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Title: Leaf endophyte load and fungal garden development in leaf-cutting ants

item VAN BAEL, SUNSHINE - Smithsonian Tropical Research
item ESTRADA, CATALINA - Smithsonian Tropical Research
item Rehner, Stephen
item FABIOLA SANTOS, J - Smithsonian Tropical Research
item WCISLO, WILLIAM - Smithsonian Tropical Research

Submitted to: BMC Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2012
Publication Date: 11/9/2012
Citation: Van Bael, S., Estrada, C., Rehner, S.A., Fabiola Santos, J., Wcislo, W.T. 2012. Leaf endophyte load and fungal garden development in leaf-cutting ants. BMC Ecology. 12:23.

Interpretive Summary: In the American tropics and subtropics leaf-cutting ants can be severe agricultural pests of crop plants. Knowing which kind of leaf material the ants prefer especially in regard to the presence of fungal endophytes that live inside plant leaves may help to deter them from consuming crops. In this research the amount of foraging by ants of leaves with high or low endophyte content was measured. It was determined that there was no significant difference between their preference for leaves with high or low endophyte content. However, the ants produce a slightly greater mass of fungal garden using leaves with a low endophyte content. Use of leaves with high endophyte content resulted in higher mortality for the ants that may lead to strategies that will protect economically important plants. This research will be used by crop managers and ecologists to reduce the presence of leaf-cutting ants by increasing the fungal endophytes in the leaves of crops.

Technical Abstract: Previous work has shown that leaf-cutting ants prefer to cut leaf material that is relatively low in fungal endophyte content. Such a preference suggests that fungal endophytes exact a cost on the ants or on the development of their colonies. We hypothesized that endophytes may play a role in their host plants’ defense against leaf-cutting ants. To measure the long-term cost to the ant colony of fungal endophytes in their forage material, we conducted a 20-week laboratory experiment to measure fungal garden development for colonies that foraged on leaves with low or high endophyte content. We found that the colony mass and the fungal garden dry mass did not differ significantly between the low and high endophyte feeding treatments. There was, however, a marginally significant trend toward greater mass of fungal garden per ant worker in the low relative to the high endophyte treatment. This trend was driven by differences in the fungal garden mass per worker from the earliest samples, when leaf-cutting ants had been foraging on low or high endophyte leaf material for only 2 weeks. At two weeks of foraging, the mean fungal garden mass per worker was 77% greater for colonies foraging on leaves with low relative to high endophyte loads. Our data suggest that the cost of endophyte presence in ant forage material may be greatest to fungal colony development in its earliest stages, when there are few workers available to forage and to clean leaf material. This coincides with a period of high mortality for incipient colonies in the field. We discuss how the endophyte-leaf-cutter ant interaction may parallel constitutive defenses in plants, whereby endophytes reduce the rate of colony development when its risk of mortality is greatest.