THE ROLE OF CLIMATE AND DISPERSAL IN THE INITIATION OF OUTBREAKS OF THE SPRUCE BUDWORM IN NEW BRUNSWICK: II. THE ROLE OF DISPERSAL

Abstract

In Part I of this paper consideration was given to the role of climate in the initiation of outbreaks of the spruce budworm in New Brunswick. Analysis of the available weather data showed that the 1912 and 1949 outbreaks developed after several consecutive dry summers. Support was given to the theory of climatic release, which explains the time and place of outbreaks on a climatic basis. However, the recorded history of the spruce budworm also shows that high populations appeared in New Brunswick shortly after "spreading" through Quebec, and this suggests that the New Brunswick outbreaks are also a continuation of this spread. In the present part of the paper consideration is given to the role of dispersal. Moth dispersal is a more effective agent of spread than larval dispersal. Moths may be transported by convectional and turbulent air currents for long distances. Light traps used to detect the incidence of moth movements, showed that large segments of a population may be transferred from one area to another. Unspent females often predominate in these movements. Moth invasion was not detected before the 1949 outbreak although there is evidence from other sources that it occurred in 1948. When deposited in dense, mature, softwood stands, the moths can create outbreaks, but when deposited in young, open, or mixed-wood stands the ensuing high populations soon decline unless bolstered by repeated invasions. Populations in New Brunswick showed gradual and general increases as early as 1947. It is thought probable that these increases resulted from the build-up of local populations through climatic release. The nearest highly-populated centers were over 100 miles to the west in 1947. Later, invasion of moths from centers outside of the Province may have hastened the process.