Intensive feeding results in a silvery-white stippled appearance sometimes referred to as white blast or silver top. Seedlings may be killed if large numbers of thrips are present. Feeding occurs on the interior of the onion neck, on developing leaves. The early bulbing stage is the most susceptible. Where many thrips have fed, the discolored areas coalesce to form large brownish, blister-like areas.
Severe infestations can lead to underweight and misshapen heads.
Damaged tissue used for sauerkraut turns into dark blotches during processing. If possible, avoid planting cabbage, cauliflower or onions close to and immediately downwind of small grains or alfalfa. Choice of variety can also help. In cabbage, onion thrips tend to prefer varieties with very tight head, but host plant resistance tests have not always been consistent. Eastman et al.