Royal Alberta Museum: Invertebrate Zoology - Bug Facts

[Caterpillars and Other Immature Insects]

Fact Sheets: Commonly Observed and Asked About Insects and Spiders Found in Alberta

White-spotted Sawyer (Monochamus scutellatus)

White-spotted Sawyer
Photo CD 0023 3291 1601, Image #07 - Credit: Terry Thormin

INTRODUCTION

The White-spotted Sawyer (John Acorn calls it the Spruce Sawyer in his book Bugs of Alberta) is a common forest beetle in Alberta. It is a member of the family Cerambycidae which are the long-horned beetles, so named for their long antennae. Because it is attracted to fresh burns or freshly cut wood, there are times when it becomes incredibly abundant.

IDENTIFICATION

This is a large beetle, up to 25 mm long, with a narrow body and antennae that can be up to twice as long as the body. It is jet black with white spots, in some individuals restricted to a single white spot where the front wings or elytra join the body, in other cases extensively spotted.

DISTRIBUTION

Because it feeds on conifers like spruce, pine, true firs and douglas-fir, it is restricted to those areas where these trees occur and is absent from the prairies.

TIME OF YEAR

Adults are present during the months of June and July, depending on the weather and region of the Province. They generally emerge later in the year as one goes further north.

HABITAT AND HABITS

The larvae construct galleries in the wood of trees in fresh burns or in freshly cut wood. It takes the larvae up to two years to reach adulthood. When attacking a fresh burn, the insect is considered to be beneficial as it increases the rate of decomposition. Adults feed on foliage and bark of conifer trees and at times can cause serious damage around large clear-cut areas.

SIMILAR SPECIES

Although there are many species of long-horned beetles in Alberta, we will only mention two others here, the two that most closely resemble the White-spotted Sawyer. The Northeastern Sawyer is very similar to the White-spotted Sawyer, except that it is on average larger, reaching 35 mm long and it is patterned with black, grey and white. The Poplar Borer is about the same size but is a light grey with yellow blotches and numerous small brown dots.

COMMENTS

Although the White-spotted Sawyer can be very abundant at times it is not generally considered a pest except to logging companies. People who own newer log cabins, however, do occasionally have larvae in the logs. The larvae make enough noise when chewing their way through the wood that it sounds like a distant saw, hence the name. When adults emerge inside the cabin they can be rather disconcerting.

In Fort McMurray, Alberta, this beetle is referred to as the Tar Sands Beetle. Evidently the beetles tend to swarm around any place where the bitumen is exposed. One theory as to why they swarm is that the bitumen gives off terpineols, the same substance that recently damaged trees give off, and this is what attracts the beetles. The Athabasca River, which goes right through the middle of Fort McMurray, cuts a deep channel through the surrounding landscape and exposes the bitumen-rich layer, which, particularly on hot days of summer, releases the terpineols.

Although White-spotted Sawyer Beetles are capable of biting, this is a rare occurrence, and the bite is not in any way dangerous because it is a simple mechanical bite without the use of any poisons or digestive enzymes. These beetles only bite when they feel threatened or they are trapped, so panicking just increases one's chance of being bitten.

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Last update: June 10 2005