Giant Wood Moth

The giant wood moth is the world’s heaviest known moth!

The sighting of a giant wood moth is big news. In 2021, an individual made international news when it was spotted at an elementary school in Australia. One might think that a moth that grows up to six inches long, with a wingspan of nearly 10 inches and a weight 60 times greater than that of a monarch butterfly would be pretty easy to spot. But when that moth lives mainly in the forests and only survives a few days after pupating, it becomes quite an incredible find. Although the research on this amazing species is somewhat limited, what we do know about the giant wood moth is fascinating.

Incredible Giant Wood Moth Facts

• Giant wood moths are the heaviest known moths in the world.
• These moths are part of a genus of macro-moths that live mainly in Australia.
• Few photos of live giant wood moths exist because they are so rarely seen.
• Female giant wood moths can lay up to 20,000 eggs.
• The larvae of giant wood moths, along with those of similar species, are known as witchetty grubs, and are sometimes eaten by indigenous people.
• Young eucalyptus trees can sometimes snap off at the bore holes made by these moths.

Where to Find Giant Wood Moths

Giant wood moths live in Australia and New Zealand, like most other species from the Endoxyla genus. According to the Australian Museum, sightings have been made primarily in the forested coastal regions of western and southern Australia, from Adelaide to Brisbane, although they have been spotted in other regions.

They depend on eucalyptus trees for their survival. Late instars of the giant wood moth larvae burrow into various types of eucalyptus trees for up to a year before emerging as adults. Sightings of adult moths are rare because they do not live long after emerging. Evidence of the moths, however, can be seen in the eucalyptus trees that the larvae bore into. They leave their exuviae, or the shed larval skin, hanging out of these holes when they emerge as adults.

Scientific Name

The giant wood moth’s scientific name is Endoxyla cinereus. Prussian entomologist and botanist, Johann Tepper, described the species in 1890, along with many other Australian insects. It is one of the giant moths of the Endoxyla genus, which includes more than 60 species, primarily from Australia. These moths are part of the Cossidae family of macro-moths.

Appearance

Giant wood moths are very large moths. Significant sexual dimorphism exists, with females of the species attaining nearly twice the size of males. Females can reach lengths of 15 centimeters, or approximately 6 inches. Their wingspans can reach 25 centimeters, or about 9.8 inches. Females grow to about 30 grams, approximately one ounce, in weight.

Compared to the familiar monarch butterfly, one of the larger known butterflies, giant wood moths are positively huge. According to the U.S. Fish and Wildlife Service, monarch butterflies can have wingspans of up to 10 inches, around 40 percent as large as giant wood moths. However, they weigh only about half a gram, just 1.6 percent of the weight of the massive macro-moth species.

Adult giant wood moths are gray. They have a black marking on the dorsal side of their thorax, just behind their head, that looks like an inverted heart. Their forewings are light gray, with faint dark gray markings in irregular patterns. The hind wings are dark gray, but not often seen, as these moths do not fly well and spend most of their short adult lives perched on a vertical surface.

Juveniles vary in appearance. The smallest instars of the larvae are less than two millimeters in length. They are so small and light, they can be carried away on the wind. Older instars appear large and chunky. They reach lengths of up to 10 centimeters, with a diameter up to two centimeters. The transfer larvae, meaning the ones that move between the ground and the holes they eventually bore in trees, are prominently banded in purple and white. These colors fade away once the larvae have bored into the trees where they live until emerging as adults.

Giant Wood Moth Behavior

The behavior of giant wood moths is quite focused. They seem to be on a mission to grow, grow, grow, and then mate and quickly die. Almost from the moment they hatch, they begin to spin silk so they can fly away on the wind and find a place to begin growing. Along with up to 20,000 siblings, they disperse. Not much is known about their life or their behavior from that moment until they rise from the ground and ascend the eucalyptus trees.

Each surviving larva climbs to a spot on a suitable tree close to where they’ve been living and finds just the right spot to bore a hole. Researchers have observed that the taller the tree, the higher the larvae seem to climb before boring their holes. They work to conceal the holes until they are safely inside, and then continue to eat and grow until the right time arrives to pupate. Temperature may help them to determine when that time comes.

