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“Invasional Mutualism” Between Honey Bees and Myrtle Rust Pathogen | Blog

Figure 1.
Foraging of urediniospores by bees on plants infected with myrtle rust. a. Honey bee forager collecting Austropuccinia psidii urediniospores from leaves of broadleaf paperbark [Melaleuca quinquenervia (Myrtales, Myrtaceae)], Bungawalbin, New South Wales, Australia; b. A. psidii urediniospores in the opening of a Geraldton wax [Chamelaucium uncinatum (Myrtales, Myrtaceae)] flower bud in Brisbane, Queensland, Australia.

Photographs by Geoff Pegg.

A paper published in March 2026 in NeoBiota is entirely unsurprising to an evolutionary biologist, but deeply awkward for anyone trying to present nature as the intelligent design of an omnibenevolent creator. It reports what appears to be a mutually beneficial relationship between the introduced Western honey bee, Apis mellifera, and the invasive fungal plant pathogen myrtle rust, Austropuccinia psidii. It is a neat example of how evolution has no foresight, no moral purpose and no long-term plan. Symbiotic alliances can arise naturally between different species when there is an immediate benefit to both, even when the longer-term consequences for one or both partners — and for the wider ecosystem — may be destructive.

Myrtle rust, Austropuccinia psidii, is an invasive rust fungus in Australia. It infects more than 500 species in the Myrtaceae family, which includes many of Australia’s culturally, ecologically and economically important native plants, including eucalypts, paperbarks and related species. In Australia, the pathogen is regarded as a serious threat to native ecosystems, with around 17% of endemic vegetation considered at risk. The other partner in this apparent mutualism is also an introduced species: the Western honey bee, Apis mellifera.

The basis of this relationship is an exquisite example of the sort of functional complexity creationists routinely try to claim as evidence for intelligent design. That, of course, raises the obvious question: why would an omnibenevolent designer design a fungal pathogen capable of damaging so much of Australia’s native vegetation, and then provide it with a convenient pollinator-assisted dispersal system?

The mechanism is ingenious, but only in the blind, short-term sense in which natural selection can be ingenious. The bright yellow urediniospores of myrtle rust are collected by honey bees much as pollen is collected. The bees pack the spores into their pollen baskets and carry them back to the hive. For the fungus, this provides a potential route for dispersal beyond simple wind movement. For the bees, the spores are not just inert particles accidentally mistaken for pollen; they have real nutritional value. The researchers found that myrtle rust spores contained more than 22% protein and all ten amino acids regarded as essential for honey bee nutrition, making them comparable with high-quality pollen.

Laboratory feeding trials also showed that honey bee larvae could develop normally on a diet based on myrtle rust spores, with survival, development time and body weight similar to larvae reared on a high-quality pollen diet. Even more concerning from a biosecurity point of view, the spores remained viable inside honey bee colonies for at least nine days, meaning that hives could potentially act as reservoirs and transport systems for the pathogen.

So, the fungus gains a mobile vector, while the bees gain an alternative protein source. But what benefits both in the short term could be damaging in the longer term. As myrtle rust damages Myrtaceae-rich habitats, it can reduce the availability of flowers and pollen. That, in turn, could encourage honey bees to rely more heavily on fungal spores or other alternative foods, while the fungus benefits from the continued movement of bees and managed hives. The result could be a damaging ecological feedback loop, with plant-pollinator networks and forest regeneration placed under increasing pressure.

This is precisely the kind of outcome that any omniscient, intelligent designer should have foreseen and avoided. It is also precisely the kind of outcome that a mindless evolutionary process can produce without difficulty. Natural selection rewards immediate reproductive or nutritional advantage; it does not plan for ecological stability, protect biodiversity, or ensure that mutually beneficial relationships remain harmless in the long term.

What is Invasional Mutualism? In ecology, mutualism is a relationship between two different species in which both gain some immediate benefit. Classic examples include flowering plants and their pollinators, or fungi and plant roots in mycorrhizal associations. The relationship need not be planned, balanced, moral or beneficial to the wider ecosystem; it only needs to give each partner enough of a short-term advantage for the interaction to persist.

Invasional mutualism occurs when two introduced or invasive species help one another become established, spread or persist in a new environment. Instead of acting as isolated invaders, they interact in a way that makes the invasion more successful or more damaging.

In the case of Western honey bees and myrtle rust in Australia, both partners are alien to the ecosystem. The honey bee, Apis mellifera, was introduced to Australia and is now widely used in managed pollination. Myrtle rust, Austropuccinia psidii, is an invasive fungal pathogen of plants in the Myrtaceae family, which includes eucalypts, paperbarks, bottlebrushes, lilly pillies and many other native Australian plants.

