Forests regularly get devastated by outbreaks of pest insects, whose numbers can go from virtually undetectable to extraordinarily high. Plants can release toxins as protection, but these were largely believed to play little role in fighting such outbreaks, since the direct effects of these poisons are typically weak. Now investigators find these defenses can be more effective than thought, as reported this week in the Proceedings of the National Academy of Sciences.
The causes of the outbreaks that can wreak havoc on forests are poorly understood. Although decades of research have generated many potential explanations, a decisive experiment to choose between them is almost insurmountably difficult to carry out, since outbreaks take place over five- to 30-year-long intervals and typically cover thousands of miles. As such, researchers typically rely on computer models to understand why these outbreaks happen.
Previously scientists proposed these outbreaks came and went based on the absence or presence of natural enemies of the pests such as predators or diseases. However, this explanation would apparently not explain unusual outbreaks of gypsy moths seen in North American woods.
In forests with a high percentage of oaks, gypsy moth outbreaks alternate between severe and mild, whereas in forests with a lower percentage of oaks, outbreaks are uniformly moderate. Simple mathematical models of gypsy moths and their natural enemies cannot explain why these kinds of forests would differ in the patterns of their outbreaks.
To learn more, theoretical ecologist Bret Elderd of Louisiana State University and his colleagues sprayed red oaks with the plant hormone jasmonic acid, which made them release toxins known as tannins. They found tannins decreased variability in the risk that gypsy moth larvae would get infected with a virus that could kill them.
When the researchers factored in this red oak defense into a computer model of gypsy moths and a natural enemy, they found they could reproduce the outbreak patterns seen in forests with both low and high levels of oaks.
“This shifts our picture of what drives insect outbreaks — our work suggests plant defenses can alter the interactions between insects and plants in these outbreaks,” says researcher Greg Dwyer, a theoretical ecologist at the University of Chicago. Such research might influence what trees forest managers plant in forests, he adds.
Future research could make this model even more complex by accounting for details such as strains of the insects evolving resistance against the viruses or plant toxins, Dwyer notes.
“Also, it’s difficult to get enough computing power to keep track of every insect and where it moves in the forest,” he said. “Having more computing power might end up with a different and possibly more interesting model.”