Making a Bee Line for Logs
by Dr. Nenad Vidovic
Head, Research and Development Department,
The Sansin Corporation
In the fall of 1999, a Toronto customer of The Sansin Corporation called to
report that a "bumble-bee resembling bug" was eating into his cedar deck. An
on-the-spot investigation was conducted in May 2000.
What We Found
Our customer had enhanced his beautiful brick house with a lovely cedar deck
looking out over a backyard full of flourishing plants and a few cherry trees. The rafters of the deck were peppered with 17 insect holes and it was
not difficult to determine that the culprit was, in fact, the large carpenter bee, one of the most common wood-boring insects in North America.
Some male bees buzzed around us in an attempt to guard their 'territory', while females were busy moving in and out of the galleries. The bees
evidently enjoyed boring through this $5,000 cedar deck.
"Whenever we are sitting out here, they fly around and threaten to sting
us," our customer related. "We have sprayed the holes with different liquids
but the bees just look mad and return a few days later, drilling ever more
holes in the deck." He enthusiastically accepted our offer to use the deck
as a field test for our eradication methods.
Mother Nature's Miracles
Mother Nature does not allow organic material to last indefinitely. Otherwise, we would soon be engulfed in a thick layer of the organic stuff
and life on the globe would die out due to the lack of important minerals.
Microorganisms are the main tool that Mother Nature uses to break down organic material. When she uses this 'tool' to attack valuable lumber, we
must protect it. One effective method is to keep the wood dry because microorganisms cannot go to work without water, But, alas, Mother Nature has
developed another group of creatures to devour organic material: namely, dry-wood insects. Most of the beetles in this group, such as the
powder-postbeetle, the death-watch beetle, the long-horn house beetle and termites,
feed and nest in dry wood, while others, including ants and carpenter bees,
use dry wood only for nesting. Even dry wood insects need some water to propagate. They get it by devouring the wood. As each cellulose chain in the
wood is broken apart in the insects' stomachs, a molecule of water is generated, producing enough moisture for the insect larvae to live happily
in the dried wood. No one could ever accuse Mother Nature of lacking ingenuity.
The type of carpenter bees that homeowners have to worry about are large species (20 to 30 mm) with a smooth and shiny, metallic blue-black abdomen
(bumble bees have a yellow and hairy abdomen and do not bore into wood).
Large carpenter bees prefer to bore into fir, spruce, pine, cedar or douglas
fir but are known to attack many other species, especially if the wood is left unfinished. Outdoor structural timber such as porch and deck
components, window sills, wooden sidings, eaves, fascia boards, garage and
shed components, is the most vulnerable; but carpenter bee galleries are also found in posts, transmission poles, timber bridges and dead trees.
After the bees mate early in the spring, they make a bee line for wood in search of a home for their 'babies'. Females are usually very busy. Several
generations may use old tunnels for many years but most often are obliged to
bore new ones. Male bees are not just couch potatoes. They fly around, guarding the nests. If a person or some other outsider shows up, they become
aggressive, making rapid flights around the intruder and threatening to sting. Ironically, they have no stinger. The females have a stinger but are
non-aggressive and will not use the weapons unless they are repeatedly provoked.
Upon finding a convenient location for a home, females begin excavating.
They bore perfectly round holes about the size of a dime as far from the ground as possible. Overhanging timber is a favorite site. Wherever
possible, the bees bore the holes on the bottom surfaces. They instinctively
avoid areas that are vulnerable to water seep when it rains and prefer 'sky
highs' that are less attractive to ants, termites and other enemies. The main entrance to a bee gallery is oriented across the grain and is up to two
inches deep. The bees then start tunneling at 90 degrees, along the grain,
making the main family tunnel. These are usually about one foot long but can
reach several feet, especially after years of use. Multiple tunnels are also
common. If entry occurs through the end of a board or log, there is no right
angle turn; the bees simply make straight-line tunnels with the grain.
Once the gallery is ready, the female begins laying eggs, starting from the
deepest end of the tunnel. She uses wood sawdust to cache walls between the
eggs so that each egg is partitioned in a separate chamber. When the larvae
hatch, they feed on parental food (pollen and nectar based) and grow into juvenile (unmated) adults. They may emerge in the fall in search of more
food but return to the galleries to hibernate. Early in the spring, the adults emerge, mating follows and so the life cycle goes on. By mating time,
the parents have died. One female delivers a generation of up to eight bees
once a year. Some species produce two generations, especially in warmer regions.
Damage to the Wood
At first sight, the only evidence of attack are the entry holes made by the
females. Damage is minor if infestation occurs only in decorative wood such
as trims, fascia boards or sidings. If structural lumber such as in joists,
windows, beams, shingles or logs is attacked, costly repairs are often necessary, especially after several years of neglect.
The presence of carpenter bees in wood sometimes attracts woodpeckers looking for larvae. The resulting damage can be extensive, especially when
the hungry birds go after the bees from one chamber to another in the wood.
After we spotted the holes in broad daylight, we came back and monitored the
insects at dusk as they were turning in for the night. As soon as most of them seemed to have returned to the
galleries, the action began.
We saturated each hole with Sure Killer, an aerosol type of insect spray.
Sure Killer is available in any shop selling pesticides. It contains 1% Diazinon insecticide.
As the treatment began, dazed bees emerged from the galleries, flew around
erratically and fell to the ground, dying. By 11 p.m., we counted 12 dead bees and found three more in the morning. After seven days, the score was
36. The Sure Killer treatment was then repeated. Finally, we injected Sansin
DIPAR gel into the holes and sealed them with tapered corks to retain the DIPAR gases inside the galleries and keep track of the holes that had been
treated. The DIPAR liquid increases the kill ratio, prevents insects from re-entering and keeps woodpeckers away from the holes.
We made a final inspection in July 2000. No bees were observed flying around. After dismembering two joists to register the size of the galleries,
we were surprised to find nine more bee carcasses in each joist (Figure 1).
What You Can Do
If bees are buzzing around, follow their flight to entry holes. Always inspect your lumber for the presence of holes at least once a year,
preferably in the spring. Look for a sawdust-like frass that may accumulate
on the surface below entry holes. If the frass looks like freshly sawn wood,
the insects are probably still alive and active. Old frass is grayish in
colour. It indicates that adult insects have fled the scene and that mated
females have probably not taken the same gallery to develop their own generation. Check for insect activity by placing folded paper ('Kleenex' is
suitable) into the holes and seeing later if it has been pushed out. Should
eradication be necessary, make sure that the insects are back in the gallery
at the time of action (if females survive, they will construct a new gallery). Modern (lead-free) paints and stains will deter the bees but not
forever. Even pressure treated wood can be attacked (Henning 1999), so it is
wise to apply wood stains.
Any insecticide that targets carpenter bees on the label can be used for eradication. Look for a product in a container with a flexible straw, so the
liquid can be injected beyond the entry curve, directly into the main gallery. Powdered insecticides also do a good job.
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The Sansin Corporation
3377 Egremont Drive
N7G 3H6 Canada