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Rare Moths of the Frostbottom

Sheriff’s Meadow Foundation not only owns property and holds conservation restrictions, it also manages some private land. The Wintucket Cove Frostbottom in Edgartown is an example of a property that Sheriff’s Meadow Foundation manages. Owned by the Vineyard Golf Club, the Wintucket Cove Frostbottom is protected by conservation restrictions and managed by Sheriff’s Meadow Foundation.

Frostbottoms are unique geological features that were formed by melting glaciers 15,000 years ago. Since cold air is heavier than warm air, it flows down these channels at night, eventually pooling in the lowest parts. Because of this, frostbottoms can have frost any day of the year. On the other hand, because there are no tall trees to provide shade, frostbottoms also get very hot during sunny days. If the frostbottom is functioning properly, temperature fluctuations of greater than 70 degrees Fahrenheit can occur during one day. These extremes make the frostbottom an inhospitable environment for most plant species. As only the hardiest plants can survive, frostbottoms are commonly dominated by scrub oak. Because frequent frosts make it very difficult to grow agricultural plants, frostbottoms on Martha's Vineyard are thought to be a habitat that was never cultivated. Since they were never plowed, they are generally in more pristine condition than other island habitats.

The combination of no plowing and a dynamic temperature regime has created important habitat for many rare species, especially moths. More than 10 species of moths that are listed in Massachusetts as being Endangered, Threatened or of Special Concern have been recorded in the Wintucket Cove Frostbottom. These moths not only survive under these harsh conditions, but actually appear to depend on frostbottoms to reproduce. Most of these frostbottom obligates have two characteristics in common:

1. They are summer flyers at the extreme northern edge of their range; and
2. Scrub oak and other oak species are common larval host plants.

These more southern species face an unusual dilemma. The moths needs warm temperatures to fly about, find a mate and start the next generation. On the other hand, since oak leaves quickly develop tannins that make them difficult to digest and limit their nutritional value, the caterpillars need new tender leaves that are rich in nitrogen.

Oak trees typically leaf out fairly early in the spring, when temperatures are still cold, particularly at night. Due to the extreme nighttime temperatures in the frostbottom, however, the scrub oaks growing there don’t leaf out until June or sometimes even July. By then, the air temperature is warm enough for the moths to fly. By feeding on these late-growing oak leaves in the frostbottom, the caterpillars get the best of both worlds: nutritional food that allows them to grow rapidly and change to the moth stage at a time when the air temperature is high enough for them to fly at night, mate and lay their eggs.