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.