News & Notes

Winter Moths and Bruce Spanworms

Monday, December 14th, 2015
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Have you seen any moths flying around lately? In the late fall (and even into winter) we often see “winter moths,” especially on warm nights. I wrote about these moths in a post last year but with a mild December we have seen an increase in reports about them so I think it is worth revisiting.


The Maine Forest Service has launched a survey asking people to report when they see Winter Moths but I want to explain the difference between “winter moths” and Winter Moths. Here we have a big problem of common or colloquial names being used and confusing two very different but also very similar species.

The term “winter moth” is a colloquial name for Operophtera bruceata, whose common and more entertaining name is also Bruce Spanworm. This is a native species, sometimes considered a pest because they are defoliators, but they go through natural cycles.

In contrast, Winter Moth (notice the capitalization) is the common name for Operophtera brumata. This is the non-native species (lacking in predators or parasites) that can cause a lot of damage.

These two species are nearly identical and are considered “not identifiable in the field.” In fact, the most reliable way to identify them is by examining their genitals:

(Eidt, 1996)

(Eidt, 1966)

The point I’m trying to make here, while encouraging everyone to report the moths they see, is that not all those ‘winter’ moths are bad. Despite being a defoliator, Bruce Spanworm’s natural cyclical boom years could provide an abundance of larvae that many of our nesting birds may benefit from. So, consider all of our native species (from Bruce Spanworm to our nesting birds) before accepting the need for control (via pesticides and tree cutting).




Literature cited: Eidt, D.C., Embree, D.G., Smith, C.C. (1966) Distinguishing Adults of the Winter Moth Operophtera brumata (L.), and Bruce Spanworm O. bruceata (Hulst) (Lepidoptera: Geometridae). The Canadian Entomologist, V. 93, 258-261.

Doug Hitchcox Head Shot - please credit  M. Kathleen Kelly (1)Meet Doug Hitchcox, Maine Audubon Staff Naturalist

A Maine native, Doug grew up in Hollis and graduated from the University of Maine in 2011. Throughout college Doug worked at Scarborough Marsh Audubon Center and was hired as Maine Audubon’s staff naturalist in the summer of 2013, a long time “dream job.” In his free time, Doug volunteers as one of Maine’s eBird reviewers, is the owner and moderator of the ‘Maine-birds’ listserv and serves as York County Audubon board member and Secretary of the Maine Bird Records Committee.


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