Basic biology and management of onion thrips on field crops

In the UK, onion thrips particularly affect leek, salad onion, stored cabbage and sugar beet. Obvious thrips presence and feeding damage can make crops unacceptable for market. Onion thrips are a difficult target for insecticides that work by contact action. 

• Warm, dry conditions promote fast larval development and are ideal for flight
• Very mild winters may favour the continued development of onion thrips inside stored cabbage maintained in ambient stores

Scientific name: Thrips tabaci

Adults are approximately 1 mm in length. They are usually brown, with two pairs of wings, fringed with long hairs.

Eggs are minute (0.3 mm long), kidney-shaped and white/yellow in colour.

Larvae are yellow/cream and start around 0.5 mm in length.

Oct–Apr: In the UK, onion thrips overwinter in the adult stage and prefer host vegetable crops, such as leek. However, they will also overwinter in other locations, such as on winter cereals.

May–Sep: Once temperatures rise, female thrips lay eggs on either the overwintering host (if it remains a suitable food source) or moves to an alternative host.

May–Sep: Following egg hatch, there are two active larval stages and two inactive stages (pre-pupa and pupa). Pupation usually takes place in the ground.

A generation (egg to adult) takes about 52 days at 12.5°C and 15 days at 25°C.

Feeding by adult and larval thrips damages the host plant via direct removal of cell contents. As individual plant cells are killed, scarring of the leaf is observed in the form of silvering. On cabbage, feeding by thrips can result in small, brownish-grey growths on the leaf surface, as well as silver-coloured lesions.

Non-chemical control

There is a consensus (across many countries) that irrigation helps reduce thrips populations.

In some situations, resistant onion varieties provided more effective control than insecticides in the USA. However, there has been no comprehensive survey of resistance in UK-grown cultivars.

Intercropping in onions has reduced infestations by at least 50% in some trials. Leek under-sown with clover has also shown excellent potential for control. For both approaches, competition between the crop and companion plants may limit their usefulness, along with associated costs.

Although thrips control has been achieved with predators, this is mainly in protected crop environments, with augmentative or inundative techniques. The technique has not been evaluated outdoors in the UK. The potential of entomopathogenic nematodes has been investigated, but no evidence of thrips control was found in leek crops.

Monitoring

Adult onion thrips can be monitored most effectively with blue or white sticky traps. It may be sufficient to trap thrips at one or two locations within a region, as the pattern of activity appears to be consistent in any year. Direct crop examination of damage (which is easier than counting thrips) is also worthwhile. Inspection of young leaves provides an indication of recent/current activity.

Thresholds

Although traps provide an early warning of activity, they do not provide a reliable prediction of infestation severity.

Insecticide resistance

Resistance to pyrethroid insecticides in field populations of onion thrips has been confirmed in the UK.