Budworms, beetles, and wildfire: disturbance interactions influence the likelihood of insect-caused disturbances at a subcontinental scale

Irruptive forest insects are a leading biotic disturbance across temperate and boreal forests. Outbreaks of forest insects are becoming more frequent and extensive due to anthropogenic drivers (e.g., climate and land-use), perhaps increasing the likelihood that forests will experience multiple insect-caused disturbances.
Across the fire-prone Douglas-fir forests of western North America, recent outbreaks of the western spruce budworm and Douglas-fir beetle have impacted large expanses of forests, with a higher degree of overlap than expected in some ecoregions. Outbreaks of both insects are positively related to host availability and exhibit density-dependent population dynamics that are affected by climate and weather.
Here, we leverage data from aerial detection surveys, estimates of host availability, climate and weather, and categorized fire severity to describe the spatial overlap between western spruce budworm and Douglas-fir beetle and assess: 1) how climate and host availability..., Aerial surveys– To characterize the spatiotemporal patterns of Douglas-fir beetle and western spruce budworm outbreaks, we used aerial detection survey (ADS). ADS data are collected by aerial observers in fixed-wing aircrafts and include the likely damaging agent (species of insect/disease or abiotic factor), binned estimates of damage severity, and forest type. ADS data are the most accurate data describing the spatiotemporal patterns of specific forest insect outbreaks at a broad spatial extent across the western US; a recent assessment of the survey program found that 85% of randomly selected Douglas-fir beetle and 100% of western spruce budworm polygons were correctly identified based on ground-truthing in Oregon, California, and New Mexico (Coleman et al., 2018).
We summarized ADS data by calculating the presence/absence and severity of Douglas-fir beetle and western spruce budworm within a 1 km2 grid. First, we listed presence when a grid cell contained any polygons or points that..., , # Budworms, beetles, and wildfire: disturbance interactions influence the likelihood of insect-caused disturbances at a subcontinental scale.

[https://doi.org/10.5061/dryad.bcc2fqzns](https://doi.org/10.5061/dryad.bcc2fqzns)

## Description of the data and file structure

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Overview—We compiled spatially and temporally explicit data (Table 1) to describe the drivers of Douglas-fir beetle and western spruce budworm outbreaks and test for potential disturbance interactions among wildfire, Douglas-fir beetle outbreaks, and western spruce budworm outbreaks. Because the outbreak distribution of insect herbivores is constrained by bioclimatic factors, we first developed a model to describe how host availability and climate predispose forests to outbreaks of Douglas-fir beetle or western spruce budworm (Outbreak Distribution). Second, because inter-annual variation in weather is known to influence insect outbreaks, we developed a model to describe the inciting factors (i.e., weather) of out...