The apparent mutualism is simple but potentially serious:

The bee benefits because myrtle rust spores resemble pollen closely enough to be collected and are nutritionally useful, containing protein and essential amino acids.

The fungus benefits because bees collect the spores, pack them into their pollen baskets and carry them back to the hive, potentially helping the pathogen disperse.

The ecosystem loses because the fungus attacks native plants, damages new growth, reduces fertility, causes defoliation and can kill susceptible species.

This is not the harmonious balance of a benevolent design. It is a short-term ecological bargain produced by natural processes. Each partner gains something immediately, but the wider consequence may be the destabilisation of plant-pollinator networks, the increased spread of a destructive pathogen and further pressure on Australia’s already vulnerable native flora.

That is exactly what evolution predicts: local advantages, opportunistic interactions and no foresight. Natural selection can produce impressive functional relationships, but it cannot look ahead and ask whether the eventual outcome will be good for forests, bees, native pollinators, or biodiversity.

The research and its implications for Australian flora were also discussed in a Pensoft® blog, published to mark World Bee Day on 20 May.

“Invasional Mutualism” Between Honey Bees and Myrtle Rust Pathogen
New research from NeoBiota uncovers an invasional mutualism between Western honey bees and myrtle rust, with potentially troubling consequences for Australia’s native ecosystems.
Surprising Interaction

Apis mellifera on leaves with myrtle rust.

Photo credit to Geoff S Pegg.

New research published in NeoBiota has found that the Western honey bee – an introduced species to Australia – and the devastating, invasive plant fungus known as myrtle rust (Austropuccinia psidii) may have formed a mutually beneficial relationship known as an “invasional mutualism.”

Myrtle rust is notorious for devastating ecologically and culturally significant native plants in the Myrtaceae family, putting 17% of Australia’s endemic vegetation at risk. While rust fungi generally rely strictly on wind to spread, researchers discovered that bees may actively forage on the bright yellow fungus spores, packing them into their pollen baskets and carrying them back to the hive just as they would regular pollen.

Through a series of experiments, the team made three significant findings. Firstly, the rust spores proved to be quite nutritious. They contained over 22% protein and all 10 essential amino acids, meeting the threshold required for bee colonies to survive. In fact, the fungus matched the nutritional quality of high-value floral pollen, like willow pollen.

Researcher taking samples from a beehive.

Beehive in the forest.

Photo credit to Caroline Hauxwell.

In laboratory feeding trials, larvae raised on a diet of myrtle rust spores grew up perfectly healthy, developing at the same speed and reaching similar body weights as bees raised on a traditional high-quality pollen diet. As the researchers explain:

These findings suggest that spore foraging may not be an aberration, but a viable foraging strategy for honey bees.

And perhaps the most alarming discovery is that the myrtle rust spores remain viable and capable of causing new plant infections for at least nine days inside a beehive which could pose significant biosecurity risks

A Devastating Ecological Feedback Loop

Taking samples from a beehive.

Photo credit to Caroline Hauxwell.

This discovery challenges the assumption that invasive species always act independently, and it carries major environmental consequences. As myrtle rust kills off keystone taxa in the Myrtaceae family, such as eucalypts, paperbarks, and other ecologically and culturally significant species, particularly in Australia, fewer flowers and less pollen become available for bees to forage on. Beyond the direct biodiversity loss, as the fungus kills these plants, fewer flowers and less pollen are available for the bees.

Under such conditions, bees may increasingly turn to alternative protein sources, such as fungal urediniospores
—the researchers explain.
This could set off a devastating ecological feedback loop.
Over time, this dynamic may destabilise plant-pollinator networks and forest regeneration, particularly in regions with high Myrtaceae endemism. While generalist foragers like A. mellifera may buffer their colony health by switching to spores or non-Myrtaceae pollen sources, the long-term ecological cost could be substantial, especially for specialist pollinators that lack such flexibility.

The authors

The risks extend beyond ecosystems. Because spores remain viable inside a hive for over a week, commercial beehives – regularly transported across the country over three to seven days to pollinate crops – now represent a significant pathway for human-assisted spread of the pathogen.
As the lead author, Sacchi Shin-Clayton (University of Cambridge) emphasises:
Apis mellifera is an introduced species used as a commercial pollination agent worldwide, and shifting honey bee colonies between agricultural sites to boost pollination has become a standard practice. This reliance on honey bee colonies and shifting between multiple sites is quite concerning, given the demonstrated interaction between A. mellifera and myrtle rust, and its longevity within colonies.

