Barry Muir | Guest Contributor
In Part 1 (Muir 2024a) of this series of five articles, I discussed climate change to set the background for what follows. In Part 2 (Muir 2024b), I discussed the impact of climate change on endophytic fungi: those ubiquitous but unseen upholders of plant and animal life. Part 3 (Muir 2024c) was about mycorrhizal fungi that live in the roots of plants, and how climate change damage to mycorrhizae appears to be altering habitats on which birds depend. In Part 4 (Muir 2024d), I talked about Mangroves and Flyways and the role of migratory birds in dispersal of seeds and fungal spores.
In Part 5, the final in this series, I discuss entomopaths – specialised fungi that are parasitic on insects and other invertebrate food sources that insectivores, including many bird species, depend upon. Entomopathic fungi are becoming more prevalent in the environment because of climate change, and causing greater loss of insectivores’ food sources.
Entomopaths (Greek entomon = insect; ‘path’ derived from Latin pathologia = the study of disease) are fungi that attack and kill insects, spiders, caterpillars and other small beasties.
The primary hosts of entomopathic fungi are aphids, leafhoppers, flies, beetles, caterpillars, thrips, and mites, beetle larvae, moths, wasps, cicadas, spiders and even pill-bugs. These are all vitally important foods for insectivorous birds.
You have probably heard of or read about two well-known entomopaths.
Cordyceps growing on a spider. Photo courtesy of Bridgette Gower, Aussie Macro Photos.
The first one, Cordyceps or “caterpillar fungus”, attacks caterpillars and then grows a club-like fruiting body out of the corpse. These were harvested in their thousands in Tibet, Nepal, Bhutan and China for their supposed medicinal benefits. Overharvesting and climate change (Hopping et al., 2018) have resulted in their now being considered endangered. Although climate change is causing some entomopathic fungi to become more prevalent in the environment, they are also a natural insect control, and balanced numbers of entomopaths are necessary in nature.
The second well-known entomopath is Ophiocordyceps that causes “Zombie Ants”, and attacks other insects. The fungus infects ants making them climb a bush or tree and chomp down on a vein on the underside of a leaf. The fungus fruit-body then grows out of the ant’s “neck” and scatters its spores where other ants can become infected. Cicadas are also victims.
Ophiocordyceps sobolifera on cicada nymph. Photo courtesy of Brendan Watts.
The spores of entomopathic fungi are either ingested by the victim, or they attach to its external skeleton (exoskeleton), germinate and grow into its victim, where they proliferate. The victim’s death is caused by tissue destruction and, occasionally, by toxins produced by the fungus. The fungus emerges from the insect's body to produce spores that spread infection with wind, rain, or contact with other insects. Dead insects may be covered with fungi.
Gibbelula fungus on a moth. Photo courtesy of Bridgette Gower, Aussie Macro Photos.
An excellent rundown on entomopaths can be found in a Cornell University website (extracted from Hoffmann, MP and Frodsham, AC (1993): Natural Enemies of Vegetable Insect Pests. Cooperative Extension, Cornell University, Ithaca, NY. 63 pp.).
Climate change, industrial pollution, vehicle exhaust fumes, and excessive (sometimes unnecessary) use of pesticides, as well as increasing numbers of entomopathic fungi, is putting insects under stress (Cinel et al. 2020, Iwona Wojda 2017). The insect stress hormones, octopamine and adipokinetic hormone, act in insects in the same way as cortisol stress hormone does in us – it makes them more susceptible to disease.
The stress manifests itself in the insects and spiders as reduced feeding and lethargy. They do, however, have some defence: their hard cuticles are difficult to penetrate; they have antibodies to fight the disease; and they may even bask in the sun to raise their temperature, just as we heat up when we get a fever.
There is even a fungus found in the USA that makes the abdomens of cicadas drop off, but the cicadas stay alive and fly about scattering spores. The fungus even produces hormones that encourage the cicadas to try to mate, transferring spores to the partners, and increasing spore spread.
Some insect species, including many pest insects, are particularly susceptible to infection by naturally occurring insect-pathogenic fungi. These fungi are very specific to insects, often to particular species, and do not infect humans, other animals or plants. Fungal growth is favoured by moist conditions, but some fungi also have resistant stages that maintain infection potential under dry conditions.
With a warming climate, entomopathic fungi can spread quickly through an insect population and cause its collapse. Because fungi penetrate the insect body, they can infect sucking insects such as aphids and leaf-hoppers that are not susceptible to bacteria and viruses.
A warming climate and the stress on invertebrates from toxic gases and pesticides is making the entomopathic fungi spread rapidly. The result is a dwindling supply of food for many animals, especially birds and other insectivores such as small native mammals.
But – there is another catch (isn’t there always?). Entomopathic fungi are being developed more and more to work for us as “environmentally friendly” pesticides because they do not contain chemicals: great – right?
The catch is that the entomopathic fungi used to kill crop pests will only kill the most susceptible and weakest insects. The ones with a good immune system will survive, breed, and become the next plague of pests which will laugh at our feeble entomopathic pesticides and we will need to develop something stronger – the fungus/insects arms race is set in motion.
Just as food for thought, some of these super-fungi developed to control pests are already spreading into wild insect populations. The result is obvious for insectivores – and scary.
References
Available on email request to the author at unit57.may@gmail.com