HOW do you tell if an insect can fly? Put it on a hotplate and turn up the
heat . . .
No this isn’t a bad joke from a cracker, but a genuine scientific experiment
dreamt up in the 1930s by entomologist Ron Fisher. He wanted to know more about
the habits of a small creature that was steadily eating its way through the
historic buildings of England—the deathwatch beetle.
By Fisher’s day, fear of the ghostly tapping in the timbers—once
thought to signal the approach of death—had been replaced by fear of the
beetle itself and the damage it was doing. Of the half a million listed
buildings in England, about 10 per cent are major historic treasures with oak
timbers, the beetle’s favourite food, says John Fidler, head of architectural
conservation at English Heritage. From Westminster’s Great Hall to Winchester
Cathedral and Henry VIII’s palace at Hampton Court, Xestobium
rufovillosum was consuming England’s architectural heritage.
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And because almost nothing was known about the beetle, attempts to eradicate
it failed miserably. No one had even seen the beetles fly, so how did they make
their way from one building to another? Or had every infestation been there
since builders carried in the timber?
For the past three years, researchers from Britain and the Netherlands have
been unravelling the secrets of the beetle’s life as part of the Wood Care
Project. Both countries have a serious beetle problem nurtured by their damp
climates and penchant for building with oak. Collecting the beetles is no job
for the fainthearted: the nave at Winchester Cathedral is England’s longest and
Salisbury’s steeple is the country’s tallest. Hunting beetles in the rafters
requires a head for heights and a keen eye. But the nerve-racking fieldwork and
patient hours in the lab have been worth it: at last the researchers know the
beetle well enough to curb its costly eating habits.
Soft, white and hungry
X. rufovillosum is a nondescript, brown beetle between 5 and 7
millimetres long. Little is known about its outdoor life, except that it tends
to live in oaks and willows. Indoors, it haunts old oak timbers, where it lives
in company with the oak rot fungus, Donkiopora expansa. Adult beetles
do little damage. The destroyers are the soft, white grubs which chew their way
through timbers until they crumble to dust. In April and May, the adults drill
their way out through a neat, round hole and begin knocking their heads on the
timber to summon prospective mates (“Knocking on wood for a mate”, New
Scientist, 6 July 1991, p 42).
Way up in the lofty heights of a cathedral tower the trouble may not be
immediately obvious. “The timbers might look perfect, but close up you can see
the pinhole exit holes. If you open up the beam you might find the whole inside
has been hollowed out,” says Fidler. Favourite places include joints and the
ends of trusses. “In fact the pieces of timber that hold up these wonderful
ǴǴڲ.”
When Steve Belmain of Birkbeck College, London, and Monique Simmonds of the
Royal Botanic Gardens at Kew began to probe deeper into the beetle’s life they
found that the adults live just long enough to find a mate and, in the case of
the females, to lay eggs. Most are dead within a month. The larvae, on the other
hand, can live up to 13 years, although some may complete their development
within a year.
Beetles, it turns out, can fly without the encouragement of a hotplate. But
they do need warmth, taking off only when the temperature rises above 17 °C.
The team also found that the beetles, especially females, are attracted to
light. They like old, damp timbers infected with the oak rot fungus and don’t
seem to have much trouble finding them. The light may guide them out of the
building but then what? “They don’t just blunder around and stumble on the right
piece of timber,” says Brian Ridout of Ridout Associates, who coordinated the
project with English Nature. “So what is it that attracts the beetle?”
At the Timber Research Centre near Delft in the Netherlands, Petra Esser and
her colleagues looked for chemical clues that might help the beetles locate a
new home. They found that as oak ages, its chemical profile alters. “We couldn’t
tell exactly which compounds change but we see a change in the overall profile,”
says Esser. “And there is a very clear change with fungal decay.” Taking a
similar approach, at University College Dublin, Derveilla Donnelly and her team
looked for chemicals in the fungus that might signal its presence to the
beetles. “There are two compounds we think are interesting, but we can’t say if
it’s the one or the other yet,” says Donnelly.
