Population Dynamics of the Spruce Budworm Choristoneura Fumiferana

Abstract

Using the latest observations, experiments, and theoretical studies, I have reanalyzed spruce budworm data from the Green River Project, and now propose a new interpretation of the population dynamics of the species. Spruce budworm populations in the Province of New Brunswick have been oscillating more or less periodically for the last two centuries, the average period being °35 yr. Local populations over the province tend to oscillate in unison, though their amplitudes and mean levels are not always the same. The local population process in the spruce bud worm is composed of two major parts, a basic oscillation, and secondary fluctuations about this basic oscillation. The basic oscillation is largely determined by the combined action of several intrinsic (density—dependent) mortality factors during the third to sixth larval instars. These factors include parasitoids and, probably, diseases (e.g., microsporidian infection), and, most important, an intriguing complex of unknown causes, which I term "the fifth agent" (a large number of larvae with no clear symptoms died during the population decline in the late 1950s). Other mortality factors, including predation, food shortage, weather, and losses during the spring and fall dispersal of young larvae, are not causes of the basic, universally occurring oscillation. Because of immigration and emigration of egg—carrying moths, the ratio of all eggs laid to the number of locally emerged moths (the E/M ratio, or the apparent oviposition rate) fluctuates widely from year to year but independently of the basic oscillation of density in the local populations that were studied. The fluctuation in this ratio is the main source of the secondary fluctuation in density about the basic oscillation, and is highly correlated with the meteorological conditions that govern the immigration and emigration of moths. The E/M ratio is the major density—independent component of budworm population dynamics. Contrary to common belief, there is no evidence to indicate that invasions of egg—carrying moths from other areas upset the assumed endemic equilibrium state of a local population and trigger outbreaks. Moth invasions can only accelerate an increase in a local population to an outbreak level, but this happens only when the population is already in an upswing phase of an oscillation caused by high survival of the feeding larvae. In other words, the "seed" of an outbreak lies in the survival of feeding larvae in the locality, and moth invasions can act only as "fertilizers." The weight of evidence is against the idea that an outbreak occurs in an "epicenter" and spreads to the surrounding areas through moth dispersal. Rather, the egg mass survey data in New Brunswick since 1952 favor an alternative explanation. If the trough of a population oscillation in a certain area stays comparatively high, as in central New Brunswick in the 1960s, or if the area is more heavily invaded by egg—carrying moths when the populations in that area are in an upswing phase, these populations might reach an outbreak level slightly ahead of the surrounding populations, all of which are oscillating in unison. If a local population oscillates because of the action of density—dependent factors intrinsic to the local budworm system, it may appear to be difficult to explain why many local populations over a wide area oscillate in unison. However, Moran's (1953) theory shows that density—independent factors (such as weather) that are correlated among localities will bring independently oscillating local populations into synchrony, even if weather itself has no oscillatory trend. I illustrate this with a simple time—series model. The same model also illustrates a principle behind the fact that outbreaks occurred fairly regularly in New Brunswick and Quebec during the past two centuries but rather sporadically in other regions of eastern Canada. Finally, I review the commonly accepted theory of outbreaks based on the dichotomy of endemic and epidemic equilibrium states and argue that the theory does not apply to the spruce budworm system.