Thrips have become a serious pest of plants grown in glasshouses or indoor heated cultivation. The most common and harmful species is the Western flower thrip Frankliniella occidentalis. This thrip does not go into diapause in the winter and will keep reproducing if temperatures are high enough. Thrips will also develop quicker on flowering plants, as they can feed on pollen. Thrip adults are very small and pencil shaped. Their life cycle develops through six stages; egg, two larval, pre-pupa, pupa and the adult.
Thrips cause damage by piercing the surface of leaves and flowers sucking out the contents causing the surrounding tissue to die. This leaves silver grey mottled areas on leaves and white distortions on flowers. Growth is reduced and leaves and fruit can shrivel. Thrips can also pass on plant viruses when they feed.
Adult thrip and damage on flowers
Biological control of thrips involves combining a range of beneficial insects and predators to protect plants from damage and stop populations developing. Measures to protect plants can be taken from early on in the growing cycle. The first and simple step is to place out yellow or blue sticky traps to monitor and catch adult thrips. This gives an early warning of activity and can help reduce egg laying. There is also a thrip pheromone lure that can be hung on a sticky trap to attract higher numbers of adults to the traps.
The next step is to protect young plants from attack and kill off any pupae in the compost or growing media. There are two predator based products that can help with this; Mighty Mite, which contains a soil dwelling predatory mite which feeds on any small pupae or larvae it finds in growing media. The other product is the Thrip Killer Propagation stick. These are supplied as small sachets on a stick, each containing several hundred Amblyseius cucumeris predators. These tiny predators are released from the bags over a couple of weeks ensuring predators are on hand to feed on any small thrip larvae, if they appear on young plants and seedlings.
Amblyseius cucumeris predatory mite
Once plants are larger they also need protecting from fresh thrip attack. One sachet of Thrip Killer predators should be hung on each plant. This will ensure predatory mites are on the plants all the time. This is important as these predators only eat the very small larvae and will not feed on thrip adults. In the summer months it is advisable to switch to another predatory mite called Amblyseius swirskii. This thrives in warmer temperatures and will eat larger thrip larvae and whitefly eggs. It is available in slow release paper sachets or new foil sachets, which last longer and produce more predatory mites.
Amblyseius swirskii sachet
If thrip adults are present on plants, the predatory bug; Orius, should be introduced, these predatory bugs feed on the whole thrip life cycle including the adults but can be difficult to establish if there are low numbers of thrip. It takes time for them to build up in a crop and begin overcoming infestations. It is important to remove sticky traps once Orius is present, as they will get caught on traps.
Thrips are a difficult pest to control, so use all the biological tools available to control them!