Q What are thrips?

A They are sap-sucking insects that attack many plants and are commonly known as thunder flies or thunder bugs. Roses, onions, chrysanthemums and carnations are commonly attacked outdoors. Thrips can also be troublesome under glass. Flowers and flower buds often support huge numbers. As they need warmth and dryness, they are a summer problem.

Caption: Thrips are a problem outdoors during the warm summer months

Q What do thrips look like?

A They are tiny (less than 2mm long) narrow insects. Depending on the species they may be yellow, black or brown. Each thrip has a pair of tiny wings, but you need a magnifying glass to see these. The female lays 60-80 tiny eggs in slits she cuts in the leaves and flowers. They don't always need to mate to lay viable eggs. The larva is a tiny (about 0.4mm), wingless version of the adult and takes only 10-14 days to reach the pre-pupa stage. They either drop to the ground and form a pupa or pupate on the leaves. Adults emerge, ready to breed, in 4-11 days. Under glass, generation follows generation all through the year as long as it is warm enough. Outdoors, there may be only one or two generations per year.

Q How do I recognise thrips' damage?

A As thrips feed, they pierce the leaves and flowers. The damaged areas dry out, and become speckled and silvery. Petals often turn brown. The insects are very small, and as they shelter beneath leaf sheaths and in buds they are seldom seen.

To find them, hold a sheet of white paper beneath the plant and tap the stems sharply. The thrips fall on to the paper where they can be seen clearly. (Be warned: some people’s skin can be irritated by thrips.)

Pulling back leaf sheaths of gladioli and onions will often reveal thrips sheltering in these sites. They also carry tomato spotted wilt virus – this is a problem with chrysanthemums.

Q What could I mistake thrip damage for?

A Spider mites and leaf-hoppers will damage foliage in a similar way. If webbing can be seen beneath the foliage, look closely for tiny spider mites. Leaf hoppers are much larger, greenfly-like insects, 5mm long, with prominent eyes and back legs. They can be found lurking beneath the foliage.

Q What kinds of thrips might I come across?

A It is hard for gardeners to tell the different types apart. The following are common species:

Onion thrips (Thrips tabaci) is very common, with a wide host range. Outdoors it can be a serious pest of onions, leeks, brassicas and tomatoes. It can survive the winter in crop debris, onion necks and in the soil.

Pea thrips (Kakothrips pisivorus) are especially common on garden peas and broad beans, during dry, hot summers, mainly in southern England. The pods and young leaves become silvery. When the attacks are severe, the plants are stunted and no crop forms. Numerous thrips are usually present. They overwinter as young thrips in the soil. They emerge in May and June as mature adults and lay eggs in and around the young flowers. The immature forms hatch after about a week, and feed for three to four weeks before seeking the soil to overwinter.

Gladiolus thrips (Thrips simplex) survives from year to year in corms that have been lifted for winter storage. They are unlikely to survive in the soil. After hatching, the thrips feed on the stored corms, first as larvae, then as pupae and finally as adults. If the stored corms are warm enough there may be several generations. Other bulbs, freesias for example, are also attacked.

After planting, thrips spread up the growing plant in leaf folds or flower buds. They continue breeding while it is warm. Later they shelter in the soil and on the corms. When the corms are lifted and stored the cycle begins again.

Rose thrips (Thrips fuscipennis) attacks rose and shrub flowers and leaves. If you find spotted, streaked and rotten flowers from spring to August, check for thrips.

Privet thrips (Dendrothrips ornatus) need a dry, hot summer to thrive. They damage lilac and privet hedges, causing the leaves to become grey and even fall. The immature thrips are yellow and narrow. The adults are winged, dark brown or black with a red band. Fortunately, the affected plants usually recover.

Q What can I do about thrips?

A Often heavy rain will wash them off the plants. In dry spells, overhead watering will have the same effect. In fact, well-grown outdoor plants, with plenty of water, resist attacks much better than those in greenhouses. This makes thrips much less of a problem outdoors than in greenhouses.

Q Can thrips be sprayed?

A Yes. However, thrips are very often resistant to insecticides. Try spraying with one based on fatty acids or soaps. These are contact insecticides that kill only the pests they come into contact with, so you need to spray the plants carefully to get good control.

Q Are there biological controls for thrips?

A There are biological controls available for use indoors, but they are unlikely to be useful outside.

Q Can thrips be avoided?

A No. They are such common and mobile pests that there is no way of avoiding them.

Q What can I do with plants infested with thrips?

A It is safe to compost infested plant material – the thrips will have left the plants to overwinter in the soil. Digging over the soil will help to reduce the number of viable thrips left.