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Geometer Moths, Inchworms, and Two Hemlock Looper Species

Linda Crampton is a writer and experienced science teacher with an honors degree in biology. She enjoys writing about science and nature.

Interesting Caterpillars and Moths

This article was inspired by an outbreak of green, looping caterpillars in a municipal park near my home. I enjoy sitting under a Douglas-fir tree in the park and working on my articles via the free Internet access. The animals that visited me as I worked were phantom hemlock looper caterpillars. Caterpillars that alternate loops and a straight body as they progress along their path are often referred to as loopers or inchworms. Hemlock loopers belong to a family known as geometer moths, or the Geometridae.

Inchworms are interesting creatures. They can sometimes be destructive pests, however. In this article, I focus on the phantom hemlock looper and the western hemlock looper caterpillars and moths. The first species (Nepytia phantasmaria) is found in British Columbia, though there have been a few reporting sightings in nearby locations in the United States. The second (Lambdina fiscellaria) is found in British Columbia, Washington, and Oregon. The latter species in particular is sometimes a problem in the caterpillar stage. The species also lives in the eastern part of North America, though the animal there is a different subspecies from the western one.

A phantom hemlock looper caterpillar on my backpack

A phantom hemlock looper caterpillar on my backpack

Differences Between Moths and Butterflies

Moths and butterflies belong to multiple families in the order Lepidoptera within the class Insecta. There are sometimes noticeable differences between the two groups of insects, but some features are generally different instead of always so. The features mentioned below are seen in the adults of each group. The caterpillars or larvae of each group have a range of characteristics.

  • Moths tend to have duller colors than butterflies. Butterflies are often brightly coloured.
  • Moths frequently have feathery antennae while butterflies have a club at the tip of each of their antennae.
  • Moths frequently have thick bodies and tend to be active at night. Butterflies have thinner bodies and are active during the day.
  • Unlike the case in butterflies, the front and hind wings of moths are generally joined by a structure called a frenulum.

The Library of Congress says that as researchers are discovering more moth and butterfly species, "distinctions between the two are becoming blurred." For example, some insects classified as moths today have features that resemble those of butterflies, including a colorful body and/or clubbed antennae. In addition, some moths are active during the day.

Features of Geometer Moths

Geometer moths are small to medium-sized members of the moth group. They have relatively slender bodies compared to other moths. They also have broad wings that are held flat over their body when they are at rest. Their body surface exhibits various patterns, but they are generally not colourful insects as adults. There are exceptions to this observation, however.

Though many adults are some mixture of black, grey, white, and brown, some species exhibit brighter colours, including yellow, orange, blue, or green. Even the less colourful individuals can be interesting to see if the patterns on their body are observed. The caterpillars of geometer moths are green, yellow, brown, grey, or a mixture of these colours.

People who have taken biology courses may have studied the colours and patterns of the peppered moth (Biston betularia) as an example of evolution by natural selection. The species is a geometer moth. The study of how its physical appearance is indirectly affected by its environment is an interesting one.

Inchworm Facts

Inchworms are moth caterpillars with a particular style of movement. They have legs only at the front and rear of their body. This explains why they have to move with a looping movement. The caterpillars were given their name because they seem to inch along the ground or to be measuring the Earth. The term "geometer moth" is thought to have a similar origin.

The legs of an inchworm are technically known as prolegs because they aren't segmented. The front ones are widely separated from the rear ones. The legs resemble short tubes.

The larvae feed on leaves and can be destructive if their population is dense. They are sometimes known as cankerworms (though they aren't worms). Some species produce silk threads that hang from trees and act as ropes. The threads enable the animals to escape from danger and travel back to their previous location or to reach the ground for pupation. During an infestation, multiple threads may be seen. They may create a web-like structure.

The Douglas-fir tree (on the right) is where I met the inchworms. The other tree is a rowan, or a mountain ash.

The Douglas-fir tree (on the right) is where I met the inchworms. The other tree is a rowan, or a mountain ash.

