Selecting the drought stressed: Effects of plant stress on intraspecific and within-plant herbivory patterns of the leaf-cutting ant Atta colombica

1. Leaf-cutting ants (LCA) are highly polyphagous but at the same time highly selective dominant herbivores of neotropical rainforests. While a whole range of leaf properties has been identified to influence interspecific host choice, the mechanisms underlying intraspecific and intra-individual herbivory patterns remain obscure. Here, we evaluate the plant-stress hypothesis by analysing whether and how drought stress of food plants positively affects LCA food plant selection. 2. In bioassays with whole plants of Piper marginatum, Atta colombica workers harvested more than twice the leaf area of stressed than of vigorous control plants. Within individual plants, the attractiveness of a given leaf increased with its stress level. 3. Drought stress induced an accumulation of proline and non-structural carbohydrates in the plant tissue by a factor of 3.5 and 2, respectively, accompanied by a decrease in the leaf water content of about 35%. Moreover, samples taken from preferred leaves within a plant contained more osmolytes than those representing the whole plant. 4. Ants were shown to detect and prefer these osmolytes in bioassays conducted with leaf discs that had been experimentally enriched with proline and/or sucrose. We therefore propose a mechanism that links the preference of LCA to drought stress via the osmolyte concentrations within the leaves. 5. Our results support the plant-stress hypothesis, confirming that stressed plants and plant parts are more attractive to LCA. This may account for intraspecific and intra-individual herbivory patterns of LCA, thus influencing populations of host species through the discrimination of drought-sensitive individuals.