A recent discovery was made in the fascinating world of insect defense. Female stink bugs, specifically the Megymenum gracilicorne species, have evolved an unusual method to safeguard their eggs from parasitic threats, by cultivating a garden of fungi on their hind legs, a symbiotic relationship that provides crucial protection against parasitic wasps.
Discovery of the fungal defense mechanism
The research, led by Takanori Nishino at the National Institute of Advanced Industrial Science and Technology in Japan, uncovered this defense mechanism while examining the tympanal organ of female stink bugs. Traditionally understood to be an auditory structure, this organ, located on the hind legs, was actually found to be a porous cuticle called a glandular mycangia, which was housing a diverse array of fungal hyphae. This fungus wasn’t there incidentally; it play a pivotal role in the stink bugs’ reproductive strategy.
How the fungus protects the eggs
When a female stink bug lays her eggs, she deliberately scratches at the fungal-filled pores with her other leg. This action spreads the fungi over the eggs, coating them in a protective layer of hyphae. As the eggs develop, the nymphs that emerge are also covered in this fungal coating until they molt. This coating is not just for show; it has a significant impact on the survival of the eggs.
An unclear evolution
The origin of the specialized hindleg organ and its role in fungal symbiosis in Megymenum gracilicorne is still unclear. Unlike other bugs with enlarged hindlegs used for fighting, these hindlegs are unique to female stink bugs in the Dinidoridae family. The researchers believe this organ may have originally evolved to smear a secretion on eggs for defense or camouflage. Over time, fungi likely colonized this organ, leading to a symbiotic relationship where the fungi help protect the eggs. Further research is needed to understand this evolutionary process across different stink bug species.
The threat of parasitic wasps
The main threat to stink bug eggs comes from a parasitic wasp known as Trissolcus brevinotaulus. These wasps lay their eggs inside the stink bug eggs, with the larvae consuming the stink bug eggs from the inside. However, the fungal layer on the eggs acts as a formidable barrier. The hyphae create a physical obstruction that prevents the wasp larvae from successfully penetrating the eggs. Interestingly, the wasps have in turn, adapted their strategies, with females having thicker antennae to help them breach the fungal defenses.
Symbiosis for survival
The significance of this fungal symbiosis lies in its effectiveness as a defensive strategy. The fungi, primarily benign Cordycipitaceae entomopathogens, do not harm the stink bugs in any way but instead provide a crucial protective shield. This form of external fungal symbiosis represents a novel adaptation, where the stink bugs use their elaborate morphological and behavioral traits to select and cultivate specific fungal associates for defense.
This symbiotic relationship is a prime example of how insects can develop intricate and specialized defenses against natural enemies. The cultivation of fungi on their eggs and bodies showcases an advanced level of evolutionary innovation, blending physical and biological defenses in a way that ensures the survival of their offspring.
