Appearance and lifecycle
The Beet Webworm has been a sporadic pest in western Canada affecting a wide variety of vegetable and field crops. The adult moth is 20 mm long with a 25 mm wing-span. They are triangular in shape when resting and the people often say that they look like a little stealth fighter plane. There is a distinct white margin at the end of the wings.
There are normally two generations per year. The first generation of moths appear mid-May to July and the second from mid-July to September. The first generation of moths lay eggs on hosts crops and larvae appear in late June and early July. Once the larvae are mature they drop to the ground, spin long tubular cocoons and pupate just below the surface. Second generation moths emerge in mid-July to September and lay their eggs. The larvae hatch, feed and enter the ground to over winter.
Newly hatched larvae are pale green and 1-2 mm long. At the end of development larvae are 25-40 mm long, olive green becoming black as they get older. Mature larvae have two white or cream-colored stripes on either side of a black center line and two rows of paired circular marks down either side of the back.
Food hosts
Larvae feed on a wide range of broadleaf weeds and crops. These include lamb’s quarters, Russian thistle, sugarbeets, canola and flax. They have also been found on sweet clover, alfalfa, mustard, sunflower and various vegetables.
Damage
Larvae begin to feed on leaves, then move to the stems and pods. They strip the surface tissue giving the plant a whitish appearance.
Thresholds
There is no official threshold for beet webworm in most field crops. During outbreaks in the past the threshold for bertha armyworm in canola has been used.
It is worth noting that there are a number of parasitic insects that attack the beet webworm. During the 2003 outbreak very few acres required chemical control even though the moth numbers were high in the spring.
Chemical control
Control should occur only if an economic threshold is reached and ideally before larvae are more than half grown. For chemical control options always consult the current Crop Protection Guide.
Picture identification
Prepared by Kent MacDonald, Ag-Info Centre, Alberta Agriculture and Rural Development |