COLLEGE STATION, Texas, Dec. 2 (UPI) -- Recent experiments prove the arrangement between ants and the knotweed species Triplaris americana is quite strong.
Triplaris americana, a Neotropical knotweed plant found in Central and South America, is often called pau-formiga, or "ant tree." It sprouts empty stems to house entire colonies of the ant species Pseudomyrmex dendroicus. The plant also feeds the insects with a sugary syrup-like liquid.
In return for food and shelter, the ants offer protection. But how much protection?
Researchers wanted to gauge the fairness of this unique mutualistic relationship. To do so, scientists watched the populations of leaf-eating insects on several ant trees grown in the lab. Some trees were allowed to exist in their natural state, while others had their ant colonies forcibly removed.
The latter group of trees hosted a 15-fold increase in the number of herbivores.
"When an ant encountered a caterpillar, a worker approached and detected it with its antennae, and then recruited more workers," researchers recalled in the paper on the experiments, published this week in Journal of Hymenoptera Research.
"Typically more than 10 workers were recruited around the intruder in less than five minutes," scientists continued. "Several workers harassed the herbivore by stinging or biting, until it dropped off the plant."
Often, the hungry caterpillars would string a silk thread as they evacuated, using it to return shortly after for more food. The ants would wait, and chase away the invader once again.
The ants also routinely surveyed the tops leaves looking for debris to remove -- bits of lichen or fungi. The behavior suggests ants protect the plants against insects and agents of disease.