Do natural enemies explain fluctuations in low-density spruce budworm populations?

Area of Science:

• Ecology and Forest Science
• Insect Population Dynamics
• Pest Management

Background:

• Understanding forest insect outbreak triggers is crucial for resource management.
• Studying low-density, pre-outbreak insect populations is challenging due to sampling difficulties.
• Spruce budworm (Choristoneura fumiferana; SBW) outbreaks significantly impact forest ecosystems.

Purpose of the Study:

• To investigate the roles of parasitism and predation in fluctuations of low-density SBW populations.
• To determine if these factors explain large-scale variations in SBW abundance over time and space.

Main Methods:

• Laboratory-raised SBW larvae were deployed across a 1,000 km gradient in Québec over 11 years.
• Parasitism and predation rates were monitored.
• Spatial covariance analyses and piecewise structural equation models were used to analyze data.

Main Results:

• Parasitism rates by two main species fluctuated asynchronously, suggesting compensatory dynamics.
• Substantial, large-scale spatial correlation was found in interannual parasitism rates, but not predation.
• Temporal variations in parasitism and predation did not significantly influence temporal SBW abundance.
• Warmer locations showed higher parasitism, correlating with lower wild SBW populations.

Conclusions:

• While large-scale drops in parasitism can occur and potentially fuel SBW increases, they did not explain population variation during this study.
• Weather effects on larval development and indirect effects via host tree physiology/phenology are more likely drivers of large-scale SBW population dynamics.
• Findings highlight the complexity of insect outbreak drivers, emphasizing environmental factors over direct biotic interactions in certain contexts.