Animals aren’t the only living things that can benefit from a poke or two. People have been injecting trees with various substances for centuries. Leonardo da Vinci injected his apple trees with arsenic as a deterrent to thieves — forgoing his own enjoyment of the fruit for the satisfaction of poisoning a few larcenous peasants.
More recent advancements benefit the trees themselves. Tree injection is widely practiced by arborists hoping to protect their charges from disease and insect pests. Pesticides, fungicides and even antibiotics are now part of the arsenal used to combat these often-fatal enemies. The process has been crucial in the fight against devastating fungal infections such as Dutch elm disease and insects such as the emerald ash borer and bronze birch borer.
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BBC Wildlife spoke to Jake Miesbauer, an arboriculture research scientist with the Center for Tree Science at Morton Arboretum, in Lisle, Illinois, about the tree injection process — also called vegetative endotherapy.
What is tree injection and what does it treat?
Tree injection involves drilling very small — 5 millimeter — holes around the circumference of the base of the tree. Little plugs or ports are put into those holes. They're connected by a hose all the way around. That hose is connected to a tank containing the agent.
Emamectin benzoate is the typical insecticide used to treat emerald ash borer [a beetle that has devastated American elms], for example. The container is pressurized. As water flows from the roots, the tree takes the insecticide up. Then the larvae of the beetle feed on the cambium containing the insecticide and it kills them.
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A number of pathogens — such as fungal diseases — as well as insect pests that feed on the cambium of trees can be treated this way.
How frequently do trees need to be injected?
With emerald ash borers, it was originally a two-year efficacy rate. It was found that emamectin benzoate is effective for three years. Other diseases or other insects and other treatments may have different timeframes.
We take a pretty minimalist approach to using pesticides. We have some large specimen trees that are prominent in the landscape. If we did not treat them, they would die. We treat those trees.
How does the tree react to the injections?
The tree senses there's something invading it. It puts chemical compounds out that resist the spread of decay. These compounds have antimicrobial properties that protect the area. That's why you see lighter areas [around the darker colored injection sites in cross-sections of the tree]. These are phenolic compounds that the tree produced to protect itself.
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Can the injection process harm the tree?
We did a study with the city of Milwaukee where we looked at how they treat their ash trees. They've been treating them since the mid to late aughts. They started treating the entire street tree population — almost 30,000 trees. These trees have been treated up to eight times. We wanted to see if there were any negative repercussions. We dissected 40 trees. We didn't find that there was anything really negative.
When you drill a hole into the tree, if there's too much pressure, it can actually blow the cambium and bark off, creating a larger wound at the injection site. We found very little decay, even in the oldest injection sites. They seal over fairly quickly. Once they seal over, the decay fungi don't have oxygen, which they need to spread. Any decay is really localised and dissipates quite quickly.
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Are tree injections more effective than external applications of pesticides?
Foliar applications or even soil drenches are also used. It depends upon what you're treating for. There are certain things that foliar applications are the only effective way of treating. Apple scab is a foliar disease. But systemic injections are the primary way of treating anything that feeds on the wood.
One of the benefits of doing systemic trunk injections is that they stay within the trunk of the tree. You run the risk of drift when you're doing a foliar spray. Most of them will kill other things as well. The systemic ones will too. If you inject something into the trunk, anything that's feeding on that cambial tissue is going to die. But it's a much more targeted approach.
top image: Ash trees killed by Ash Dieback Disease on the South Downs in West Sussex, UK. Credit: Andy Soloman/ Getty Images
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