By Abelardo Correa and Mary Moreira

Resource Person - James Wetterer

For WWNFF Ð Environmental Science Institute
 

Introduction

Leaf-cutting ants belong to the Tribe Attini, the Òfungus growing . Leaf-cutting ants typically harvest live plants (flowers, leaves, buds, and fruits) to use as compost for growing a specialized type of fungus which they grow for the food. There are two genera of leaf-cutting ants: Atta and Acromyrmex. Atta colonies can grow to contain several million workers and several Atta species are important pest. In contrast, Acromyrmex species have much smaller colonies, typically with fewer than 100,000 workers. Whereas many Atta are well-studied, most Acromyrmex species remain relatively unknown.

Acromyrmex volcanus is a leaf-cutting ant, which is endemic to the Atlantic slope of Costa Rica, though it probably extends north into Nicaragua and possibly south into South America. Very little is known about the A. volcanus and there is only one published study (Wetterer 1993). This earlier studied noted that A. volcanus at La Selva only in old growth forest. All nests at La Selva were located up in trees. The present study was motivated by the observation of A. volcanus in two highly disturbed areas of the La Selva property. We wished to document whether the foraging and nesting ecology of these colonies differed from those in the old growth forest.
 
 

Methods

We searched for Acromyrmex volcanus activity on the disturbed and old growth areas on the La Selva property. At each site we observed their foraging behavior and attempted to follow their trail to find their nest. This process was often difficult because after trail disturbance, some ants would stop activity for 3-4 minutes and then most of the ants seem to have difficulty relocating the trail again. The length of the trails studied were sometimes 10 or more meters from nest to foraging site, often through dense vegetation. Some of the trails went up trees and we estimated the height.

On July 26, 1999 at 2 of the sites, we collected 25 consecutive loads from the colony. We identified the type of plant parts that we collected and later compared them to a previous study done on 6 colonies of Ac. volcanus in undisturbed areas of the rainforest

We excavated the colony at one site to collect samples of the fungus, and some ant specimens.
 
 

Results

We found a total of four Ac. volcanus foraging sites. All of these sites were located in disturbed areas of the rainforest. We labeled these: Site A (SA): behind House 2; Site B (SB): next to the workshop, Site C (SC): in the arboretum near #6, and Site D (SD): near the laboratory. On the second day of the study, we could not find any ants at Site B, even after numerous returns throughout the day and searches in a large area around the path where they were originally seen. Difficulties arose because Ac. volcanus is easily affected by rain and trail disruption.

We found that the Site C colony was located in a very tall Pterocarpus officinalis tree (# 587 in the arboretum). The site D colony was also found in a very large tree - a Pentaclethra macraloba. The Site A colony, however, was located at the base of the trunk of a much smaller Pentaclethra macraloba tree, in a pile of leaf detritus . In the previous study (Wetterer, 1993) all of the colonies at La Selva were found in trees.

In his study of Ac. volcanus, Wetterer states Ò I never found Ac. volcanus foragers cutting the leaves of small trees.Ó From our observations we found that Ac. volcanus was cutting leaf buds of the Zigia longifolia tree at site A. At the other 2 sites, C and D, the ants were scavenging flower petals from the Dypterix panamensis tree on the ground.

Table 1. Foraging selectivity of Acromyrmex volcanus.
Foraging selectivity - (Wetterer 1993)

                                Present Study

Discussion

Our observations differed in several ways from those of Wetterer (1993). First, rather than being limited to rare old growth forests in the lowlands as observed by Wetterer (1993), Ac. volcanus appears to also live in highly disturbed areas. Second, we found one colony nesting at the base of a tree. The significance of this observation deserves further study.

We found one colony cutting the leaves of a very common forest tree. This is a very abundant resource of the forest. The other 2 colonies were scavenging flower petals from another common tree which was in flower. This source probably reflected availability of this resource within close proximity to the nest. Two of the nests were measured as to distance from nest to food source. Site A was about 40 meters from the base of the tree plus an estimated 10-15 meters further in the tree. (This is a striaght line measure which does not reflect the actual distance traveled by the ants.) Site C ants traveled about 30 meters in a fairly straight path along a concrete path plus at least 20-30 meters more up the tree.

Note: We opened the nest at Site A located at the base of the tree about 10 AM. The next morning the nest was checked around 6:30 AM and the ants had vacated that nest and moved somewhere else.

Applications: This activity can be replicated with students to emphasize the following skills: observational skills over an extended period of time, data gathering from multiple sites, behavioral factors and data interpretation.

Literature: Wetterer

Future Direction: Some of the questions to explore with Ac. volcanus would be:

How is the trail system arranged?

How often do they modify trails and how do they arrive at this decision?

Why do they pick leaf buds and not other parts of the tree?

Why do they maintain a relative long distance from each other when bringing loads to the nest?

Why do they choose not to engage in conflict with other ants even if size favors them in a fight?

Who are their predators?

Why are so many workers involved in scouting?