Park Inchworms and a Douglas-Fir Tree

Kensington Park is a large municipal park that has three sources of public Internet. The park contains multiple recreation facilities as well as areas with grass and cultivated plants. The inchworms visited me as I worked under a Douglas-fir tree near a path. Both of my caterpillar photos were taken under the tree. Just behind the trees at the back of the photo above is a small building associated with the pitch and putt course, which was the source of the wireless Internet that I was using.

Though the moth species in the park is referred to as the phantom hemlock looper, it’s found on other trees besides the western hemlock, including the Douglas-fir (Pseudotsuga menziesii). The name of this tree is often hyphenated to show that it’s not a true fir. True firs belong to the genus Abies. Both types of tree belong to the pine family, or the Pinaceae.

The female cones of the Douglas-fir have bracts with three points at their tip located between their scales, as shown in the photo below. A story that I've heard since I first learned about the tree is that the bracts represent the two legs and the tail of a mouse entering the cone.

The first part of the tree’s name is capitalized because the plant is named after a person. David Douglas (1799–1834) was a notable Scottish botanist. He spent part of his life in the United States and died in Hawaii.

Underneath the Douglas-fir tree

Underneath the Douglas-fir tree

Phantom Hemlock Looper Features

Physical Appearance

The caterpillars of the phantom hemlock looper are mostly green in colour. They bear stripes and horizontal bands of a lighter or darker green than their surroundings or that are yellow-green or white. They also have black spots on their head and rear end.

Eggs and Larva

There are four stages in the life of a hemlock looper: egg, larva (or caterpillar), pupa, and adult. The eggs of the phantom hemlock looper are laid on the underside of the needles of the insect's coniferous host. The insect spends the winter in the egg stage. The caterpillars emerge from the eggs near the end of May. The larva go through several instar stages as they develop. An instar is a developmental stage between two moults. The caterpillar loses its outer layer as it grows and then expands to fill the new one.

Pupa and Adult

After a couple of months of feeding on leaves, a caterpillar forms a pupa or chrysalis in August. The caterpillars burrow into the ground before they pupate. Adults emerge from the pupae in September and October (or in case of the one in my photo below, in late August). After mating, the female lays her eggs on the underside of coniferous tree needles, and the life cycle begins again.

I’ve been unable to discover why the insect is known as a “phantom” hemlock looper. It’s a question that interests me. The species name of the insect (phantasmaria) is intriguing.

A phantom hemlock looper under the Douglas-fir tree; another individual can be seen peeking into the photo at the top

A phantom hemlock looper under the Douglas-fir tree; another individual can be seen peeking into the photo at the top

Behaviour of the Caterpillars

The Douglas-fir tree that I observed maintained its green needles and healthy appearance throughout the summer in which I took the photos in this article. The animal in the photo above was photographed on the last day of July. They were some dead and shrivelled caterpillars on the ground under the Douglas-fir at that time, but there were also some live individuals present. As they moved, they appeared to be probing the ground. Perhaps they were trying to find a suitable place to bury themselves and produce a protective cocoon as they turned into a pupa. It was interesting to see that the two that were nearest to me occasionally stood on their hind legs and reared their upper body into the air as though they were inspecting their environment.

During outbreaks, abundant moths flying in infested areas may give the impression of a snowstorm.

— USDA (with respect to the western hemlock looper)

Western Hemlock Looper Facts

Three subspecies of Lambdina fiscellaria exist. Lambdina fiscellaria lugubrosa is the western hemlock looper. The caterpillars are mainly grey, yellow, or brown, depending on their age. They often affect hemlock trees but can also be seen on other types of trees, including the Douglas-fir. The larvae are most likely to be seen in June and July.

After a season of feeding, the mature larvae drop to the ground on silken threads. The silk of caterpillars is produced by modified salivary glands. The insects pupate in crevices in tree bark or inside mounds of moss, lichen, or leaf litter on the ground. The pupa has a mottled green and brown appearance. Metamorphosis occurs in the pupa as the caterpillar turns into a pale yellow moth.

