Spatiotemporal Patterns of Eastern Spruce Budworm in Maine

This layer shows all 602 budworm trap sites that were placed within the forest.

Three traps were placed per site and were pulled after a period of time.

When pulled, the moths that were caught in the traps were counted and an average of all three traps was taken to display the data we see to the right.

The counties layer and the budworm trap layer were dissolved together, with the join coint showing traps per county.

Higher counts are shown in purple, while smaller counts are shown in lighter shades.

The multivariate cluster tool was used to generate clusters based off of similar population trends from 2014-2023.

Clusters are not in sequential order, but rather formed with features expressing similar population trends over the 10 years studied. 

Cluster 3 has the fewest features but the highest moth counts, so these areas should be monitored closely.

White Spruce has a somewhat even spread, with slightly higher concentrations occurring in the northern part of the state.

Black spruce tends to prefer cool upland soils, but is also found along streams and bordering swamps.

The highest concentration of black spruce occurs in the northwest part of the state right near Canada.

The most abundant spruce variety in the state, it prefers well-drained, rocky upland soils.

The highest concentrations of this species can be found in the areas surrounding Baxter State Park, which is home to Mt. Katahdin, the highest peak in Maine.

Buffered zones of 100m, 500m, 2000m in MFS-1515 shows significant increases where spruce-fir dominant forest is also present. Especially in 2000m zone at the border with Canada, abundant spruce-fir biomass is largely accessible to grow a SBW population.

This region is also adjacent to the Canadian outbreak, providing easy access for large moth populations.

Similarly, the result in this location indicates a budworm growth despite being in the no data (black) pin. The close proximity and SBW trap locations to spruce-fir dominant forest shows the correlation the high increase of SBW.

IRV - 1511 shows SBW increase greater than 50 , even among less dominant host trees.

This could be due to there being 3 pheromone traps per moth site and obtaining the moth average; we do not have data on where they were actually placed, so they may have ended up in a singular red or orange pixel stand of dominant spruce-fir, despite the majority of pixels being non-dominant.

The top 5 locations that report significant increase are spread out in Northern Maine. We have focused on places in the western portion, where host tree composition data was available.

SI-1614 lacks data in many areas. These areas could represent water or areas of deciduous trees, both being unfavorable conditions for the budworm to thrive.

This location had an almost total annihilation of SBW, despite having spruce-fir dominant forest in its immediate 100m buffer.

The neighborhood forest stands are composed of less spruce-fir biomass, and possibly SBW do not have enough food to grow its population.

The consistent low host biomass in this trap location (PC-512) show decrease from 61.7 to 6 SBW on average in 2023. It confirms the thesis that SBW will not flourish in forests low in balsam fir and spruces species.

The spatial distribution of all decreases and the top 5 decreases were spread randomly through the north and central state of Maine.

While we identified the top 5 sites with decreases, let's compare the forest composition of some additional traps that had less significant decreases.

LV-1214 had a year over year decrease, but less significant. This could indicate an established population that can endure several years of growth and contraction. There are enough host trees in the 100m and 500m buffer for the moths to feed on.

Areas such as this should have more monitoring stations placed to track this potential established moth colony.

These locations are defined by having a 2023 moth count greater than 50, and also having had a total moth count increase greater than 50 from the previous year.

Four locations of further interest were identified, which all occurred in the northwestern-most region of the state.

The trap shows increase in mature forest location where canopy is the thickest.

Reason for irregularity in this location might be data discrepancies of 2021 and budworm averages of 2022-23, where black areas might be related to timber harvesting.

Huge increase of SBW in this site where mature forest is present, and nearby forest is also mature, prone to SBW defoliation. Might be the area of focus for targeted mitigation practices.

Significant decrease of budworm may be due to the fact that there is non-mature forest to live on.

The drop budworm average in 2023 in mature forest (SBW favorite forest type) might be because the traps are placed among mature deciduous or mixed forest trees, which are non-host species.

Despite being in a region of mature-dominant forest, we saw significant decreases at SI - 126. These decreases could be due to other factors such as elevation or climate.

Similar unexplained decreases in areas of mature-dominant forest are seen at PC - 512.

Southern and coastal Maine gets more mild summers and winters from the influence of the Atlantic...

In Map 5 we specifically only isolated increases and displayed every year we have SBW moth counts for. It’s clear the SBW has a strong preference for cooler temperatures in northern Maine around Aroostook County.  To quote a paper from Elsevier, “Defoliation intensity can be lower with warmer minimum spring temperatures when the temperature increase is insufficient for the larvae to end their diapauses [hibernation/arrested development].” As the region continues to see temperatures rises, might we see a continued northern-migration of the SBW, or will they adapt with the changing climate?

...whereas the more northern continental climate of Maine has mild summers but colder and snowier winters. The northward colder regions of Maine are where we see the SBW reside.

For this series of maps, we isolated for increase/decrease in SBW from one year to the next using a tipping point of 50 moths counted per trap. In 2016 there were no increases from under 50 in 2015 to over 50 in 2016.

But in 2017 we see several locations along the Maine/Quebec border with rises above our 50-count threshold.

It’s clear the SBW has a strong preference for cooler temperatures around Aroostook County.

To quote a paper from Elsevier, “Defoliation intensity can be lower with warmer minimum spring temperatures when the temperature increase is insufficient for the larvae to end their diapauses [hibernation/arrested development].”

As the region continues to see warmer springtime temperatures, might we see a continued northern migration of the SBW, or will they adapt with the changing climate?

As for population fluctuations attributable to maximum temperature, these results are inconclusive. It seems regardless of springtime temperature variations over 2016-2023, the increase/decrease is more oriented with northward continental position than it is with warmer/cooler areas of the north.

Here we are looking at all 602 moth trap sites to get an idea of the wide net that has been cast.

These pinpoints are areas where there's been a consistent growth in population over 3 years (18, 19, 20).

And notice the preference for much lower elevations.

Overlaying the DEM is a tree species DTM (Digital Terrain Model) that is isolating only for Balsam Fir, the tree of choice in Maine for the SBW. The legend shows percent of on-the-ground biomass coverage, and anything with less than 10% is filtered out with "no color."

A closer look shows that Balsam Fir prefers the lower elevations too.

Unfortunately, our DTM for the Balsam Fir is limited in its scope to just a northwestern block of Maine. But if we can assume the eastern half of Maine has about the same level of Balsam Fir coverage, then also grant that it continues to thrive best in lower elevations, this is a strong indicator for why most of our SBW increases are seen in the northeast in lower elevation terrain.

Only 3 locations out of a possible 602 saw a 5-year increase!

Of these 3 spots, only one was 1.3 meters above average for the region, while the other two locations are significantly below average: -2.3 and -6.4 meters.