Identification of damage caused by field thrips in field crops

Field thrips affect pea and field bean crops from early emergence throughout the growing season. They feed inside the tightly rolled leaves of the growing point. They also damage sugar beet foliage. 

• Periods of slow growth in cold, dry springs, especially on stony soils, increase risk 
• In good conditions, crops usually tolerate damage and grow away 

Scientific name: Thrips angusticeps 

Adult field thrips and pea thrips are indistinguishable without microscopic examination. They are dark, shiny, and narrow-bodied, and reach about 2 mm in length. Two pairs of wings, usually folded along the back, are used to migrate large distances.  

Eggs are minute, kidney-shaped and are embedded into the tissues of flowers and pods. The immature stages are similar in shape to the adults but have no wings and are bright yellow with a conspicuous black tip at the rear end. 

Oct–Feb: Overwinter in soil as short–winged, flightless adults 

Mar–May: Emerge from soil to feed on young crops 

May–Sep: Adult field thrips fly to other crops  

Mottled patches and distortions on leaf surfaces are a symptom of thrips damage.  

Beans: The leaves may appear shiny and speckled, with sooty black markings. The undersides of bean leaves develop a rusty-brown discolouration.  

Peas: Field thrips cause the foliage to thicken and pucker, with a translucent spotting developing on the surface of leaves.  

In many situations, peas and beans outgrow the initial attack. However, occasionally, when the attack is severe, peas may produce blind shoots that form no flowers, develop multiple secondary shoots, and develop as small bushy plants. This is called ‘pea dwarfing syndrome’ and plants will not recover fully. 

Bean leaves may die off completely and severely arrest the growth for a week or two.  

Non-chemical control 

In high-risk areas, sow late-emerging crops. Thrips are predated by spiders, ladybirds, predatory flies, and lacewings. 

Monitoring 

Crops should be monitored frequently, from emergence. Carefully unfold the leaflets of affected seedlings. In peas, the period from pod emergence until the pods are full is important. Examining late-emerging bean crops is often unnecessary, as they usually escape damage.  

Thresholds 

Treatment in peas and beans is justified as soon as damage is seen. In beans, since most damage occurs while the crop is young, treatment after mid-May is not worthwhile.  

Insecticide resistance 

There is no evidence of resistance in this species.