Defoliation by spruce budworm at Maine’s border

Although we have still not picked up defoliation by spruce budworm in Maine, surveyors in New Brunswick have, including some right across our border.  Observers recorded very light and scattered defoliation on the New Brunswick side of the St. John River between Madawaska and St. Francis in ground plots.  Aerial survey and additional ground plots picked up a total of about 3,700 acres of light, scattered defoliation in the northern third of New Brunswick.    

The detections near the St. John River were a result of roadside surveys conducted by the province.  Host branches from about one in four ground plots were found to have trace to light defoliation.  This is not the sort of damage that would be picked up by a casual observer, nor through the level of operational monitoring conducted in Maine at current levels of spruce budworm populations.    

The eastern spruce budworm, Choristoneura fumiferana, is a native moth that feeds on spruce and fir needles as a caterpillar (larva).  This species has cyclical populations that build when the host trees mature.   Populations reached epidemic levels in Maine, leading to tree growth loss and mortality, three times in the last century.

In Maine, spruce budworm feeds on the spruces (native white, red, black and introduced species) and balsam fir.  Within that group, the budworm has favorite foods.  It does best on balsam fir and white spruce whose buds begin to expand early in the spring when the caterpillars are ready to feed.  Red and black spruce buds swell after those of white spruce and fir, so the species are not as well suited for feeding by young spruce budworm caterpillars.  Maine has about 5.8 million acres of spruce and fir dominated forests, an area equal to the size of New Hampshire.

Defoliation has intensified on the Quebec side of the border as well, with increases in area and intensity seen in the Bas-Saint-Laurent Region, which lies closest to our northwestern border.  The Province of Quebec has been mapping defoliation from this pest for more than a decade.  In 2017, more than 17.6 million acres of forest were defoliated across the entire province. 

MFS Cooperator SBW pheromone trap results as of October 16, 2017; approximately 62 percent of sites have not been processed. Click image for larger version of map.

The Maine Forest Service (MFS)-led cooperative pheromone trap effort for spruce budworm is wrapping up for the year.  As in the last several years, around 20 organizations participated in the program. Several cooperators have retrieved their traps and sent in samples.  Others will be collecting samples next month, at the same time as a Cooperative Forest Research Unit-led overwintering larvae (L2) survey. In addition, volunteers with the Healthy Forest Partnership, Budworm Tracker Program have brought in their traps for the year and are sending in their catches.  Together, the data from these sites should give us a decent picture of spruce budworm populations in Maine, including potential impacts from moths coming in from the infestation in Canada. 

To date, about 38 percent of the MFS-Cooperator sites have been received and counted. Based on that sample, catches are on par with last year—down significantly from the summer of 2015. Across 160 sites, the average catch is about 7.5 moths per trap (average on the same sites was about 5.5 in 2016). Catches range from 0 to 68.3 moths/trap. The highest catches are found in a 40-milewide band south of the northern boundary of the state.

Sites in the eastern third of the state are trending towards higher catches than those in the western two thirds—this is apparent from Route 9, north to the St. John River. We anticipate receiving moths from more than 250 additional sites, and the picture could change before the dust (composed of moth scales in this case) has settled from those samples.

In addition to participating in the CFRU-led L2 surveys, MFS will conduct targeted surveys of host stands in the coming months in regions closest to the observed defoliation in Canada, and in response to high trap catches.

The coming epidemic of spruce budworm in Maine’s forests will probably be less severe than that in the 1970s-1980s.   However, a significant loss of trees and wide-ranging impacts to Maine’s natural-resource based-economy, forest structure and composition, wildlife and society are expected. Forest and social conditions in Maine have changed greatly since the last outbreak, so it is hard to nail down specifics of what the next will bring.  The low populations, and lack of detected defoliation to date, gives forestland owners and managers, as well as others who will be impacted, additional time to prepare.

Read new MFS report on Forest & Shade Tree – Insect & Disease Conditions for Maine.