Close up of a wood wasp

Sirex noctilio woodwasps inject venom and symbiotic fungus into pine trees along with their larvae

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Grinch bugs: the heartless insects making a meal of Christmas

In the lead-up to 25 December around eight million conifer trees including spruce, pine and fir are cut down to decorate homes, businesses and public spaces around the country.

Picking out the perfect Christmas tree is an important tradition for many families. 

But the trees that make it as far as your home are the lucky ones - at least they've been spared the Grinch-like insects that would have considered your decor their dinner.

Museum Curator and wasp expert Dr Gavin Broad introduces us to a couple of heartless species that want to steal your Christmas (trees).

No meat on the menu

Megastigmus strobilobius is a small wasp which eats the seeds of Christmas trees before they ever get the chance to grow.

'These lovely, tiny, Grinchy wasps eat the seeds of Norway spruce and a number of other spruce species,' Gavin says.

'Each wasp will develop inside a single seed before it emerges into the world.'

At around 10 millimetres in length, Megastigmus uses an elongated ovipositor (like a modified stinger) to drill into the seeds and deposit their eggs.

Megastigmus strobilobius develop in the seeds of Norway spruce and other spruce species



'The larvae will eat the nutritious material inside both fertilised and unfertilised seeds which might otherwise have gone on to make a spruce tree.'

They are similar to their parasitoid wasp cousins, which develop in or on the bodies of other insects, eventually devouring their hosts alive - although for Megastigmus, meat is totally off the menu.

'They are parasitoid wasps that have gone back to being vegetarian,' Gavin explains.

'They are closely related to proper parasitoids but this particular genus has given up a high meat-protein diet and made a switch to feeding on plant tissue alone.'

Axing Christmas

Gilpinia hercyniae, better known as the spruce sawfly, are plant-eating relatives of wasps, with a reputation for decimating spruce tree plantations.

'Sawflies are primitive plant-eating relatives of wasps which often feed like caterpillars on the foliage of trees,' says Gavin.

The male of Gilpinia have furry antennae that look a little like Christmas trees. Males are extremely rare in G. hercyniae as females usually reproduce parthenogenetically (with no need for fertilised eggs).


Sawflies get their name from the sawlike ovipositor that females use to open holes in the plant into which she can lay eggs.

'These really nifty little saw is how Hymenoptera first started laying their eggs, then the ovipositor evolved to be able to deliver venom as well as eggs into a host insect.

'At rest, the ovipositor will sit flush against their body, only hinging down when it's needed.

'Spruce sawflies crawl down and pupate in the soil at the base of trees, and can spend a number of years in their cocoons waiting for the right conditions to emerge.

'They have become established wherever there are spruce plantations and are actually quite a serious forestry pest.'

When the conditions are right (usually after a warm, dry summer) sawfly numbers can see a massive increase. They are capable of totally defoliating huge amounts of mature trees.

One devastating outbreak in the 1930s in the Gaspé Peninsula in Canada stripped nearly 66% of spruce trees (or 40 million cubic metres) in the region.

As so many spruces are used in forestry, spruce sawflies have been studied in great detail. Several methods have been devised to control them, including the release of parasitoid wasps and flies, as well as a virus that has effectively wiped out the insects in Britain. The combination of parasitoids and virus is also used to control the population in North America, where it was a serious - though introduced - defoliating pest until the 1940s.

Venomous guests

The grinchiest bug of all may be the Sirex noctillio woodwasp, which injects a venomous cocktail of toxins, as well as a symbiotic fungus into pine trees, eventually killing them.

Woodwasps are capable of devastating pine plantations especially in countries such as New Zealand, where they have been accidentally introduced.

Although they look fearsome, they can't sting people as their ovipositors are specially adapted for pine bark.

'Sirex oviposit in trees, where their larvae develop on the dying wood, helped along by a fungus, Amylostereum, that breaks down cellulose,' Gavin explains. 'The tip of a Sirex ovipositor is enriched with zinc to help with the drilling.

'Their venom is interesting as these herbivorous wasps are usually thought of as not being venomous, and it's only relatively recently that these venomous compounds have started to be investigated.

'They also may be a factor in the evolution of venoms that allowed parasitoid wasps to evolve to sting insects instead of trees.'

Do wasps have hearts?

Like all arthropods, wasps have open circulatory systems, meaning they don't have blood vessels or hearts, as such.

Instead of relying on a heart, blood and lungs to distribute oxygen throughout their bodies, insects have hemolymph, a fluid that fills their entire body cavity and bathes their internal organs.

Oxygen is admitted directly by little breathing holes in their shells which form tracheae to distribute oxygen throughout the body.

While they have no need for a heart, wasps have tiny waists and sometimes pump their abdomens to circulate hemolymph through such a narrow connection.

So depending on how you want to think about the mechanism of a heart, if you're describing wasps' circulatory system then proportionally they probably have bigger hearts than we do.

But if you're concerned about tree-eating wasps or sawflies emerging in your home, there's little need to worry. Both of these species are nowhere to be found in chilly weather and are safely ensconced by the time carols are playing in shops.