“Don’t Panic!”

The words sit emblazoned on the cover of The Entomologist's Guide to Gardening. “When facing a vast, confusing, and hostile caterpillar attack on your trees," the guide continues, "the best and most practical response is to stay calm, keep a relaxed attitude, and go with the flow.” Good advice, if you ask me, and yes, in case you are wondering, I have been asked several times last week about the voracious appetites of caterpillars!

Those of you who are well read and of a certain age will recognize the inspiration behind my introduction. I'm echoing Douglas Adams and his gentle 1979 satire, The Hitchhiker’s Guide to the Galaxy, which has many nuggets of wisdom between its covers. Alas, there is no book titled The Entomologist's Guide to Gardening. But if there were, I hope it would have a chapter or two dedicated to caterpillars.

Often, one first encounters caterpillars during a defoliation event. You notice holes appearing in the leaves of your trees, which leads you to discover a writhing mass of caterpillars treating your tree like an all-you-can-eat buffet. Alarm bells go off in your head... Something is eating your plant! The very tree you planted to support biodiversity! Kill them!

Don’t panic.

At this point, I recommend following Doug Tallamy’s 10-step rule: take 10 steps back from the plant. From that distance, the small holes and chewed leaf edges are virtually invisible, with no effect on the tree’s overall health or your yard’s appearance. Therefore, in most cases, you should do absolutely nothing about the caterpillars in your trees. 

Those caterpillars chewing on your oak, cherry, birch, willow, or other native trees are not a sign of something wrong. In fact, they're a sign of something right, and they illustrate why we encourage Natural Neighbors participants to learn to live with leaf damage.

By the end of August, it's a rare Oak leaf that is left untouched by herbivory. Don't panic! Take ten steps back and assess; this Post Oak is doing just fine. Photos: Rich Couse

Trees and caterpillars have been interacting for millions of years. Native trees have evolved alongside the insects that feed on them, and herbivory is a normal part of their lives.

A mature, established tree can tolerate a surprising amount of leaf loss. Trees produce more leaves than they strictly need, and a season of caterpillar feeding rarely poses a serious threat to a healthy tree.

What looks like damage to us is evidence of a functioning food web. That food web begins with plants, and native plants are already optimized for this essential function.

It takes between 6,000 and 9,000 caterpillars for a single pair of chickadees to raise one brood of chicks. Photo: Generated by Gemini; Prompt: Rich Couse

Caterpillars are one of the most important sources of food for birds, especially when birds are raising their young. A single clutch of nestlings requires hundreds of caterpillars to grow to maturity.

Chickadees, titmice, warblers, wrens, nuthatches, and many other birds actively search trees and shrubs for caterpillars. Caterpillars also feed spiders, predatory insects, amphibians, reptiles, and small mammals.

So when we eliminate caterpillars because they have eaten some leaves, we are not simply protecting a plant. We are also removing one of the most important sources of food from the ecosystem. And before you say, “I never see birds eating caterpillars,” I will say, “you’re not looking hard enough!”

90% of the caterpillars we see are night-flying butterflies, a.k.a. moths. This Halloween-themed orange-tipped oakworm moth caterpillar will eventually metamorphose into the fuzzy darling pictured here. Caterpillar Photo: Rich Couse. Moth photo: Vivki DeLoach, Wikipedia commons.

Of course, the caterpillar in your tree is not going to remain a caterpillar. Eventually, it will transform into an adult moth or butterfly.

And while butterflies get all the glory, moths are just as important — indeed, more important because there are far more of them. In the Order Lepidoptera, which comprises butterflies and moths, moths represent at least 90% of the species. So the odds are 9 to 1 that a caterpillar is a moth species, not a butterfly. And not only are adult moths important pollinators, but like caterpillars, they figure in the diets of an enormous number of other species.

That means the holes in your leaves are part of a much larger story: a native tree supporting insects, insects supporting birds, and birds contributing to the larger biodiversity of your region.

One of the reasons we encourage planting native trees and shrubs is that they support our local wildlife in ways that many introduced plants simply cannot. Some caterpillars are highly specialized. For example, many moths and butterflies can only feed on particular plant species or groups of plants. And some native plants are astonishingly good at supporting the insects they co-evolved with.

