Defoliating insects feed on the leaves or needles from trees. They can be beneficial or harmful to the health and productivity of forests. Integral to all forest ecosystems, they feed on coniferous and deciduous hosts. They are significant forest pests in forests across Canada. In B.C., several major defoliator species concern forest managers.

Depending on the duration and the severity of defoliation, tree growth may be stunted. Top-kill and eventual tree mortality can result. Feeding is often most severe on understory or suppressed trees, so defoliating insects may serve as natural thinning agents. In some ecosystems, maintaining understory is vital to maintain a continuous crop of trees.

Tree mortality due to defoliator feeding can contribute to forest succession. Meanwhile, harvesting, regeneration, and stand-management activities can affect the interactive competition, population fluctuations, and spread of defoliator populations.

Another aspect of defoliation is nutrient cycling in the form of dead insects, insect frass, and foliage dropped to the ground from wasteful feeding. Damage due to defoliation may have significant effects upon: 

  • The continuous availability of timber
  • Aesthetics in parks and recreational areas
  • Property values where standing live trees are integral components of the property
  • Watershed and wildlife values
  • Human health

Early detection and identification of an outbreak, and identification of the causal insect, are critical initial steps in managing forest defoliators. Projected damage may be severe enough to warrant treatments, either directly through application of suitable insecticides, or indirectly through harvesting or silviculture treatments.

Be sure to consider potential defoliator outbreaks throughout all phases of forest management. Defoliators should be considered at all levels of planning and when creating prescriptions, even though they may not be treated in all situations. 

Important defoliators in B.C. that are actively managed are highlighted in the following sections.

Western spruce budworm (WSBW or IDW)

The western spruce budworm is an important native defoliator of Interior Douglas-fir. Periodically, population levels reach outbreak proportions. In 1987, more than 800,000ha were under attack.  Aerial spray programs have been conducted to reduce defoliation.  

Douglas-fir tussock moth  (DFTM or IDT)

The Douglas-fir tussock moth is a destructive native defoliator of Douglas-fir. Outbreaks of tussock moth occur every 10 to 12 years, causing significant damage and mortality to stands in B.C.'s Interior. The hairs on the caterpillars can threaten the health of humans and domestic animals.

Western hemlock looper (WHL or IDL)

The western hemlock looper is a native defoliator of western hemlock, western redcedar, Douglas-fir and Interior spruce. This insect periodically reaches outbreak levels, causing severe damage.