Stinkbugs Use Fungi as Living Shields for Their Eggs Against Parasitic Wasps
- Gauri Khanna
- 11 hours ago
- 4 min read
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Female stinkbugs of the species Megymenum gracilicorne cultivate symbiotic fungi on specialised glands in their hind legs, then deliberately coat their eggs with the organisms to create a protective barrier approximately 2 millimetres thick.
Laboratory experiments demonstrated that parasitic wasps attacked 62 per cent of unprotected eggs but only 10 per cent of fungi-covered eggs, suggesting the fungi provide physical rather than chemical defence.
The relationship is unusual amongst insect symbioses because stinkbugs lose their fungal partners after moulting and must reacquire them as adults, offering researchers a model system to study how insects select beneficial microbes.
An Accidental Discovery in the Laboratory
Takema Fukatsu, an evolutionary biologist at the National Institute of Advanced Industrial Science and Technology, spent three decades studying insect symbioses before stumbling upon one of nature's more peculiar defence strategies. His focus on Megymenum gracilicorne, a drab stinkbug and the only local representative of the understudied Dinidoridae family, initially centred on understanding a mysterious anatomical feature rather than fungal partnerships.
The species possesses an enlarged section on its hind limbs that researchers presumed functioned as a tympanal membrane, an eardrumlike hearing structure common in many insects. Yet only females displayed this organ, whilst males retained slender back legs. The sexual dimorphism puzzled Fukatsu, as hearing organs typically evolve in both sexes.
The investigation took an unexpected turn when retired stinkbug specialist Shuji Tachikawa contacted Fukatsu to share decades-old observations. In the early 1970s, Tachikawa had documented a now-vanished Tokyo population of M. gracilicorne in which females carried a white substance on their hind legs that also appeared on their eggs.
Decoding the Purpose of Gossamer Fibres
When Fukatsu's team collected M. gracilicorne specimens from wild cucumber vines along a rural riverbank, they observed gossamer fibres on sexually mature females—fungal patches smaller than rice grains. Scanning electron microscopy revealed that the structure's surface was not smooth like a hearing membrane but rather polka-dotted with tiny holes from which fungi emerged. Dissection showed each hole contained a gland secreting an unknown substance.
Laboratory observations revealed a remarkable behaviour. As females laid rows of eggs, they paused to scratch the fungus on each leg with the claws of their other foot, then rubbed the newly laid egg. Fungi subsequently grew over the eggs, enveloping them in a layer about 2 millimetres thick within three days. Cameron Currie, an evolutionary biologist at McMaster University, noted the striking appearance of the phenomenon, describing it as looking like pathogenic fungal overgrowth.
DNA sequencing identified several fungal species present on the eggs, all considered benign to insects. The researchers hypothesised the fungi might defend against Trissolcus brevinotaulus, a parasitic wasp that deposits its own eggs inside stinkbug eggs.
Physical Barriers Prove More Effective Than Chemistry
To test their hypothesis, the scientists placed five sexually mature female wasps—raised in laboratory conditions—into a small chamber containing approximately 20 stinkbug eggs. Half the eggs retained their fungal coating; the other half had it brushed off. The wasps approached eggs by tapping them with their antennae, but displayed clear aversion to fungi-covered specimens. They carefully circled around these eggs and groomed themselves after contact. Across multiple trials, wasps parasitised 62 per cent of clean eggs compared to only 10 per cent of fungi-covered eggs.
The researchers concluded the fungi repel wasps through physical shielding rather than chemical defence, finding no evidence of protective bacteria or deterrent compounds. Fukatsu and colleagues speculate the fungi likely feed on secretions from the gland cells, benefiting both parties in the arrangement.
A Symbiosis That Resets Each Generation
Additional experiments revealed that newly hatched stinkbugs retain some fungi initially but shed them after moulting. This loss represents an unusual pattern amongst insect symbioses, according to Martin Kaltenpoth, an evolutionary ecologist at the Max Planck Institute for Chemical Ecology. Adult stinkbugs must subsequently acquire fungi and permit colonisation of their hind legs, a process raising critical questions about microbial selection.
Kaltenpoth emphasises the importance of understanding how insects ensure they acquire beneficial rather than pathogenic fungi. The stinkbug-fungus system offers particular research value because, unlike most exclusive and long-lasting insect symbioses, stinkbugs allow various fungal species to colonise their hind legs. This flexibility enables researchers to conduct comparative experiments between symbiotic and non-symbiotic fungi, potentially identifying genes or traits that foster beneficial relationships.
Kayla King, an evolutionary ecologist at the University of British Columbia, notes the partnership's unusual nature: the fungi provide physical rather than chemical defence, distinguishing it from most insect-microbe relationships.
Implications Beyond Entomology
Fukatsu's team plans to investigate microbial selection mechanisms by disabling genes potentially used by gland cells when secreting substances that influence fungal colonisation. Such genetic insights may ultimately enable techniques to isolate and manipulate environmental microbes, applications relevant to fields including drug discovery. Fukatsu also notes that a related stinkbug species features in traditional Asian medicine, suggesting the hind limb secretions or associated fungi might possess therapeutic properties.
Currie views the findings as a reminder of fungi's essential role in human life, from beer and bread production to industrial applications. The dependence may be comparable to that of stinkbugs, he suggests, highlighting the often under-appreciated importance of these organisms across biological systems.