigatus: a fast, dark-brown predator bug, 1-3mm in size with characteristic red eyes. The cycle from egg to adult takes about 2-3 weeks; the adult bug lives for three to four weeks. The speed of this cycle as well as their lifespan are dependent on the temperature. In association with Degenerans or Cucumeris, Orius is capable of keeping the thrips population down to acceptable levels, or even eradicate them completely. It’s highly recommended that you release Orius at the first sign of thrips. When there are no more thrips left to hunt, Orius will also eat other insects in the grow room, such as red spider mite, aphid and white fly. What’s more, Orius is the only natural enemy that besides attacking the larvae also preys on the adult thrips. Orius sucks the prey dry and is a ravenous and aggressive hunter that has even been known to hunt prey without bothering to eat them afterwards.
- Chrysopa carnea: a lacewing, the lava of which is a very aggressive ‘suck ‘til dry’-style hunter. These larvae are very active for 12-13 days and in this timeframe can make significant inroads in to infestations of the pests they prey on (whitefly, thrips and aphid). Although Carnea is less active against thrips and is mostly used for controlling aphid populations, the larvae of this lacewing species are frequently applied too, as Carnea can deal outstandingly well with fluctuations in temperature and air moisture. Preventative release of this lacewing does not make sense though. Only release them when pests are visible, as the larvae are very mobile and will wander off if not released close to a population of prey.
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