Just ten to fifteen days after the animal entered pupation, the adult emerges. The female lays her eggs in September or October. The eggs don't hatch until winter is over. The adults die soon often mating has occurred and the eggs have been laid.

The phantom hemlock looper, Nepytia phantasmaria, is sometimes unusually abundant in western hemlock looper outbreaks.

— United States Department of Agriculture

Potential Damage Caused by the Larvae

The U.S. Forestry Service says that “outbreaks (of the western hemlock looper) are generally brought under control by the action of parasites, predators, and diseases”. The leaves of an infected tree turn brown and may look as though they’ve been exposed to a forest fire. If a treatment is applied to the affected trees, it may consist of a bacterium named Bacillus thuringiensis kurstaki (Btk), which infects and kills insects in the order Lepidoptera.

In the video above, the effects of an invasion by both phantom hemlock looper caterpillars and western hemlock ones in the Metro Vancouver area are shown and described. (Metro Vancouver consists of the city of Vancouver plus some surrounding cities.) In general, the western hemlock looper seems to be a more important problem in the area. The expanse of dead and dying trees shown in the video looks dramatic. Interestingly, though, the scientist in the video says that the invasion is probably nothing to worry about and that it triggers regeneration in the forest. This may not be true everywhere that the caterpillars are found, but it's interesting that it sometimes is.

According to Babita Bains, a provincial forest entomologist in British Columbia, the caterpillars of the phantom hemlock looper and the western hemlock looper can both cause tree problems, especially the latter species. The damage may not be serious, depending on the severity of the infestation. The entomologist has said that “trees typically survive light and moderate severity defoliation, however, severe defoliation can result in growth reduction, top kill and tree mortality.”

At one point in the video above, someone refers to the caterpillars as "worms," which may have arisen from the term inchworms or cankerworms. Caterpillars are insects while worms aren't.

An adult phantom hemlock looper in the park on August 29th

An adult phantom hemlock looper in the park on August 29th

Intriguing and Sometimes Destructive Insects

I think hemlock loopers are interesting animals, especially in their larval form. I enjoy observing them. I would be very unhappy if the caterpillars caused significant damage to trees that were important to me, though. If you read some of the references below, however, you’ll see that at least some of the professionals involved in dealing with the caterpillars are not alarmed by the presence of the insects. Several sites say that although trees can be killed by a heavy infestation of the insects, the trees are often old ones. They also say that the infestation tends to last for a few years and then disappears. New trees appear to replace the dead ones.

Anyone concerned about a hemlock looper infestation should seek the advice of a qualified professional in their region. The appearance of the insects might not be a reason to panic, though, at least in some areas. Biologically, hemlock loopers are intriguing organisms, whatever their effects.


  • Moth and butterfly features from the Library of Congress
  • Geometrid moths from the Missouri Department of Conservation
  • Phantom hemlock looper facts from Natural Resources Canada (a government organization)
  • Douglas fir information from the Government of British Columbia
  • Information about western hemlock loopers from the Government of British Columbia
  • Western hemlock looper facts from the USDA (United States Department of Agriculture)
  • A report about the western hemlock looper from the U.S. Forest Service
  • Hemlock looper outbreak from the District of North Vancouver (This district is separate from Metro Vancouver, despite the similar names.)
  • An outbreak of both species of hemlock looper caterpillars in British Columbia from the Peak newspaper

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Linda Crampton


Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on August 09, 2021:

Linda, you're welcome.

Linda Crampton (author) from British Columbia, Canada on August 09, 2021:

Thank you, Miebakagh.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on August 09, 2021:

Linda, thanks for the additional information.

Linda Crampton (author) from British Columbia, Canada on August 09, 2021:

I appreciate your comment, Ravi. The term "glowworm" often refers to the female of a specific beetle species (Lampyris noctiluca). She looks like a larva instead of an adult beetle and emits light.

Ravi Rajan from Mumbai on August 08, 2021:

Thanks, Linda. I learned something new today.I used to always get confused between moths and butterflies but your valuable article has given me good insights.BTW one question - where exactly a glowworm can be classified - inchworm or moth?

