Onion leaves damaged by thrips

Thrips

Learn how to tackle thrips on your houseplants and in your greenhouse, with the help of our guide expert.

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Thrips are tiny insects, also known as thunderflies. Not all thrips are harmful to plants, but there are several different species of thrips that can cause damage to foliage and flowers on a wide range of plants, both under cover and in the garden.

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How to identify thrips

Thrips are tiny, up to 2mm long. In their immature stage, thrips are flightless and whiteish-yellow in colour. Adult thrips have two pairs of wings and are brownish-yellow or black-brown in colour. Both adults and the immature thrips, known as nymphs, feed on the upper surface of leaves and on flowers. Thrips are hard to see with the naked eye and may need identifying with a magnifying glass. In the greenhouse or conservatory, you can hang yellow sticky traps among plants to assess whether thrips (and other) pests are present, and whether action is needed.

Damage caused by thrips

Close up of thrips damage on onion leaves
Close up of thrips damage on onion leaves

Leaves eaten by thrips often first become dull or matt in appearance, later developing a silvery mottling on the upper side and may become distorted. On close inspection the leaves are likely to be dotted with black. Flowers develop a slightly ‘streaked’ or flecked appearance, may have a distorted shape, or fail to open.

Plants affected by thrips

Several species of thrips are specific to a plant genus or group, including gladioli, honeysuckle, onions, peas, and privet. Other species such as Western flower thrips attack a wide range of plants including chrysanthemum, fuchsia, pelargonium, and verbena, as well as tomato and cucumber. Thrips can be a problem year-round under cover and on garden plants in the warmer months.

How to deter thrips

Thrips thrive in hot, dry conditions so boosting humidity levels helps discourage this pest. In the home or greenhouse, stand pot plants on saucers or trays of pebbles and water, or mist frequently with tepid water. In the greenhouse or conservatory damp down the floor by spraying with water in the morning and afternoon. Place capillary matting on benches which retains moisture.

Good hygiene helps prevent pest problems. Once or twice a week, go over indoor plants and remove all dead or fallen leaves and flowers which can harbour pests. Thoroughly clean out greenhouses and conservatories once or twice a year, washing the glass inside and out, including surfaces and staging, and fumigate the structure using a chemical-free product.

Take care to buy only healthy plants to avoid bringing the pest into your home and garden.

How to kill thrips without chemicals

Under cover, a biological control – which is the use of a specific living organism, such as a parasitics wasp, to predate on a pest – is an environmentally friendly and effective way to tackle pests. This can only be used in any indoor environment, not outdoors. Thrips can be combatted with several species of predatory mite including Amblyseius. Biological controls can’t be used as a preventative measure as the pest must be present for the control to work. Predator populations can take several weeks to build up so don’t expect immediate results.

Chemical-free (organic) sprays that are based on surfactants or fatty acids also kill thrips. These have a short period of persistence so will need several applications. Do not use sprays if applying a biological control as it would also kill the beneficial predatory mites.

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Chemical controls to kill thrips

The systemic neonicotinoid insecticide acetamiprid is available to combat thrips. However, this type of insecticide is harmful to bees and other beneficial insects and is best avoided wherever possible. Always follow instructions when using chemicals and do not spray plants in flower as beneficial insects, bees and others are likely to be killed. Apply sprays late in the day when insects are less active.