Oak trees are an excellent example. Oaks support an extraordinary diversity of caterpillars, which in turn provide food for birds and other wildlife. When we look at a tree only as a landscape feature, as something that should remain perfectly green and untouched, we can miss the much more important role it is playing as habitat. A few chewed leaves show that your tree is doing its job.

A group of Red-humped Catterpillar moths feeding on bayberry. Photo: Robyn Graygor

Fine. You got me. There are circumstances when intervention may be appropriate.

A severe infestation can sometimes stress a young, newly planted, or otherwise vulnerable tree. Certain introduced insect pests can cause significant damage, particularly when they occur in large numbers or attack a plant repeatedly. And outbreaks even of native caterpillars, which are a natural part of forest dynamics, can spill over onto ornamental or specimen trees.

The key is to identify the problem before treating it.

Before taking drastic measures and reaching for an insecticide, or calling an exterminator, ask:

• What insect is actually causing the damage?
• Is it a native species or an introduced pest?
• How much of the foliage has actually been lost?
• Is the tree mature and otherwise healthy?
• Is the plant likely to recover without intervention?
• Is the insect causing a genuine threat to the plant, or simply using it as food?

In most cases, the best course of action is simply to watch and wait. Then, if you do need to intervene, start with the least harmful approach. When intervention is warranted, our goal should not be to eliminate every insect. The goal should be to address a specific problem while causing as little collateral damage as possible.

1. Identify before you act

Take a photograph and try to identify the caterpillar or insect. We recommend using iNaturalist or another reliable identification resource such as Bugguide.net. Knowing what you are looking at can completely change your response. A bunch of caterpillars that look alarming may turn out to be a native moth species that is an important part of the local food web.

2. Do nothing, if possible

Take 10 steps back. If the tree is healthy and the amount of feeding is modest, leave the caterpillars alone. This is often the most environmentally responsible choice. Plants are food. Nature simply does not operate without leaves being eaten. A completely untouched tree is not necessarily a healthier tree; it may simply be a tree that isn't supporting much insect life, most likely because it is non-native.

3. Remove individual caterpillars by hand

I know what you’re thinking… Ewww! Wear gloves if you have to. If there are only a few caterpillars on a particularly vulnerable plant, hand removal may be enough. Simply pick them off and relocate them to another suitable plant when practical. For larger infestations, pruning off a small number of heavily infested branches may sometimes help. These approaches are far more targeted than spraying an entire tree.

4. Encourage natural predators

This is my favorite method because it is the way Mother Nature meant it to be. A healthy yard already has an insect-control system. Birds, spiders, predatory beetles, assassin bugs, parasitoid wasps, and other predators help keep herbivorous insects in check. You can support them by providing a diversity of native plants, avoiding pesticides, leaving the leaves, saving the stems, and stewarding natural habitat. Note that many predatory insects are also pollinators, visiting flowers for pollen and nectar. Sometimes the best pest-control strategy is to make your yard a better place for predators.

5. Avoid broad-spectrum insecticides 

Broad-spectrum insecticides don't distinguish between the caterpillar eating your leaves and the beneficial insect eating that caterpillar. They can also kill pollinators, predators, and other insects that were never part of the problem.

Products marketed as "natural" or "organic" are NOT harmless to wildlife. If treatment is truly necessary, use the most targeted product and method available, follow the label carefully, spray only in calm conditions and during the cool morning hours, and avoid spraying flowering plants or areas where pollinators are actively feeding. And make sure the person you are paying to do the job follows those guidelines.

Milkweed is meant to be food, and you can rejoice that species that are at risk of extinction, such as this Monarch butterfly caterpillar, are feeding on yours. Photo: Luanne Johnson

One of the most important shifts we can make as stewards of our land is to stop thinking of every hole in a leaf as a problem. A landscape designed entirely around appearance leaves little room for the insects, birds, and other wildlife that make an ecosystem function.

Your yard doesn't have to look untouched. In fact, a little evidence of wildlife is something to celebrate. Leaves that have bites taken out of them tell us that all is right with the world, that something other than us is not only living, but thriving there. So the next time you see a caterpillar chewing on one of your trees, Don’t Panic! Look a little closer. You may not be looking at a pest problem at all. You may be looking at biodiversity at work.

If you would like to learn how to support biodiversity, visit the Natural Neighbors website by clicking HERE, request a visit, and we will help get you started on your journey! We will share our knowledge and resources at no cost to you and find many other ways for you to support all the biodiversity in your yard.