Linda Crampton (author) from British Columbia, Canada on August 08, 2021:

Thank you for the interesting and kind comment, Misbah. I love the thought of ladybirds tickling the skin!

Blessings to you as well. I hope you're healthy now and are feeling good.

Misbah Sheikh from — This Existence Is Only an Illusion on August 08, 2021:

Linda, Thanks a lot for creating this beautiful and creative hub. It's very informative and interesting as always. I enjoyed reading it and have learned a lot from your hub today. When I was a kid, I used to catch and run after butterflies because their bright colors never failed to draw me in, but I can't recall ever attempting to catch a moth. I'm also a fan of ladybirds. To feel the tickling, I always put them on my hands, so they can crawl. Thanks for sharing this beautiful hub.

Blessings and Love to you, dear friend

Linda Crampton (author) from British Columbia, Canada on August 08, 2021:

I would like to explore Nigeria. I expect it has some interesting sights and facilities, even though they may be different from the ones in my area.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on August 08, 2021:

That's great. But such faclities were hard to come in my area.

Linda Crampton (author) from British Columbia, Canada on August 08, 2021:

Hi, Mel. Thank you for the interesting and insightful comment.

A nearby park that provides free access to the Internet is a great asset. I often take advantage of it.

Mel Carriere from Snowbound and down in Northern Colorado on August 08, 2021:

Very intriguing and educational as usual, Linda. When nature is out of balance, when these caterpillars' natural predators are suffering because of habitat loss, that might result in more serious infestations of trees. Inchworms themselves are not the problem, the problem is people, I think.

I enjoyed the insight you gave us into your creative process, imagine a park with Wi-Fi access! Great work.

Linda Crampton (author) from British Columbia, Canada on August 05, 2021:

Thanks for commenting, Denise. I know that the inchworms in my area and at nearby sites in the United States are frequently found on Douglas-fir, but I don't know much about the situation where you live. I think all Inchworms are interesting animals, though.

Blessings to you as well.

Denise McGill from Fresno CA on August 05, 2021:

This is truly interesting information. I didn't realize that the inchworms I used to see frequently were really caterpillars. Now I know. I haven't seen one in many years here in the Central Valley of California. Perhaps I'm just not looking in the right places. I don't have any Douglas firs near me so that could account for it.



Linda Crampton (author) from British Columbia, Canada on August 05, 2021:

Hi, Manatita. Observing nature during walks is always interesting. I appreciate your comment.

Linda Crampton (author) from British Columbia, Canada on August 05, 2021:

That sounds like an interesting activity, Miebakagh.

manatita44 from london on August 05, 2021:

I believe we have ringworms at home in the Caribbean. As to moths and butterflies, I tend to think of them as the same thing. They do look a bit different in your video, I suppose. Glad you are able to observe them on your walks.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on August 05, 2021:

I learn about caterpillars naturally at a childrens park.

Linda Crampton (author) from British Columbia, Canada on August 04, 2021:

Hi, Peggy. It is a lovely park to observe nature. The caterpillars are an interesting sight.

Peggy Woods from Houston, Texas on August 04, 2021:

You certainly have a lovely spot in the park to contemplate nature. It was informative and interesting reading about these types of moths and the caterpillars. I am glad to know that they do not usually kill the trees during an infestation.

Linda Crampton (author) from British Columbia, Canada on August 04, 2021:

Hi, Mary. I wouldn't like to sit under a tree in the situation that you've mentioned! Thanks for commenting.

Mary Norton from Ontario, Canada on August 04, 2021:

We used to have trees in front of our school that got infested, and we avoided passing under the tree for fear of these worms

Linda Crampton (author) from British Columbia, Canada on August 03, 2021:

Hi, Heidi. The moth that you saw sounds interesting! I very much agree with the first sentence of your last paragraph. Nature is amazing.

Linda Crampton (author) from British Columbia, Canada on August 03, 2021:

Thank you, Nithya. I appreciate your kind comment. The park is a lovely place to write and to observe nature.