Despite this, current biosecurity strategies for managing myrtle rust do not account for the movement of commercial beehives, leaving a critical gap in disease management approaches.
We propose that honey bees be explicitly considered in both epidemiological models and the formulation of management and containment strategies
—the researchers urge.
Recognising pollinators as potential vectors of invasive plant pathogens is an essential next step – one that could prove critical for protecting Australia’s vulnerable native forests. This World Bee Day is a timely reminder that our relationship with bees is more complex than it might seem. Protecting them and the ecosystems they move through will require us to understand that complexity better.

Publication:
Shin-Clayton S, Mortensen AN, Beggs JR, Buxton MN, Hauxwell C, Bateson MF, Jochym M, Pegg GS, Pattemore DE (2026)
Honey bees as potential vectors of the invasive rust pathogen Austropuccinia psidii: nutritional mutualism and implications for pathogen spread.
NeoBiota 106: 75-90. https://doi.org/10.3897/neobiota.106.169027

Abstract
Myrtle rust fungus, Austropuccinia psidii (Basidiomycota, Pucciniales), infects more than 500 species in the Myrtaceae family and is a significant threat to keystone plant taxa, particularly in Australia. Urediniospores are primarily dispersed by wind, but there are several reports of animal-mediated dispersal events. Recent observations of Apis mellifera (Western honey bees) collecting urediniospores of A. psidii raise the possibility of a novel mutualistic interaction between these two alien species. Here, we test two key conditions required for such a mutualism: (1) that spores remain viable after entering hives and (2) that they provide nutritional benefit to the pollinator. We show that A. psidii spores remain viable for at least nine days within honey bee colonies and match or exceed the nutritional quality of known high-protein pollens. Trials using artificial rearing of bee larvae further demonstrate that A. psidii spores support normal development and survival in honey bee brood. These findings suggest that spore foraging may not be an aberration, but a viable foraging strategy for honey bees. Our results support the hypothesis of a mutualism and dispersal of these plant pathogens, whereby nutritional benefits to A. mellifera facilitate the long-distance dispersal and epidemiology of A. psidii. This has implications for models of pathogen spread, especially in regions where managed hives are regularly transported over long distances in agricultural pollination services landscapes. Recognising pollinators as potential vectors of invasive plant pathogens is essential for developing more effective biosecurity and conservation strategies.

Shin-Clayton S, Mortensen AN, Beggs JR, Buxton MN, Hauxwell C, Bateson MF, Jochym M, Pegg GS, Pattemore DE (2026)
Honey bees as potential vectors of the invasive rust pathogen Austropuccinia psidii: nutritional mutualism and implications for pathogen spread.
NeoBiota 106: 75-90. https://doi.org/10.3897/neobiota.106.169027

Copyright: © 2026 The authors.
Published by Pensoft Publishers. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

So here we have another example of nature behaving exactly as evolutionary theory says it will behave, and not at all as creationism would require it to behave. There is no foresight, no moral purpose and no concern for ecological harmony. There is only the immediate advantage available to organisms capable of exploiting it. The bee obtains a usable food source; the fungus gains a potential dispersal mechanism; the wider Australian ecosystem may pay the price.

For creationists, this is an uncomfortable problem. If they insist that complex, functional interactions between organisms are evidence of intelligent design, then they must explain why their putative designer intelligently designed a pathogenic fungus to mimic pollen, feed honey bee larvae and use an introduced insect to help spread itself through some of Australia’s most important native plant communities. And if they argue instead that this is not design but corruption, degeneration or the consequence of “sin”, they have abandoned their own argument that functional complexity points to design whenever it suits them.

For an evolutionary biologist, however, there is no mystery. Natural selection does not produce what is kind, sustainable or ecologically wise; it produces what works well enough, here and now, to enhance survival and reproduction. A relationship that is mutually beneficial today can become destructive tomorrow. A short-term advantage can generate long-term instability. Evolution has no mechanism for pausing to consider the health of Australian forests, the survival of vulnerable Myrtaceae, or the eventual consequences for pollinators and plant communities.

That is why examples like this are so revealing. They show, yet again, that the living world is not the product of a perfect moral intelligence designing for balance and benevolence, but of blind, opportunistic, natural processes acting on immediate advantage. The result can be beautiful, intricate and ingenious — but also wasteful, cruel and ecologically disastrous. In other words, it looks exactly like evolution, and nothing like intelligent design.

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