Stinkbugs have evolved fungus-filled leg organs to protect their eggs

How do you like your eggs – hardboiled, scrambled or covered in fungus? For the stinkbug Megymenum gracilicorne, the latter is very much preferred.

While some fungi can cause serious diseases in insects, others have a much more amicable relationship with these stinkbugs. The fungus feeds on a layer of carbohydrates on the outside of their eggs, forming a layer that protects it from parasitoid wasps such as Trissolcus brevinotaulus.

These wasps normally lay their eggs inside those of other insects, with the larvae then feeding on their victims from the inside out. But these stinkbugs have a novel way of protecting their young: a layer of fungus prevents the parasitoid wasps from laying its own eggs and protecting the developing stinkbug eggs.

What’s more, researchers have discovered that the stinkbugs have evolved a special organ on their back legs in which to house their fungal ally.

Professor Takema Fukatsu, one of the co-authors of the new research, said that the discovery was “totally unexpected”.

“Some insects, such as bark beetles, are known to have external cavities known as mycangia where they can host symbiotic microbes,” Takema says. “But the shape, architecture and number of mycangial pits of Megymenum gracilicorne are extraordinary.”

“It is difficult to find an equivalent of these fungus-filled organs in any other organism.”

Dr John Noyes, one of our Scientific Associates who specialises in parasitoid wasps, adds that this style of parasitoid wasp defence hasn’t been seen before.

“Everything’s possible in nature and this is a unique system, as far as I’m aware,” says John. “There are insects which produce wax filaments to prevent parasitoids attacking them or their eggs, such as scale bugs, but nothing like this.”

“It goes to show just how much we still don’t know about insects.”

The findings of the study were published in the journal Science.

How fungi and insects can work together

This type of relationship between the fungi and M. gracilicorne is known as a symbiosis, a mutually beneficial relationship between different organisms.

Many different insects and fungi are known to have a symbiotic relationship. Some termites, for example, farm fungi to eat while a variety of insects have fungal symbiotes that help them eat wood.

One such group of insects, known as ambrosia beetles, take fungi with them as they colonise new trees. The fungi are released from the beetles’ mycangia and grow into the wood, helping to break it down and releasing the nutrients within for the beetles to feed on.

The hindleg organs of M. gracilicorne, however, are much more densely packed with fungi than the mycangia of ambrosia beetles. Each stinkbug organ contains as many as 2,000 fungus-growing pores!

In fact, the structure is so unlike any known mycangia that historically most scientists assumed it was part of the organ that the stinkbug uses to hear, known as a tympanum.

How do fungi protect stinkbug eggs?

As M. gracilicorne females approach the egg-laying stage of their life, they start picking up fungi from the environment and storing them in their specialised hindleg organs.

While the exact species of fungi they collect varies between populations and over time, it is often Simplicillium and Cordyceps fungi. These genera both contain well-known insect-killing species, but the stinkbugs appear to be able to select the less deadly species to collect.

As the stinkbugs lay their eggs, the scientists saw the females using the tip of their back legs to scratch its fungus-filled hindleg organs. They then smeared the egg with the fungus, which grew to cover the eggs within a few days.

Experiments exposing the T. brevinotaulus wasps to eggs with and without the fungus revealed that they could only lay their own eggs when the fungal layer had been partially or fully removed. Though it’s not currently certain how this works, it’s believed the fungus provides a physical barrier that prevents the wasps from laying.

“As well as providing a tough layer for the wasps to get through, it’s also possible that the fungus blocks the chemical cues they use to find their hosts,” John suggests.

“Female parasitoid wasps learn these chemicals from the egg they hatch from, and look for them again in later life when they’re ready to lay their own eggs. They have sensitive organs for this purpose, but the fungus might stop the signals getting through.”

Though a full coating of fungus seems to stop T. brevinotaulus getting to the stinkbug eggs for now, John says that it’s possible they may learn to adapt in the future. Other parasitoid wasps have learned to attack eggs from underneath to avoid defences on the top of the egg, and this species might evolve to do something similar.

It’s also possible that other wasps are engaged in a similar evolutionary arms race, as the researchers identified several other stinkbugs with similar hindleg organs. Takema says that they hope to investigate these species more in future research, as well as find out how the organs develop.

“We’ve seen that while Megymenum stinkbugs are associated with specific fungi, members of their wider family are not,” Takema explains. “We’ve already seen that certain genes are upregulated in the hindleg organs, so we’re looking to see if they can explain the differences in the composition of the fungi found there, as well as how these associations have evolved.”