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Moth Week: On the Trail of an Elusive Micromoth

July 26, 2019 12:00 AM
By: DCNR

Moth Week: On the Trail of an Elusive Micromoth

​Scientists estimate there are as many as 500,000 moth species. This week is National Moth Week, intended to celebrate the beauty, life cycles, and habitats of moths.

Moths are not just drab brown insects. Some things to know:

  • Moths help to pollinate plants, feed birds and bats, and in some places in the world, even people
  • Moths are among the most diverse and successful organisms on earth
  • Their colors and patterns are either dazzling or so cryptic that they define camouflage
  • Shapes and sizes span the gamut from as small as a pinhead to as large as an adult’s hand
  • Most moths are nocturnal, and need to be sought at night to be seen -- others fly like butterflies during the day

The Pennsylvania Natural Heritage Program (PNHP) includes a group of scientists that gather and provide information on ecological resources in the commonwealth to guide conservation and land-use planning.

Inventory Ecologist Pete Woods has been looking at moth species that could be impacted by the invasive pest, the emerald ash borer, which has caused the decline of ash trees in Pennsylvania and the U.S.

Emerald Ash Borer Impacts on Moths in Pennsylvania

The arrival of the emerald ash borer in North America has placed at least 32 moths, and 68 other invertebrates, at serious risk of extinction. These are species that use ash trees as a host plant, exclusively or nearly so.

PNHP staff have been trying to determine which of those 100 species are present in Pennsylvania, and the list of documented species now stands at 27, including 13 moth species.

Additional species of ash-eating moths are known from adjacent states, and Pete has been on the trail of one (or maybe two or three) of those species for several years.

Micromoths in the genus Marmara have a habit that is unique among moths: their caterpillars eat the inner bark of woody plants.

The tiny, flattened caterpillars live between the outer bark and the wood, and they leave a long, sinuous feeding trail, called a mine, that is visible from the outside.

Inventorying Moths in Pennsylvania that Rely on Ash Trees

Steps in the inventorying process have included:

  • Locating moth trails on ash saplings in 2017 at several sites. There was uncertainty about which species was making the trails, because there are three species of Marmara that eat ashes -- so obscure that none of them has a common name.
  • Rearing an adult. For two years, attempts were made to rear them in jars, with no success.

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Marmara caterpillar mines in an ash sapling. When the caterpillar was young, it made the fine lines in the top half of the photo.  When it was larger, it made the wide mines lower down.

  • Deciding to let them finish their development in the wild, and then collect them as pupae. This involved finding a C-shaped flap of bark where the caterpillar spun its cocoon, peeling off the flap and collecting it. In two weeks, the moths emerged.

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At the end of its mine, this caterpillar cut a flap of bark to protect it while it pupated.

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The white blotch on the underside of the flap is a silk sheet that covers the moth’s cocoon.

Moth Identification

As far as moths go, this is a very, very small moth, about 3 millimeters from head to tail, with brown stripes on a white background, long fringes at the edges of the wings, and red eyes.

Even under a microscope, and comparing it to the published illustrations, it still could not be identified.

The next step will be dissection!

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This moth is either Marmara fraxinicola, or M. corticola.

Saving Moth Habitat

Why put so much effort into finding and identifying these tiny moths?

This moth population could be doomed. Large ash trees in that region have already been killed by the emerald ash borer, and it is expected that the seed bank will be depleted in about a decade.

After that time, there will be no new seedlings that the moths can use.

But there is one hope that these three species have a better chance at survival than the other ash-dependent invertebrates.

It is possible to save individual mature ash trees, or groups of trees, by injecting them with insecticide.

The insecticide kills emerald ash borer larvae, as well as any other insect that tries to eat the tree, so these trees are a toxic trap to almost any ash-eating insect. But not to the bark-mining moths, because they only use saplings.

The saplings produced by the toxic reproductive trees are not themselves toxic.

The hope is that groves of treated ash trees will continue to produce seeds and seedlings, which will continue to support the three species of ash bark miners, until emerald ash borer is controlled and ashes return to our forests.

Surveys are needed to look for these moths at sites that are being treated with insecticides, to learn which sites are acting as refugia for these tiny moths.

Those sites should be prioritized for continued injections of trees.

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The emerald ash borer (Agrillus planipennis) may be causing a small mass extinction of moths and other invertebrates.

Citizen Science for Moths

Part of the purpose of National Moth Week is to encourage citizen scientists to help collect information about moths.

To help, check the national events calendar, or the DCNR Calendar of Events.


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