Heidi Thorne from Chicago Area on August 03, 2021:

Though they can be destructive as you note, some moths are just beautiful. I took some pics of one giant moth (like maybe 6" wingspan) that landed in our yard. It was a different species than the one in this article, but impressive nonetheless.

There's so much of nature that we don't see, it's almost overwhelming to comprehend. Thanks, as always, for helping us see what's hidden!

Nithya Venkat from Dubai on August 03, 2021:

The photos are lovely and the trees look so beautiful. It must be great to sit under a majestic tree and write an article about what you are actually seeing. It was interesting to read about the phantom hemlock looper and western hemlock looper. I hope the Douglas-fir survives and gets past the infestation. Thank you for sharing this interesting and informative article with a detailed explanation and photos. I am so glad your article turned up on my feed so that I could comment.

Linda Crampton (author) from British Columbia, Canada on August 03, 2021:

Hi, Bill. It is a lovely spot to write. The caterpillars seem to explore anything in their path. I've found them on my possessions before when I work under the tree.

Linda Crampton (author) from British Columbia, Canada on August 03, 2021:

Thanks, Fran. I hope you have a good week.

Bill De Giulio from Massachusetts on August 03, 2021:

Very interesting, Linda. You are fortunate to have a park nearby that has free wi-fi. What a great spot to write while enjoying nature. I seem to recall seeing a similar, very green, looper or caterpillar here in the northeast. Fascinating creatures. Love the picture of the hemlock looper crawling on your backpack.

fran rooks from Toledo, Ohio on August 03, 2021:

Alicia, I always learn so much from your detailed articles. Thank you for your article.

Linda Crampton (author) from British Columbia, Canada on August 02, 2021:

Thank you very much, Chitrangada. I love being surrounded by nature. The park where I took the photos is popular for sports activities, but my favourite areas are the ones with trees, grass, and other cultivated plants. Birds and insects like the park, too. There's always something interesting to see.

Chitrangada Sharan from New Delhi, India on August 02, 2021:

What an interesting way to write articles, in the park, where you can actually see these little creatures, observe their features and movements, and so on.

Loved the pictures.

I enjoyed reading the well written information about the Geometer moths, Inchworms and others. Thank you for sharing another educative article.

Linda Crampton (author) from British Columbia, Canada on August 02, 2021:

Hi, Flourish. My guess about the "phantom" name is the same as yours, but since I couldn't find evidence to support the idea I didn't include it in the article. I'm going to continue exploring the life of the moth and will update the article if I find new information. Thank you for commenting.

Linda Crampton (author) from British Columbia, Canada on August 02, 2021:

Thank you, Rosina. I appreciate your visit and comment.

FlourishAnyway from USA on August 02, 2021:

The phantom part now has me guessing along the lines of what they do to the trees they infest and the stringy ghost-like (?) webs they leave behind as they descend. Nice article with a first-hand spin.

Rosina S Khan on August 02, 2021:

It was interesting to know about hemlock loopers and their various stages. I find it intriguing when the Library of Congress says that the distinctions between moths and butterflies are becoming blurred.

Thank you for sharing, Linda.

Linda Crampton (author) from British Columbia, Canada on August 02, 2021:

Thank you very much, Miebakagh. I appreciate your kind and interesting comment.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on August 02, 2021:

Very interesting article indeed, and coupled with first hand biological information. Actually, very descriptive and with beautiful original field photos. Biological work is best and excellently done in the field, or natural environment.

Linda Crampton (author) from British Columbia, Canada on August 02, 2021:

I appreciate your comment, Pamela. I like to observe caterpillars because they don't fly away like most of the adults. I think they're interesting to study.

Pamela Oglesby from Sunny Florida on August 02, 2021:

A “phantom” hemlock looper is an interesting name. The caterpillars seem fairly small but very green, so you can see them. I am glad a heavy infestation does not typically happen too often. T

his is a very interesting article, Linda. We have moths here in FL, but I think they are a different variety. Thanks for some great information.