From hatchling to adulthood, the life of the giant wood moth is solitary. They are not an aggressive species nor an interactive one at all, it seems, until they emerge as adults and quickly seek a mate. Females perch on a vertical surface, such as a tree, building or fence. Males find them. Mating must take place as quickly as possible, because they have only days to live and the female still has to find a place to lay her thousands of eggs.  

Diet

Little is known about the diet of giant wood moths in their early larval stage. They possibly eat the roots of the trees around which they land after hatching. Older larvae ascend the trunks of several different species of eucalyptus trees and bore into them, spending as much as two years inside the trees before reaching adulthood. During that time, they eat the wood of these gum trees and continue to grow until they eventually pupate and emerge. Once they have emerged as adults, they are unable to feed, and therefore die within just a few days.

Reproduction

Giant wood moths spend the vast majority of their lives in a series of larval stages. They live only a few days after reaching adulthood. Adult females are scarcely able to fly. They climb up the trunk of a tree or other surface and wait for a male to come to them. After mating, each female lays up to 20,000 eggs. She uses a long, flexible ovipositor, an organ specially designed for placing eggs, to deposit her eggs inside cracks in the bark of trees or hidden crevices in other materials. They are contained in a thick, sticky secretion.

Once the larvae hatch, the first instar is tiny. They measure less than 2 millimeters in length. These tiny larvae spin silk, and they are light enough to disperse on these gossamer strands in the wind. The next larval stage is spent in a location as yet unknown to scientists, but many of the larvae seem to land in and around the roots of eucalyptus trees that will provide their sustenance as they approach adulthood. How many of the thousands of early larvae land in the right spots for future growth? Given the rarity of sightings of this species, it is hard to know.

Life Within the Trees

After a long period spent in the undergrowth, possibly living within and eating the roots of a variety of eucalyptus trees, surviving larvae climb these trees and bore into their trunks. They bore holes higher on older, sturdier trees, and lower on young saplings. They cover the holes that they bore with silken tents lined with pieces of bark from the tree. This camouflages and conceals their work until they bore fully into the tree. After that, they seal most of the hole behind them with a gelatinous substance. They then live in the holes in the holes in the trees up to two additional years. They continue eating and growing, before emerging as adults at about three years of age. Adults are unable to feed, and will die within a few short days, so they must find a mate quickly and begin the reproduction cycle anew.  

Predators

As relatively defenseless larvae, giant wood moths could be eaten by any number of predators. This is especially true when they are transitioning from their early habitat in and around tree roots to the holes that they bore in tree trunks. However, researchers have noted that even though the larvae are easy to spot against the bark of the eucalyptus trees, and brightly colored with purple and white bands, few predators seem to take interest. It is believed that they may be toxic or taste too bad for predators to eat.

Once the larvae are safely in their bores, few predators can successfully extract them. Black cockatoos are one group of predators that can dig giant wood moth larvae from eucalyptus trees. They have strong beaks, capable of opening the toughest nuts, and can rip the bark off trees to get to hidden larvae. In fact, these birds are responsible for much more damage to eucalyptus trees than the larvae cause.

Lifespan

Giant wood moths live approximately three years. However, they spend only a few days of that lifetime as adults. Once they emerge from the trees where they pupate, they are unable to feed. They must quickly find a mate, and then the females must lay their eggs before they die.

The population of the giant wood moth is unknown. They are very rarely sighted and have only occasionally been studied since they were first discovered. The greatest risk to these insects may be degradation of their habitat, including loss of the eucalyptus trees that they depend on for survival.

Recent Sighting in the News

A giant wood moth was sighted at a school in Queensland, Australia in 2021. Construction workers found the humongous moth while working on a new section of the school. The workers took pictures of the moth before moving it to the nearby rainforest. The school’s principal noted that they had seen many types of wildlife on the school grounds, including koalas, wallabies, and snakes, but they had never seen a giant wood moth. News of the sighting was reported in publications worldwide.