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How Do You Identify Small Brown Moths?

by Tony Iwane on February 27, 2018

Hydriomena moth. (Photo by Tony Iwane, California Center for Natural History)
Hydriomena moth. (Photo by Tony Iwane, California Center for Natural History)

moth1What is this small moth that appeared last week in my yard?  -Carol Krasilnikoff, Millbrae, Jan. 29, 2018

From the looks of it, your moth visitor is a member of Hydriomena, a genus that sometimes goes by the common name of oak highfliers. If I were to guess the species, I’d say it’s likely Hydriomena nubiliofasciata, but (sigh) as with so many arthropods one would need to dissect and examine its genitals to know for sure. In their mighty Moths of Western North America, authors Jerry A. Powell and Paul A. Opler note that adult H. nubiliofasciata moths “exhibit an incredible amount of variation in color and markings,” but I’d say the date of activity, the overall wing shape and the wavy pattern on the forewings (with a lack of pattern on the hind wings) are suggestive of this genus and species.

A pale oak beauty moth caterpillar (Hypomecis punctinalis) -- commonly known, like all geometer moth caterpillars, as an inchworm. (Photo by Pierre Bornand, iNaturalist / CC-BY-NC 4.0)

A pale oak beauty moth caterpillar (Hypomecis punctinalis) — commonly known, like all geometer moth caterpillars, as an inchworm. (Photo by Pierre Bornand, iNaturalist / CC-BY-NC 4.0)

If you saw this one moth in your yard at the end of January, I imagine there were plenty more nearby. While on a short hike in Martinez on Feb. 3, I saw hundreds of these moths flying among oak and bay trees, an indication they had all eclosed (entomological speak for “emerged”) at roughly the same time. While most insects are not active in the winter, some Hydriomena actually do eclose from their pupae in January and get to flying about and finding mates all the way into May. As their common name suggests, the caterpillars of some Hydriomena caterpillars feed on oak trees, so they’re not tied to smaller, more herbaceous plants that grow in spring, like so many other butterflies and moths are. This means they can get a headstart chomping on the leaves of a nice robust coast live oak. Oh and fun fact, Hydriomena are members of the Geometrid family of moths, a name that literally means “measure the earth” — the reason, as you may guess, we call Geometrid caterpillars inchworms.

So what caused this uptick in moth activity? Well, even though moths are mainly nocturnal and can therefore fly at lower ambient temperatures than can butterflies, warmth is always helpful for active ectotherms, and the record-breaking warm temperatures in the Bay Area around that time likely triggered an eclosion explosion. Adult moths have but one purpose: to find a mate and successfully reproduce. The odds of this happening increase when temperatures are conducive to flight and the moths are all out and about at the same time, so a nice warm spell is the perfect trigger to get the party started.

A collection of adult Hydriomena moths from early February. (Photo by Tony Iwane, California Center for Natural History)

A cluster of adult Hydriomena moths from early February. (Photo by Tony Iwane, California Center for Natural History)

All this moth activity provides not only an opportunity for the moths to breed, it’s also a dining bonanza for insectivores. Birds looking to supplement their mainly plant-based winter diets will find the moths a rich source of protein and other nutrients, perfect as many gear up to migrate again in the spring. And our local bats, spiders, lizards, and frogs are also tucking in to this winter feast.

On the night of February sixth a few friends and I took advantage of the warm weather and put out some moth lights in Martinez. Not only did the lights attract an abundance of Hydriomena, we also saw some stunning Feralia februalis and Phyllodesma moths — which are also oak eaters who fly in winter. In addition to moths, butterflies like our native California tortoiseshell eclosed a few weeks ago as well, triggered by the warm weather. As seasonal temperatures continue to fluctuate due to climate change, it will be interesting to see how insects (and everything else) responds.

Tony Iwane portraitA resident of Oakland, Tony Iwane is the Outreach Coordinator for iNaturalist and an interpretive naturalist for the California Center for Natural History. Check out his photos on Flickr and his nature observations.

Ask the Naturalist is a reader-funded bimonthly column with the California Center for Natural History that answers your questions about the natural world of the San Francisco Bay Area. Have a question for the naturalist? Fill out our question form or email us at atn at baynature.org!

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