All Stories

Ever Wonder About Inchworms?

While walking my dog, I met a green, alien invader hovering in mid-air. Lacking the composure to note its taxonomic particulars, I swatted the critter away from what little hair I have.


My subsequent Internet perusal indicated that both the Bruce spanworm moth (Operophtera bruceata) and the winter moth (Operophtera brumata) have pale green caterpillars, known as loopers or inchworms. Both moth species have two pairs of prolegs at the back end and a white stripe down each side of the body, which grows to about an inch, or maybe more like 1.8cm.


Some people call Operophtera bruceata winter moths too, but I will call them the Bruces, in homage to the Monty Python skit about Australian philosophers. The two species are similar at all life stages, but the adults may have slightly more distinct features. Females of both species are flightless, but the length of the tufts they have instead of wings, differ. During the evenings, in the late fall or early winter, the males flap around sniffing for female perfume (pheromones). The Bruces are attracted to either species. Winter moth males are more particular about female pheromones. Hybrids have been found, so I blame the Bruce males. Some say that the wing patterns are distinctive, but others say this is unreliable. Another way to tell them apart is the male genitalia.


I did not have any adults handy, so I contacted Entomology Curator, Karen Needham at the Beaty Biodiversity Museum to give me odds on what my green worm might have been. She conferred with UBC Professor Emeritus, Judith Myers, who said it was probably a winter moth.

History, Natural and Not

Native to British Columbia and other parts of North America, the Bruces seem to get along well enough with the local flora and fauna. Winter moths, on the other hand, do not. They are originally from Europe and were first reported in Nova Scotia, Canada in 1950. They arrived in BC around 1977, on Vancouver Island, where they presumably came via another route from Europe. They now occur on southern Vancouver Island and the Lower Mainland. They seem to be less than ideal house guests, perhaps because of a lack of natural predators and maybe because of all the non-native trees. On the American East Coast, where they arrived in about 2000, the very hungry caterpillars have been chewing up many valuable species.


Larvae emerge in March or April from eggs laid in the bark of trees to tough out the winter. The larvae produce silk parachutes to balloon higher up to the tree’s buds, or to another host altogether. When the larvae are young, they creep inside the young buds and eat them. As they grow older they move to foliage. So, the critter I met was an accidental tourist. But, this chance encounter led me to investigate this whole story about an alien invasion.

Read more about bugs: check out "The Mystery of the Moth" a Science World Blog article by Search: Sara Stern Gallery curator, Rhoda Klein.