This page has two distinct sections. The first is a pictorial review of the life history of the Douglas-fir Tussock Moth in British Columbia. The second section summarizes the management strategies that have been developed in British Columbia for this insect. To go directly to the management section click here, otherwise enjoy the pictorial review.
|When you see a small patch of severely defoliated Douglas-fir trees in the fall where all ages of needles have been stripped from the branches, you could be looking at a current infestation of the Douglas-fir tussock moth.|
A close inspection of the lower branches should reveal the overwintering egg masses of the Douglas-fir Tussock Moth (DFTM). The flightless female lays her egg mass in August close to the cocoon from which she emerged. The eggs are the overwintering life stage. In late May/early June the larvae emerge and spin down on long silken threads. The gentle spring breezes waft the larvae away on their silk thread. The fortunate ones become entangled in a Douglas-fir tree and commence to feed on the new foliage.
The young larvae are wasteful feeders and their random chewing of the needles results in brown tips broadly spread among the branches. The tussocks along the back of the larvae develop gradually as the larva moults from one instar to the next.
The mature caterpillar is more than 3 cm long. It has 4 white rust-tipped tussocks along its back. There are two pencil like tufts of black spines that point frontwards from the thorax and a single tuft of black spines that points posteriorly from the end of the abdomen. The long fine hairs protect the larva from predation from other insects such as ants.
A close up view of the tussocks shows also the strong white spines that surround the red lateral spots along the body. More about these later.
How does a large caterpillar control its long body on a branch? All caterpillars have special prolegs on the abdomen that are used to grip the branch or needles. Here you see a view of the 4 pairs of prolegs on the underside of a DFTM caterpillar. The prolegs have chitinous hooks or crotchets that help to grip smooth surfaces. Note the 6 'true' legs on the right, 4 pairs prolegs on the left plus a pair of posterior terminal claspers.
|When the caterpillar is fully fed, it is ready to begin the transition to the adult stage. The caterpillar lays down a layer of silk and then begins to weave a cocoon over itself.|
|As the larva turns inside this cocoon, its spines stick through the silk and snap off. These very fine hairs and spines are mostly trapped trapped within the silk. The irritant chemicals on the hairs will provide protection to the pupa inside the cocoon just as they protected the caterpillar. The fine hairs can also become airborne and people coming in contact with the irritant chemicals on these spines can develop a serious itchy skin disorder known as tussockosis.|
|While the cocoon is being spun, most of the larval spines are broken off and the prepupa is almost naked compared to the larva which began the construction.|
|The completed cocoon on foliage.|
|The prepupa stage moults into a pupa. Note the considerable sexual dimorphism of the pupae. The male is smaller and has visible wing development. The female pupa on the other hand is much larger, black in colour and without wing pads.|
|The emerged female has no wings. She stays close by the cocoon and attracts her mate by releasing a powerful pheromone (sex attractant).|
|The male has well formed wings. The pair of plumose antennae on the head carry thousands of tiny chemoreceptors. The male rests on the foliage until he detects the pheromone of the female. He then flies upwind searching for the female and if he arrives first, he will be able to mate with the female.|
|Once mated, the female begins to lay her eggs around the cocoon that protected her as a pupa. The spines that protected the caterpillar have also protected the pupa and the adult. Finally, the eggs are laid upon the cocoon and they too will be protected. The female does not eat or drink as an adult. All the energy reserves she uses for transformation to the pupa, emergence as an adult, and in laying her eggs were derived from the voracious feeding in the larval stage.|
Management of the Douglas-fir Tussock Moth:
The linkages provided herein are to the B.C. Ministry of Forests Forest Practices Code Booklets. Take the time to fully explore this information as it represents the current best assessment of the management strategies for this insect. Table and figure numbers refer to those in the Defoliator Management Guidebook rather than following a normal sequential order in this text.
Distribution of the Douglas-fir Tussock Moth
Detailed review of outbreaks since 1916 shows that the DFTM periodically reaches outbreak levels in the IDF zones in the Okanagon Valley, Kamloops and Cache Creek areas (Fig. 1). DFTM outbreaks often predispose the trees to successful attack by the Douglas-fir beetle.
Summary of population dynamics
Hazard Rating of stands for the Douglas-fir Tussock Moth Stand conditions that have been demonstrated to support DFTM outbreaks are summarized in Table 9. Mature forests on hot dry sites, where the stand has a mature component of Douglas-fir and a mixed sized understorey, seem to favour outbreaks. Most outbreaks are terminated by the outbreak of an epizootic nucleopolyhedrosis virus (NPV) after the DFTM has stripped the trees of their foliage and the insects are stressed by starvation.
Predictive sampling techniques and population monitoring
Long-term management strategies
See especially the summary in Table 10.
The schedule of events to detect, monitor, and manage Douglas-fir Tussock Moth are summarized in Table 11.
John A. McLean
February 10, 1997