Feature

Wasp Saw

How much wood would a wood wasp drill?

Brian Langham

The old maxim of combat that if you want to succeed you should closely study your opponents strategies may pay dividends for the forest industry. It may even be that lessons learnt from the drilling mechanisms of some annoying forest pests could ultimately lead to more efficient methods of processing wood. Indeed, scientists found the insects they studied shared some of the superior cutting characteristics of the much promoted Japanese azebiki pull saw.

The Sirex "wood wasp" drills holes into pine trees and lays eggs 4-5mm deep. A predator of Sirex, the Megarhyssa wasp, was brought to New Zealand 20 years ago to control the Sirex pest. It drills into the trees to a depth of 100mm and lays its eggs on the Sirex larvae -- a food source for its own larvae when they emerge.

But how such small, seemingly fragile insects are able to penetrate the timber so efficiently caught the attention of Industrial Research's Marcus King and Julian Vincent of the University of Reading, UK.

King says they started their research with the question "how does nature cut wood?"

"Julian had heard of the drilling feats of the wood wasp, which the Forest Research Institute told us were a big problem in New Zealand."

The drill of the predator wasp, the Megarhyssa (pictured), is 100 mm long but only a tenth of a millimetre in diameter. Despite this seemingly fragile thinness, it is able to drill up to 100mm into the wood within 40 minutes.

This "fragile" apparatus is estimated to drill up to 1,400 holes in its one-season lifetime.

"This is a very efficient drill by anyone's standards," Marcus King says.

What's more the Megarhyssa drills without producing any noticeable sawdust, he says. This has real significance when it is realised that currently over 10% of the useful tree is wasted as sawdust.

The scientists found that the Megarhyssa hardened the cutting teeth on its drill by using magnesium it had ingested from its surroundings, while the Sirex used zinc.

The scientists found that the insects drills attacked the wood cell by cell, using the tip of the drill to break through the cells wall. The upward pointing teeth hook against micro-fibres within the cell wall and pull upward until the wall fails.

By measuring the forces applied by the insects, the scientists now have a measure of the ultimate strength required to fracture a wood cell. This is useful, in that it has always been difficult to measure at a micro level the force required to break through an individual cell in situ.

"You can take the cell out of the wood, but this is different from when the cell is part of a bundle," King says.

When looking at better ways of cutting wood it is also important to understand how a blade breaks wood cell by cell, he says. The insects drill tools are working at this micro level.

The scientists can now further explore whether there is anything special about the angles or design of the teeth on the insects drills which can be used to achieve similar efficiencies in an industrial situation. King is optimistic that it could be possible to design a saw which works in a similar way to that of the wasps drill. This would break the wood fibres in tension rather than shearing them as current saw technology aims to do.

Brian Langham is with Industrial Research Ltd.