Armed with chemical extracts from Dublin and Delft, Belmain and Simmonds
tried them out on beetles. Placing captured beetles in a wind tunnel, they
wafted the various odours their way. Beetles were clearly more attracted to the
scent of old oak and the odour of oak with fungus than to young oak. “Females
were more responsive than the males—and there were differences in the
behaviour of mated and unmated females,” says Simmonds. “The males are only
interested in finding mates. But the females are more interested in finding a
place to lay their eggs that will support their larvae.”
Eight-legged enemies
One of the biggest surprises to emerge from the project is just how effective
the beetle’s natural predators can be. The space inside an old roof is home to
spiders and predatory clerid beetles that take about a third of the adult
beetles that emerge in spring. “Just two or three species of spider mop up a
good number of beetles,” says Ridout. “They regulate the population.”
So what use are all these details of deathwatch life? Strangely, despite the
beetle’s toll of ancient timber, treatment so far has been as unscientific as
Fisher’s flying experiment. In 1875, a reader of The Queen magazine
suggested a novel cure. Take a loud ticking watch and place it close to the
source of the sound, he suggested. “The insect will soon kill itself in its
efforts to out-tick the watch.” His remedy was probably no worse than most of
those used today and a lot safer.
These days the usual treatment for deathwatch beetle is to blitz the
afflicted area with highly poisonous chemicals, either painting on a toxic
paste, or “fogging” the roof space with a chemical spray that covers the wooden
surfaces with fine drops of poison. In severe infestations, contractors chop out
damaged wood and rip out plaster to get at timbers in the walls. In some cases,
the chemicals course through cracks in the wood and run down the walls, damaging
decorative plasterwork or ornate paintwork. “Sometimes the treatment is worse
than the problem,” says Fidler. So, it’s not surprising that those charged with
the preservation of historic buildings would like to be sure such drastic
treatment is essential—and works—before getting in the
contractors.
Chemicals rarely reach the grubs deep inside their massive beams, even when
injected into the timber, and the emerging adults know better than to drill
their way out through a layer of poison. “The chances of killing them with
chemicals are approximately zero,” says Ridout. Unfortunately, the beetle’s
natural enemies, the spiders and clerid beetles, are all too likely to succumb.
“Orthodox treatment removes the predators without doing much to the pest,” he
says.
All the strands of information gleaned from the project suggest that there
are better ways of dealing with infestations than indiscriminately spraying
poisonous chemicals. First, Ridout points out, it’s a good idea to check that
there really is a problem. This may seem obvious, but in some instances
contractors have been called in to treat nonexistent infestations. “You might
have holes all over the timber but the beetles might have gone a century
before,” he says.
The Dutch have developed a simple way of checking. With sensitive recording
equipment, the Dutch team can pick up the distinctive sound of larval jaws
chewing through the wood. “The signal is very specific to the larvae,” says
Esser. If there are grubs at work, the listening device can pinpoint them
exactly and it even gives an idea of how many grubs there are. This means it is
possible to target exactly which parts of the timber need attention.
Adults only
Even if larvae are present they may not be doing much harm. Most infestations
are small and kept under control by spiders, says Ridout. The real problems
begin when damp gets in, encouraging oak rot fungus to spread into uninfected
wood and beetle numbers to rocket. This is the time to take action—against
the damp and the deathwatch beetles.
With the grubs tucked deep inside the timbers, the only sensible targets are
the adults. “Left alone beetle populations tend to be low and then they dip
until they aren’t viable any more and die out. We want to push them to the same
point,” says Ridout. The way to do this is to remove the adults when they emerge
in the spring, before the females can lay their eggs.
This is where knowing your enemy pays dividends. When female beetles are
looking for somewhere to lay their eggs they are attracted to
light—including light-coloured sticky traps and ultraviolet
“insectocutors”. As the adults only fly when the temperature reaches 17 °C
you can predict when they will emerge and be ready for them with traps. They
might even be lured out with the aid of a few heaters. These simple traps could
made be still more alluring by baiting them with essence of oak-with-fungus. If
the chemists can pinpoint precisely which chemicals the beetles find so
attractive, they could make the traps irresistible. It might even help to
release a few extra spiders to polish off more of the beetles, suggests
Simmonds. Only as a last resort should it be necessary to cut out pieces of the
historic timber or inject the wood with pesticides.
The challenge now is to convince the guardians of historic buildings that
they should swap their saws for spiders and their poisonous chemicals for
perfume. Base the battle against the beetle on biology and the roof won’t come
down.