onion thrips
Beginner
Scientific Name: Thrips tabaci
Identification
- Small (< 3mm), soft-bodied insects
- Cream to light brown in colour
- Thrips have sucking-rasping mouth parts and cause tissue damage when they feed on the leaves
- Feeding removes chlorophyll, leaving silvery spots on the leaves
Often Confused With
Leafminer damage
Botrytis leaf blight
Period of Activity
Thrips overwinter on grasses (especially winter wheat) and alfalfa. They migrate into fields in late May, early June. Thrips favour hot, dry weather.
Scouting Notes
Monitor for thrips as you walk through the field. Sample 25-50 plants across a representative section of the field. Record the number of thrips per plant, making sure to pull onion leaves apart to reveal the youngest leaf tissue.
Thresholds
The threshold for dry cooking onions, leeks and Spanish onions is 1 thrips per leaf.
Advanced
Scientific Name: Thrips tabaci
Identification
Thrips are a small soft-bodied insect, less than 3 mm (0.12 in.) in length - about the width of a pencil lead. They are creamy to light brown in colour. With a hand lens, their four slender, fringed wings can be seen folded across their backs. Without magnification, thrips can be identified by their small size and rapid movement. Because of their small size they can be easily overlooked. It is important to examine the leaves closely, paying particular attention to the leaf axils.
Often Confused With
Leafminer damage
Botrytis leaf blight
Biology
In Ontario, both adults and nymphs overwinter on winter grains, clover and alfalfa. Once in an onion field, thrips seek cover between onion leaves, where they feed and lay eggs. Females insert white, bean-shaped eggs into the leaf tissue. The eggs hatch in 5 to 10 days. The nymphs grow in size and pass through four distinct stages. Development form egg to adult requires from 10 to 30 days, depending on temperature. Once mature, females begin to lay eggs. The females reproduce asexually (without mating). Consequently, increases in the thrips population can occur very rapidly, especially during periods of hot, dry weather. Eggs, nymphs and adults may be found together at any time during the summer and produce many overlapping generations.
Period of Activity
Early in the season, thrips prefer grasses, especially winter wheat, alfalfa and clover. They can also be found in abundance along roadside ditches. As the weedy roadsides dry up and the winter wheat and alfalfa are harvested, thrips migrate into onion fields.
Scouting Notes
Monitor for thrips as you walk through the field. Sample 25-50 plants across a representative section of the field. Record the number of thrips per plant, making sure to pull onion leaves apart to reveal the youngest leaf tissue.
Thresholds
The threshold for dry cooking onions, leeks and Spanish onions is 1 thrips per leaf.
Management Notes
- Early detection of thrips is key to successful control. Use high water volumes when spraying to ensure insecticides penetrate into the leaf axils.
