Northern leafcutters cultivate a cold-tolerant fungus

Atta texana

Leafcutter ants are normally thought of as tropical animals. These ubiquitous insects are commonly featured in Amazonian nature documentaries, for example, and they constitute one of the most damaging pests of South American agriculture.

But the ants, in spite of our latitudinal stereotyping, do extend into the temperate zone. Atta texana occupies the humid climes of Texas and Louisiana, forming the northern edge of the distribution of all leafcutters. But why do leafcutter ants stop there? Why wouldn’t they live in New York, or Manitoba?

Northern cultivars are more tolerant of cold than southern ones (Figure 1D from Mueller et al 2011)

Last week Ulrich Mueller’s lab at the University of Texas published an ambitious study in PNAS examining one factor that might limit the northern range of the ants: the ability of their symbiotic Attamyces fungus to withstand cold temperatures. (Remember that leafcutters are farmers, feeding only from a fungus cultivated in their underground nests.) Mueller et al gathered fungal samples from dozens of colonies at varying latitudes and tested their survival when challenged with cold. The results were clear: fungus from northern nests survived longer under cold temperatures than fungus from southern nests.

That Attamyces from different latitudes have different cold tolerances suggests that winter temperature exerts significant selection pressure on the system. Thus, Mueller et al’s study is indirect evidence that the low temperatures of the North American winter might be the key barrier preventing the pesty leafcutters from chewing their way across the midwestern soybean belt.

The chronic contrarian that I am, though, I can’t just let Mueller et al’s findings sit at that.

I’d like to offer a further relevant observation having to do with the seasonality of the fungus’s food. In temperate North America, most green vegetation dies back in October and doesn’t reappear until March or April. That’s 5 months without the fresh vegetation that Attamyces requires. So it might also be that the system simply starves out if stretched too far north.

In way of support, consider Trachymyrmex septentrionalis, a related fungus-growing ant:

Trachymyrmex septentrionalis weathers longer and colder winters than Atta texana

Trachymyrmex does not cultivate its fungus with live vegetation like Atta but with bits of dead vegetative detritus. These food sources are available for more months of the year. And here’s the kicker. Trachymyrmex is found as far north as Illinois and Long Island. If they required fresh-cut leaves I’m not sure they’d make it.

Anyway, I am just a guy with a hunch typing away on a blog. Mueller et al have a peer-reviewed paper and piles of data. Make of it what you will.

Source: Mueller, U. G. et al 2011. Evolution of cold-tolerant fungal symbionts permits winter fungiculture by leafcutter ants at the northern frontier of a tropical ant–fungus symbiosis. PNAS published ahead of print, doi:10.1073/pnas.1015806108

6 thoughts on “Northern leafcutters cultivate a cold-tolerant fungus”

  1. mrilovetheants

    I’d be curious to know the earliest the Trachymyrmex septentrionalis would start foraging each year. Some plants like witch hazel and skunk cabbage bloom in January and February. Would anyone consider this a cold season ant?

  2. I am not sure about the fresh greens and die back idea though I think it is part of the puzzle…in my limited experience in North Central Texas, some things die back but others come out perhaps in part due to the relief from the summer heat. For instance, some of the speedwells and bluets come out in January and die back by April or May.

    In my field sites in Fort Worth, I find Trachymyrmex and not Atta, though Wheeler found Atta in my sites in the early 20th century (Wheeler came to UT in 1899).

    1. Ah, but do leafcutters gather those little winter herbs, even if avaialble? If ambient temperatures don’t permit frequent and regular foraging for fungus-food, as occurs farther south, then their availalbiltiy is not significant.

      As I noted above, T. septentrionalis goes without foraging for many months in the northern parts of its distribution. Here in the St. Louis area, they only are active from late April through late September, and indeed, their peak activity is May, with their presence quite inconspicuous the rest of the season. The month of May coincides with the abundant frass-fall from the spring caterpillars on new tree foliage (the same caterpillars that are the sine-qua-non for the Neotropical migrant passerines to complete their spring travels).

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