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Master Gardener: Think twice before destroying garden caterpillars
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Master Gardener: Think twice before destroying garden caterpillars

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I have a ton of little black caterpillars devouring my echinacea. I am afraid they will kill the flower, but I feel bad about trying to kill them all. What should I do? — D.H.

Thank you for this question because this is a dilemma many gardeners struggle with, especially if we are trying to utilize natural/organic methods for pest control.

The caterpillar you are talking about is likely the larvae of the checkerspot moth. These little ones can be quite destructive, especially since they are usually found in large groups devouring one of our favorite flowers. The female checkerspot moth deposits around 100 eggs on the underside of a leaf. Once the eggs hatch, they start to eat, and they can eat. You might see 50 on a single leaf.

The damage they do is quite noticeable in that they skeletonize the leaves. This is a term that means the caterpillars eat all the leaf tissue leaving only the veins. They would probably eat the veins too, but they are a little tough for these young caterpillars.

One day a few years back, I went out to water our garden and noticed an echinacea covered in these checkerspot moth caterpillars. They were going to town on one of my favorite flowers. Of course, my initial reaction was to head to the garage to grab some bacillus thuringiensis, which is an organic pesticide that specifically targets caterpillars. But then I stopped to consider whether that was really what I wanted to do. … It wasn’t. So I did what any of you would probably have done: I went back out to the garden and had a conversation with these little caterpillars. OK, so maybe none of you would have done that, but that’s what I did.

I told these little ones that if they confined their efforts to this one plant and didn’t spread to any of the others, they were free to live their life. But if they expanded the footprint of their buffet, we would have a problem. Interestingly, they stayed on that one plant, completely defoliating it of any leaf tissue.

But here is the deal. Even though that plant looked terrible for the rest of the growing season, it came back the next year no worse for the wear. And so this is where gardeners are. We usually know how to get rid of a garden pest, but should we? Sometimes the answer is yes, but sometimes the answer is maybe not.

All caterpillars are on the way to becoming something else. We love the butterflies and moths but oftentimes don’t feel the same about them while in the caterpillar stage. This means we have to make some choices. Let’s talk about some other garden caterpillars you are probably familiar with.

First up: cabbage loopers. OK, I’ll admit there is not much need to discuss this one. If you find these little ones on your broccoli, cabbage or other plants, it usually boils down to you or them because they will also skeletonize the leaves on this plant, destroying the plants’ ability to grow the food you were hoping for. In this case, bacillus thuringiensis will do the trick. Bacillus thuringiensis is a soil-based bacteria that makes the caterpillar sick when eaten. They stop eating and problem solved. This is a good thing in my book because there is no love lost between me and the cabbage looper.

Another garden pest that can cause us trouble is the tomato hornworm. These caterpillars can grow to about the size of your index finger and are really quite spectacular to look at. The usual treatment for tomato hornworms is to just hand pick them off and destroy them. If you are too soft-hearted for this, bacillus thuringiensis also works well.

But — and this is a big but — tomato hornworm caterpillars are on the way to becoming a beautiful moth. These moths are quite spectacular, growing to a wingspan of between 3 to 5 inches. So, if you find these on your tomato plants, once again you have a decision to make. Do you just live and let live, or do you eliminate the threat to your garden?

Rather than destroying these caterpillars, you might want to move them to one plant if you have multiple plants and let them devour the single plant. Their feeding only lasts about three weeks, and then they go into the soil to pupate.

This strategy comes with a warning because each female moth can lay up to 2,000 eggs in its lifetime. This means the hornworms will be back.

While we are talking about caterpillars eating plants, I would be remiss if I didn’t mention the monarch butterfly. As most of you know, monarchs lay their eggs on milkweed plants and their caterpillars devour the milkweed. But most of the milkweed, if not all the milkweed gardeners are growing, is being grown for the monarch caterpillars. Would we want to kill these caterpillars because they are destroying our milkweed? I don’t think so, or at least I hope not. We love the monarchs, and it is great fun to be part of their life cycle.

So, I guess the bottom line is, before we attempt to eliminate any and all of our troublesome caterpillars from our gardens, let’s just pause for a moment and decide what role we want to play in the overall ecosystem. Each insect has its place; we just need to decide if we can share.


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You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th St., or by emailing us at mg@tulsamastergardeners.org.

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