Metarhizium robertsii Reveals Why Fungi Defy Trade-Offs

In the natural world, excelling at one task often comes at the expense of another. Biologists refer to these compromises as trade-offs, and they appear almost everywhere they investigate.

Yet a handful of fungi appear to defy that pattern. One species can prey on insects, decompose plant material in soil, and enter living roots as a beneficial resident.

Scientists have long been puzzled by such adaptability. New research now identifies one underlying explanation.

The discovery may alter how farmers manage pests. It also speaks to the broader question of how organisms develop entirely new roles.

A fungus with three jobs

Some soil fungi have remarkably unusual lives. At one point, a fungus can act as a deadly insect pathogen; at another, as a decomposer; and at other times, as a plant partner that delivers captured nitrogen directly to roots.

This final ability is particularly unusual. After killing an insect, the fungus extracts nitrogen from its remains and supplies it to a plant in exchange for sugars.

The same fungus also acts as a recycler in soil. By consuming dead matter, it releases nutrients that other organisms can use.

Its hunting behaviour has the feel of a horror film. Spores land on an insect, penetrate its outer shell, and consume the animal from the inside.

For years, scientists questioned how a single species could perform such contrasting roles. Destroying an insect and supporting a plant seem to demand opposing abilities.

The missing trade-off

Researchers at the University of Maryland (UMD) set out to find the biological compromise they expected to be present.

They assumed that fungi more effective at forming plant partnerships would sacrifice some of their insect-killing ability.

After all, specialists usually pay a price for their narrow focus. An organism adapted to perform one job exceptionally well is commonly less adept at others.

Instead, the results revealed something unexpected. The two abilities increased and decreased together, both linked to one concealed characteristic.

“We expected to see a trade-off – that becoming a better plant partner would come at the cost of being a good killer or vice versa,” said Raymond St. Leger, a Distinguished University Professor in the Department of Entomology at UMD.

“Instead, the two abilities rise and fall together, and what links them is the fungus’s underlying nutritional flexibility.”

Testing Metarhizium robertsii

The team centred its research on Metarhizium robertsii. Farmers already use this fungus worldwide to manage insect pests, while laboratories are testing its potential to enhance crop growth.

Its established practical use made it a strong candidate for the study. This strain is widespread, extensively researched, and straightforward to compare between generations.

St. Leger and postdoctoral associate Huiyu Sheng examined eight strains spanning the species’ evolutionary tree. Their work relied as much on fungal genomes as on observed behaviour.

Genomic information placed every strain within that family tree. The resulting map indicated how the two lifestyles had diverged over millions of years.

The researchers measured the speed at which each strain killed insects and its ability to establish itself in roots. They also assessed toxin potency and growth on 95 distinct food sources.

Two survival strategies

The strains separated clearly into two groups. Their split could be traced to the point at which the lineages first diverged, in some cases more than six million years ago.

Older lineages acted like sleepers. Although they kill slowly, they devote resources to reproducing within their hosts and generating millions of spores.

These spores depend on patience. They remain in place until another insect comes close enough.

Younger lineages behaved more like creepers. They grow rapidly on both insect skin and plant roots, kill swiftly, and often use paralysing toxins.

Creepers invest little in producing spores. Rather than relying on them, they extend fine fungal threads from a dead insect towards nearby roots.

Both approaches are evidently successful. Each has persisted on its own terms for millions of years.

Metabolism links the fungal abilities

One characteristic separated these groups more strongly than any other. Researchers call it metabolic breadth: the range of nutrients that a strain is genuinely able to consume.

Fungi able to use a broader selection of sugars, amino acids, and organic acids performed better overall. They infected insects more easily and entered roots with fewer difficulties.

“Instead of forcing fungi to choose between being insect killers or plant partners, evolution appears to have favored strains that are simply better at making use of whatever resources they encounter,” St. Leger said.

“Their versatility begins with metabolism.”

One toolkit, many targets

The findings recast these insect-killing fungi as organisms with wide-ranging capabilities. Their capacity to attack insects and assist plants stems from a shared nutritional toolkit.

Scientists describe strains of this kind as environmentally competent. The term reflects how one metabolism can sustain dramatically different lifestyles.

The mechanisms that break down an insect also help process what roots provide. One toolkit can therefore operate across many targets.

This research also gives biologists a clear model for a far larger question: how do microbes evolve the flexibility to move between such different modes of life?

What this means for farming

The findings have significant agricultural implications. Farmers may eventually be able to pair particular fungal strains with specific objectives.

Strains with broad diets could suppress pests quickly while also colonising roots and supporting crop growth in fields.

Strains that produce abundant spores may serve another role. Their persistence could make them more suitable for gradual, long-term pest management.

Broader implications of the study

The benefits extend beyond convenience. Fungal pest controls can reduce the use of chemical sprays that harm soil and water.

“What we’ve learned could help growers use fungal pathogens more effectively,” St. Leger said.

Selecting the appropriate strain for each purpose could make these tools substantially more reliable. This precision is what the researchers find especially promising.

There is also a simple lesson in the findings. A broad appetite can create opportunities that narrowly focused specialists may never access.