Forest Tent Caterpillar (Malacosoma disstria)

Biology & Life cycle:

• Insect pest native to North America
• Attacks a wide range of hardwood tree species – trembling aspen, white birch, oak, ash, maple, as well as other broadleaf deciduous trees (e.g. Maydays)
• Based on historical data, typically occurs in outbreaks lasting 2-4 years, approximately every 10 years
  • Duration of outbreaks is affected by weather, parasitoid/predator populations and the structure of the host forest/stand
• Adults are brown/tan coloured moths with 3 darker bands across the forewings
  • Adults actively fly in late June to early August (depending on the region), and may be carried on wind currents several hundred kilometers
  • Lifespan of the adult moths is less than 2 weeks
  • Single generation per year
  • Emerge, mate and lay eggs
    • Typically in mid to late-July on Prairies
  • Eggs are laid in small bands that completely encircle twigs
    • Egg bands are located in the upper fringes and periphery of the canopy
    • Egg bands are covered with a foamy, sticky protective coating
      – appears shiny at first
    • Bands can be ½ inch to 1 inch (10-20mm) long
    • Larvae do not emerge until the following spring
  • Larvae emerge from egg bands in spring, as leaves open on trees
    • Larvae (caterpillars) are dark brown with broad graying-blue bands along the side of the body, with white, key-hole shaped marks in a row along the back
    • Larvae are hairy and reach about 2 inches (50mm) in length (at maturity)
      • Early larvae appear black and hairy, without major distinctive colouration initially
    • Larvae feed for about 6 weeks, following which they spin a yellowish cocoon to pupate

Monitoring:

• Fall or early spring monitoring can allow for detection of egg masses
• Regular monitoring can help with early detection of nest formation or groups of larvae
• Masses of larvae on tree trunks or in branch crotches may be highly visible
• Defoliation will become apparent and can be widespread (covering many, many acres)

Management:

• Natural factors can help to reduce population levels
  • Late spring frosts that damage foliage, combined with high populations can cause mass starvation
  • Very high temperatures in summer when moths are flying can reduce life span and reduce egg laying
  • Parasites can parasitize cocoons and their young will feed on pupae
  • Feeding by predators will keep populations low
• Egg bands can be removed and destroyed in fall or early spring, in small scale situations
• Masses of larvae may be remove by clipping the visible groupings, or by brushing off the masses from trunks
  • Masses may be drowned in a water/bleach/soap mixture
• Chemical sprays applied in late-May or early June to control earlier instar larvae, prior to major defoliation
  • There are a number of different sprays registered, including several biological controls (containing Bacillus thuringiensis kurstaki – Btk)
    • Biological controls (like BTK) must be applied to the foliage for larvae to eat to be effective
    • Contact sprays must hit larvae – masses of larvae can provide some protection to inner larvae, making these sprays less effective
    • Spray thresholds are suggested to be when the number of egg bands on a tree exceeds the trunk diameter of the tree in centimeters at 1.5m above the ground