Thistle Caterpillar

Figure 1

Figure 2

Thistle caterpillar (Vanessa cardui) has been an occasional pest of sunflowers. Sporadic outbreaks in the Prairies have resulted in localized damage of sunflower crops.

Host Crops

Larvae (Figure 1) feed primarily on Canada thistle foliage, leaving the stem and midrib. Feeding temporarily inhibits the weeds but new growth usually develops in the fall. They also feed on about 60 other hosts, including sunflowers and canola.

Biology

Adults, commonly know as painted lady butterflies (Figure 2), migrate into the Prairies from overwintering sites in Mexico, arriving in early June. There is no evidence that they can survive our cold winters. This butterfly normally prefers to lay eggs on Canada thistle plants but under some conditions, will lay eggs on other plants as well. Larvae feed on the leaves producing a loose webbing.  The larvae are up to 30 millimetres (1.25 inches) long and dark purple to black in colour. They have long spines on each segment of the abdomen.

June	July	August	September
Butterflies arrive and egg laying and larval feeding begins	Larval feeding continues, pupation starts	Pupation continues, new butterflies emerge	Populations die-off

Scouting Techniques

While scouting sunflowers, if populations seem heavy, sample about 100 scattered plants, noting the per cent defoliation on each.  Divide the total per cent defoliation by the number of plants sampled to obtain an estimate of per cent defoliation for the field.

Economic Thresholds

The threshold is 25 per cent defoliation in sunflowers provided that most of the larvae are still under about 3 centimetres (1 1/4) inches) long.  If the majority of larvae are fully grown, most of the feeding damage will have already occurred.

For further information, contact your GO representative.