Something Wild: How Trees Fight Back

Sep 23, 2016

There’s been a lot of talk about Gypsy moths this year, especially in southern New England, where trees in some areas have been hit pretty hard by this voracious caterpillar. And it has sparked a lot of discussion about how people might help reduce the damage, but it’s worth remembering that the trees these caterpillars feed on are not entirely helpless.

Gypsy moths caterpillars will feed on basically anything – oak, birch even pine. In fact, their scientific name, Lymantriam, comes from the Greek word meaning "bringer of ruin." They can be particularly devastating in North America because they’re a non-native species and so they have few natural predators here. It's something biologists have been working to mitigate for years.

The introduction of gypsy moths can be traced to 1869, when French entomologist Leopold Trouvelot brought some eggs to Massachusetts to inter-breed with silk-producing moths. But, rather predictably, his charges escaped. So here we are 150 years later still grappling with them - talk about your butterfly effect. Scientists have tried a few different things to stem the tide of the gypsy moth (NHPR's Outside/In produced a great story about this subject!). But our trees have developed their own strategy for dealing with pests like this. It's a bit like an Ironman suit, but instead of iron, think tannins. When trees sense a loss of leaves, they start to produce more tannins. Tannins are a natural anti-oxidant, acting as a kind of preservative, helping to prevent decomposition. And if you're a tree and suddenly find yourself without any leaves, unable to photosynthesize, decomposition is a serious concern. 

An arboreal early warning system!
Some arboreal defenses are more subtle than others.
Credit Courtesy Giphy

  As a side effect, these tannins make any remaining leaves bitter and less palatable to an invasive insect. While the tannins don't stop the gypsy moths from feeding on that particular tree, they do feed less on that tree and gravitate more towards those with lower tannin content. But here’s the really cool part. Some species of trees can warn other trees that pests are coming and then suit up for battle.

A given tree is often linked to dozens of others via the interconnected soil, fungi and root system that keeps them alive. So when one tree begins producing extra tannin other trees can sense it via this root system. Some trees communicate through the air releasing chemicals that are picked up by other trees down wind. Though, as far as we can tell, they can only communicate within species: maples with maples, oaks with oaks. But when the warning goes out it can slow the advance of the gypsy moths, and even redirect it.   

The best defense is a good offense

This chemical communication system can be used for other purposes, too. Trees use it for defensive methods, but some plants use it for offense. The hay-scented fern (dennstaedtia punctilobula), for example, which can be found all over the state. It secretes a chemical into the soil that suppresses the germination and growth of the seeds of some other species of plants. It’s perfect soil for the fern, but if an acorn or a maple samara lands there, it won’t survive. This is good for the ferns because it prevents more tree seedlings from growing and shading them out.

PRODUCER'S UPDATE: Dave recently visited Kingswood Regional Middle School in Wolfeboro, NH who are using the Something Wild podcast to learn more about the natural world and inspire further research into the wonders around them. They shared with us some illustrations they made of this week's episode - click through the slideshow at the top of the post to see some of them!