Soybean Pest Alert - Thistle Caterpillars

There seems to be a sporadic outbreak of thistle caterpillars this year in many soybean fields across the province. Growers are encouraged to scout their fields to assess any damage. The following bullets provide some information about the insect.

• Thistle caterpillars are the larvae of the painted lady butterfly. The butterflies migrate from the south each spring to lay their eggs on host plants. The caterpillars are approx 1¼ inches long and are black and yellow with spines running along their sides and backs.
• The larvae prefer to feed on Canada thistle foliage, but will also feed on other host crops such as soybeans.
• The caterpillars form webs by tying leaves together with silk, which provides them with a protective environment to feed and grow. Large amounts of frass (excrement) can often be found within the webbed leaves.
• The life span of the caterpillars is about 2 – 4 weeks until pupation in which the cocoon hangs from the host plant until the butterfly emerges 7-10 days later. There can be two generations per year.
• Economic thresholds are 20-25% defoliation of the soybeans foliage.

Image caption: 

Thistle Caterpiller

Image caption: 

Thistle Caterpiller leaf damage

For more information contact:
Steve Hamill
Cereal & Oilseed Development Officer
PEI Department of Agriculture
Phone: (902) 314-0233
Email: slhamill@gov.pe.ca