Gif by Zachary Hajian-Forooshani
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Asian tramp snails, Bradybaena similaris, crawl on a coffee leaf with rust fungus.

Gif by Zachary Hajian-Forooshani

How this invasive snail could save your coffee from destruction

An orange trail of poop reveals an unlikely ally against one of coffee’s worst threats.

If you think Daylight Saving Time is the biggest drag on your morning alertness, you may want to consider coffee leaf rust.

Coffee leaf rust is the most economically significant coffee pest in the world. Since the 1980s, occasional outbreaks of this fungus have crippled coffee production in the Americas, at times doubling or tripling its price. And the problem has escalated since 2008, as have the massive resources invested in fighting it.

A new discovery raises an intriguing possibility for controlling coffee’s costliest pest, and it comes packed inside a small shell. According to recent research from the University of Michigan at Ann Arbor, Asian tramp snails, Bradybaena similaris, can consume large amounts of coffee rust before the disease can damage the plant. Leave one snail on a rust-covered leaf, and it can hoover up 30 percent of the fungus in 24 hours.

While coffee leaf rust has known predators and pathogens, gastropods—snails and slugs—have never been among them. There is also limited evidence of slugs eating other types of plant fungus at all. This also appears to be the first time that Asian tramp snails have been recorded changing their diet from plants to fungus.

Yet this potential biocontrol, a tactic of managing pests with other organisms, comes with a catch. Asian tramp snails are typically invasive and a crop pest in their own right. But the researchers may have stumbled upon a way to keep the snails’ diets focused on coffee rust.

Orange poop trail of salvation

Zachary Hajian-Forooshani, a doctoral student at the University of Michigan, and his advisors John Vandermeer and Ivette Perfecto, were conducting field research in Puerto Rico’s verdant central mountains in 2016, when they spotted thin trails of bright orange snail poop clinging to the underside of broad, green coffee leaves. The color of the excrement perfectly matched that of the coffee leaf rust.

Curious, the researchers collected samples of both Asian tramps snails and the native Caribbean snail, Bulimulus guadalupensis, which often shared the coffee leaves. The group left each snail overnight with leaves bearing coffee leaf rust in a controlled lab environment. After 24 hours, only the tramp snails had cleared the coffee leaf rust from the leaves.

The following year, the group returned to the same coffee plantation to conduct additional experiments. While collecting the snails this time, they noticed a high abundance of Lecanicillium lecanii, a fungal parasite and known enemy of the rust. The co-occurrence led them to wonder if Asian tramp snails consumed both the fungal rust and its pathogen, which is known as a mycoparasite.

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A typical coffee leaf infected with the coffee leaf rust fungus, left, alongside a leaf that has apparently been cleared of the orange rust spores by snails. A trail of orange snail excrement is visible on the left edge of the leaf on the right.

But deciphering this three-way relationship is tricky. Coffee leaves with more rust spots also have more L. lecanii mycoparasites. So, the snail’s hunger could be focused on the rust, while the L. lecanii mycoparasites are just bystanders, or it could have a taste for both.

So the team collected coffee leaves infected with rust, but this time included some leaves that also harbored the L. lecanii mycoparasites. Once again, they left individual snails with single coffee leaves in dark containers for 24 hours.

A statistical analysis of the snails eating habits indicated a significant preference for leaves that contained both the rust fungus and L. lecanii mycoparasites. Moreover, the snails appeared to consume more rust when the leaves had a higher abundance of the mycoparasites.

Hajian-Forooshani explains that many organisms can change what they do and how they do it, when placed in a novel situation.

“This is generally accepted as a reality, given the spontaneous emergence of pests around the world,” Hajian-Forooshani says, “as well as the unforeseen interactions of invasive species and biological control agents within ecological communities around the world.”

The coffee guilds

The relationship between coffee leaf rust, Asian tramp snails, and L. lecanii mycoparasites might be an example of a phenomenon called “intraguild predation,” in which a predator consumes prey that has been infected with a pathogen. Theoretical research has suggested that intraguild predation can work to prevent pest outbreaks.

The intraguild interaction may stabilize the mycoparasite population, so that it provides better long-term control or suppression of the coffee leaf rust, says Stacy Philpott, a professor at the University of California, Santa Cruz who specializes in agroecology, biodiversity, and biocontrols for coffee pests.

“That could be an important ecological role played by the snail,” says Philpott, who was not part of this study. Her lab has witnessed a similar intraguild for coffee protection, via a relationship between ants and two plant-eating insects, aphids and scales.

Scales and aphids both feed on coffee plants. Ants, in turn, harvest sugar from these insects and protect them from other predators. While this sounds bad for coffee and even though farmers often mistake these bugs as being harmful, the ants are often aggressive towards other more damaging herbivores, resulting in a net benefit to the plant.

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The irony of Asian tramp snails providing an agricultural service by eating coffee leaf rust is that the snail itself is considered a severe agricultural pest, munching on citrus crops, grapes, legumes, cabbage, and mustard greens around the world. For this reason, among others, the researchers interviewed for this story all advised great caution in exploring the use of the invasive snails as a biological pest control agent.

At the moment, too little is known concerning what Asian tramp snails might bring to the ecosystem. For example, the snail is eating the mycoparasites along with the rust, but what happens if it consumes too many? Could it permit the rust to grow faster and farther than before, destabilizing the coffee plantation ecosystem?

“Classic approaches to pest control have often failed to understand this complexity, causing major natural disasters,” said Estelí Jiménez-Soto, an agroecologist at the University of California, Santa Cruz, whose work often focuses on both coffee and biocontrol measures. It also remains unknown whether this intraguild predation would be applicable as a biocontrol across or outside of Puerto Rico.

Hajian-Forooshani says future field surveys will be conducted to assess how widespread the gastropod consumption of coffee rust is on farms on the island and across Latin America.

During their last visit to Puerto Rico, the researchers noticed a few other, as-yet-unidentified slugs with what appeared to be coffee leaf rust spores inside them and which also left bright orange excrement. It remains possible that coffee leaf rust is more widely eaten by gastropods than previously thought and that this phenomenon has simply escaped scientific notice until now.

Another major question is whether the coffee leaf rust spores eaten by the snails remain viable after being excreted. If the spores can survive and propagate, then the snail’s predation may provide little benefit to coffee farmers (and drinkers).

But if this discovery does result in a viable form of biological pest control, it would certainly be welcome. Other control methods for rust must be constantly fine-tuned to keep up with the fungus evolving resistance to pesticides and fungicides. That’s where the snails may have an advantage.

“It is challenging to develop resistance to being eaten,” Hajian-Forooshani says.