Data from: Mechanisms underlying spruce budworm outbreak processes as elucidated by a 14-year study in New Brunswick, Canada

We conducted a 14-year intensive study of spruce budworm (Choristoneura
fumiferana (Clem.)) survivorship at three study plots in largely balsam
fir (Abies balsamea (L.) Mill.) stands in New Brunswick, Canada, to
elucidate certain key mechanisms underlying spruce budworm outbreak
cycles. The study covered a peak-to-declining phase (from 1981 and 1994)
of the budworm outbreak cycle that had started in the early 1960s.
Frequent sampling was carried out in each plot-year to construct a
practically continuous survivorship curve, and the annual variation in
population density was estimated. We found a high level of correlation
between the studied phase of the outbreak cycle and annual variations in
the survivorship over the postdiapause period, suggesting that
postdiapause survivorship was the chief determinant of the cycle. We found
the annual changes in population density in the present study to be
closely similar in pattern to those from the provincial budworm surveys
conducted in much larger areas. This implies that the mechanism underlying
the population process found in the few study plots in largely balsam fir
stands also applies to the process in much larger areas of diverse stand
types. The main source of postdiapause mortality is found to be natural
enemies. The impacts of parasitoids and disease are evaluated by rearing
budworm samples in the laboratory. Hymenopteran and dipteran parasitoids
are by far the major sources of mortality, and microsporidians are the
most prevalent pathogen. Occurrences of other entomopathogenic fungi and
viruses were insignificant throughout the study. Seasonal changes in
laboratory survivorship are compared with the corresponding field
survivorship to estimate the effect of predation. No major mortality
factor is found to singly play a predominant role in determining the
outbreak cycle. Conversely, some minor factors are shown to have played
significant roles. Thus, the importance of recognizing the action of
natural enemies as a complex is emphasized for understanding the budworm
outbreak cycle. Finally, centered around the roles played by the
chronological succession of natural enemies in the present study, the
results of budworm research in New Brunswick since the mid-1940s are
synthesized to outline basic mechanisms underlying the outbreak processes
as a guide for further studies.