Tuesday, May 28, 2019

Brown-tail Moths (History, Science, and a Poem)

Brown-tail moth caterpillar winter communal nests, made from oak tree
branch-tip leaves. In the spring, the caterpillars emerge, about 1/2 inch long,
eating everything as they descend. On breezy days they will let go, to drift
away on the wind, hoping to land on another tree. In effect, in late May
it will be raining poisonous caterpillars. This is annoying.
HISTORY: A bit more than one hundred years ago Maynard and Stow, in eastern Massachusetts, had a moth problem. Two moth problems, actually. Gypsy moths had been deliberately brought to Medford, MA in 1869 in an attempt to create a hybrid with silkworm moths that would be the basis of a winter-hardy silkworm industry. This failed. Accidental releases created a wild population that rapidly spread across New England, and continues to expand south and west. Less well known now was the Brown-tail moth. This species was accidentally introduced from Europe to Somerville, MA in 1897 and rapidly expanded its territory south to Long Island and north into Canada. (WHAT IS IT WITH MASSACHUSETTS AND INVASIVE SPECIES?!! Water caltrop (an aquatic plant) was brought from China to Cambridge in 1874, deliberately spread, and now plagues our local rivers and ponds. Winter moths were Canada first, but ground zero for the U.S. infestation was in or near Boston, expanding slowly west.)

Early spring Brown-tail moth
caterpillar, 3/4 inches long.
Gypsy moths caused the most severe foliage damage, but Brown-tail moths were the most dangerous to people. The problem was that barbed hairs (bristles, actually), of the caterpillars contain and deliver chemical compounds with hydrolase, esterase and hemolytic activity, the net result being a poison ivy like rash when in contact with skin, and respiratory problems if inhaled, especially for people with asthma. The hairs are shed, and remain toxic for years, so activities as simple as lying on the grass, mowing the lawn, sweeping a deck or raking leaves caused exposure. Rashes can be present from just hours, to weeks. There are no antidotes for the toxins, so symptoms are treated with anti-itch products. In severe cases, oral corticosteroids (inflammation response suppressors) can be prescribed.

Brown-tail moth caterpillar. Two red spots on back, near tail,
differentiate if from other hairy caterpillars (Internet download)
The Brown-tail moth life cycle is hatch in August, grow during summer and fall, over-winter as caterpillars in communal webs created at branch tips, resume feeding in April or May, pupate in July, emerge as winged adults in August, then quickly mate, lay eggs and die. In addition to toxic bristles on the caterpillars, the adult female sheds brown bristles from her tail (hence name) to protect the egg cases, and the molt prior to pupating protects the pupae. The caterpillars can be identified by presence of two red spots on the back, toward the tail end. This differentiates from other fuzzy caterpillars such as Eastern tent and Gypsy moth caterpillars.

Brown-tail larvae have been reported as feeding on more than a score of tree and shrub species. This generalist behavior is considered unusual. Combined with its tendency to reach extreme outbreak densities, this species is a major pest of fruit orchards, ornamental trees and hardwood forests. Partial list of plant species: apple, cherry, beech, elm, grape, hops, oak, pear, raspberry, rose and willow. In a mixed maple/oak forest, there is a strong preference for oak. An early description of the introduction to the United States in the 1890s identified pear and apple trees as most greatly afflicted, but mentioned that once trees were entirely bare of leaves, the larvae would descend to the ground in great numbers and move toward any leafy plant, including garden vegetables.

Maynard's Moth Department crew and wagon, circa 1910. Ladders were used
to get to higher parts of trees. Toxic chemicals were sprayed from the end of a
long pipe. Click on photos to enlarge (Maynard Historical Society).
In Maynard, the moth plague was so severe that the town had a Moth Department, with staff and equipment, to spray trees, remove Gypsy moth egg cases and in winter, clip branches that had Brown-tail moth communal nests. The annual budget was less than that for roads & sidewalks or the fire department, but larger than the police department allotment.

Circa 1906 there was an attempt at biological control of Gypsy moths by the introduction of Compsilura concinnata, a parasitic fly. The parasite was not species-specific, so it impacted many native moth and butterfly species, and while it was not particularly effective against Gypsy moths, it was spectacularly effective against Brown-tail moths, the reason being that Brown-tails were one of a very few species that over-wintered as caterpillars, which are what the fly larvae live within. Voila! By the early 1930s the Brown-tail had become extirpated from all of the afflicted territory with the exception of a few islands off the coast of Maine, and the tip of Cape Cod. And so the status remained until around 2000, when Brown-tails reappeared in increasing large numbers in southern coastal Maine, from Portland to Bar Harbor. One possible reason for the resurgence is a parasitism of the C concinnata fly by a species of Trigonalid wasp, a situation referred to as hyperparasitism.

MORE SCIENCE: Brown-tail moths are not unique in evolving poisonous spines. The term is "urticating hairs" which are actually hollow bristles or spines that contain toxins. For certain species of caterpillars, human reactions range from mild stinging and itching to intense pain, allergic reactions, kidney failure and death. Tarantula spiders also have detachable hairs which they will scrape off their abdomen into the face of an attacking predator.

Head to tail about one inch.
Female abdomen end is
covered in brown bristles,
hence the name. Bristles are
shed to cover egg clusters.
Brown-tail moth caterpillars, like other caterpillars, will shed their skins (molt) six to eight times between hatching and reaching full size. Immediately post-molt the caterpillar has few bristles, but quickly grows more. Each molt, bristles are shed as part of the discarded skin. These break off and are distributed by wind and any actions that disturb ground surface, such as mowing lawns and raking leaves. Touching surfaces with hands and then touching skin elsewhere can transfer loose bristles. The toxins remain potent for up to three years.

As of 2019, along the coast of southern Maine - north of Portland, south of Bar Harbor, but spreading in all directions - experiments in prevention are being funded by the state of Maine and local communities. Some of the pesticides are banned because run-off into the ocean affects marine life, specifically, lobsters. Small trees, such as apples and other fruit orchard trees, can be managed by cutting the branch tips that have the winter nests. This is not feasible, however, for mature oak trees, which are the preferred sites for females to lay eggs. Instead, testing being done with injecting pesticides into the tree trunk, with the idea that in theory this will be transported upward into newly forming leaves. Multiple injections per tree are needed (every 4-6 inches of circumference), and there is harm being done to the tree. This can cost hundreds of dollars per tree. Organically certified biorational controls are being researched, as are searches in the original habitat (Europe, western Asia, northern Africa) for species-specific parasites. Spraying the bacteria Bacillus thuringiensis (Bt) is partially effective.

After vacationing in an afflicted area all clothing should be washed in warm to hot water. Tops and bottoms of shoes should be wiped with damp paper towels. Same to inside of suitcases. Even where native, there are historical reports dating back to the 1500s of severe outbreaks in cities, with trees of all types eaten bare of leaves. Southern parts of England are reporting 2019 as a very bad year. 

And a poem:
                                    IT'S RAINING CATERPILLARS
In fall, 
we hatchlings
climb upward, 
eating as we go.

At branch tips, 
we gather, 
bind leaves about us, 
and freeze.

All winter, 
we dream not.
Night, ice and snow
are our blanket.

Come spring,
we awaken,
climb downward, 
eating as we go.

There are caterpillars,
green inchworms,
which lower themselves 
to the ground on silken threads.

To travel, 
we let go,
cradled by the wind, 
falling toward
an uncertain future.  

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