Insect herbivory is a strong regulator of plant populations. In Patagonia, the problem of pine invasions originated from plantations is relatively recent, but it is a growing problem. In South America, leaf-cutter ant herbivory is considered an important problem for the establishment of Pinaceae plantations. It is possible that the native leaf-cutter ant, Acromyrmex lobicornis, could control Pinaceae invasion in early stages of the invasion. In this study we evaluated the foraging preference of A. lobicornis ants for Pinaceae seedlings of the most widely planted exotic conifers in the region. We also studied if the foraging preference found, is related to differences in mechanical and chemical defenses of these conifers. We performed a cafeteria experiment where we chose ant nests that were similar in size and foraging activity. We selected trails from each nest and we planted native tree seedlings (control) paired with seedlings of different Pinaceae per trail (240 seedlings total). Seedlings were planted at two distances from the entrance of the nest, and also two distances from the trail to evaluate the effect of distance. We registered the number of needles and branches removed, the height of the seedling, and the percentage of each seedling consumed.
Results/Conclusions
We found that in no more than 2 weeks, leaf-cutter ants consumed almost all the seedlings offered, and that they preferred nonnative pine seedlings, especially Pinus contorta and Pinus ponderosa seedlings, being the less preferred the native Austrocedrus chilensis seedlings. We did not find big differences in ant preference if seedlings where closer or far from the entrance of the nest or of the trails. Mechanical (leaf toughness) and chemical defenses (i.e. resin content and total monoterpene, alpha and beta pinene and carene content) explain partially the foraging preference pattern found. These results suggest that the leaf-cutter ant A. lobicornis could contribute in the region to the control of exotic Pinaceae, in early stages of their invasion. Also, information on mechanical and chemical defenses of these conifers may be use to select Pinaceae species or varieties that are less likely to create large scale invasions, which generate unwanted environmental and economical problems. This information may be use to make forestry practices with nonnative trees more sustainable; since foresters could select Pinaceae species that are more defended but less prone to invade, or species that tend to invade but can be controlled by native herbivores.