Spruce spanworm

Systematic position. The spruce spanworm (Semiothisa signaria) belongs to the order Lepidoptera, family Geometridae. This moth species is a significant defoliator, known for its specific dietary preferences among coniferous tree species in northern forest ecosystems.

Crops damaged. The larvae, or caterpillars, primarily target spruce (Picea) and fir (Abies) trees. While they often occur in natural forests, they can also cause economic damage in nurseries, ornamental conifer plantings, and commercial timber stands where population densities can reach critical levels.

Biology and life cycle. The species undergoes a complete metamorphosis, including egg, larval, pupal, and adult stages. Moths typically emerge and mate during the summer. Eggs are laid on needles, where larvae hatch and begin feeding immediately. The pupal stage is generally spent in the forest floor, providing protection during the dormant season.

Character of damage and harm. Caterpillars feed on needles, often resulting in jagged edges or complete defoliation of shoots. Heavy infestations can lead to significant canopy thinning, reduced tree growth, and loss of photosynthetic capacity. Such stress makes trees highly susceptible to secondary attacks by bark beetles or wood-boring insects.

• Monitoring adult populations using pheromone traps to predict infestation severity.
• Applying biological insecticides (such as Bacillus thuringiensis) to target larval stages.
• Using systemic or contact chemical insecticides when larval populations exceed economic thresholds.
• Enhancing biodiversity in plantations to encourage natural predators, including parasitic wasps and insectivorous birds.
• Removing heavily infested or dying trees to maintain overall forest health.