Douglas-fir tussock moth — biology and history

Tussock moth outbreaks begin as localized epicentres, spreading and coalescing into larger areas of defoliation. Areas of tussock moth defoliation are very elevationally delimited, occurring in low-elevation stands only. The delimited pattern of defoliation is largely due to:

  • The inability of flightless adult females to disperse
  • The windblown dispersal of 1st and 2nd instar larvae

By combining the historical occurrence of outbreaks, both in terms of area affected and periodicity of outbreaks, with stand parameters influencing hazard, the relative risk in a particular area can be estimated.

The outbreak cycle

The building phase of a tussock moth outbreak takes one to two years. Detection of increasing insect populations during the building phase is critical, and unless detected at this stage, significant damage could occur. High population levels persist for one to four years, then collapse due to natural control agents which include parasites, predators (mainly birds and ants), pathogens, and starvation due to the forced consumption of older, less nutritious foliage.

Another factor in the collapse of the population is caused by a species-specific NPV (nucleopolyhedrosis virus), which is always present in the population at low levels. The virus is spread through insect-to-insect contact, causing populations to decline rapidly. Six to eight years elapse before populations again reach damaging levels.

Population density, year in the outbreak cycle, and the current incidence of disease in the population will affect next year's damage levels. Egg sampling can be used to predict the level of defoliation for the coming year, but this level will be reduced if the outbreak is in its third or fourth year. If dead larvae are commonly found, or if egg masses are small, distorted and incompletely covered with hairs, the population is infected with virus and no significant additional defoliation will occur.