Growing the Desert Garden

Welcome to the Desert Garden, with garden coach Tyler Storey, where we talk about everything having to do with gardening and landscaping in the Desert Southwest. From composting to Cercidium and agaves to arugula — we'll cover everything you want to know to grow your own beautiful Desert Garden.

Friday, September 19, 2008

From the Inbox: Caterpillar Poop

From a Correspondent:

Can you help me identify this stuff on my plants? I have a raised bed and containers full of edibles. The unidentified problem things look like small peppercorns and collect on the leaves of my basil, mint, and arugula plants. It has not (so far) collected on my green beans, tomatoes, peppers, cilantro or thyme. Although they look like peppercorn, they're not hard. I wipe them off with a paper towel and they mush into a dark green slime, and look kind of like rabbit poop. I always notice them in the morning. They collect on the leaves, and especially in the hard to reach nooks of the folded leaf sprouts near the root of the plant. It's often, but not always clustered. So, there might be like two or five on a bigger leaf. They remove very easily, but there are so many that I can not possibly keep up.
Are these larvae of some kind? We have lots of moths, so maybe they're moth larvae? Or, is it the poop of some large bug or caterpillar? It's just now beginning to collect on my treasured wild arugula, so the damage remains to be seen. However, I'm worried, because several months ago, when I started noticing them on my basil and mint, those plants started to deteriorate. The leaves became holey, they stopped growing, or produced much smaller new leaves than previously. These symptoms may not be related...but I suspect they are.
Any thoughts?
Los Angeles, California

Good morning Julie, and thanks for providing such a detailed description.

Your third guess was a charm: what you're seeing is caterpillar excrement, left behind as the caterpillars eat the leaves of your basil, mint, and arugula, which tend to be great caterpillar favorites. We frequently see caterpillar poop as a hard blackish pellet, but if they're eating lush plants with plenty of moisture, the bug poop will be as you describe.

Caterpillars have extremely simple digestive systems, and what goes in the one end is much the same when it comes out the other end, just ground up a bit; caterpillar-made pesto, as it were. Little-known fact: caterpillar poop from caterpillars eating basil tastes like basil, from those eating mint, like mint, etc. At least that's what I'm told. Check it out for yourself. Go ahead, you first.

There are two effective methods for limiting the damage to your plants. The first is to go out at night, while they're feeding, and use a flashlight to locate them on the leaves and along the stems. Then pick them off and discard them away from the garden; they make great chicken food. This can be somewhat labor intensive, but it also costs nothing.

Because it sounds as if you might have a more extensive problem, your next best method is to spray your plants' leaves with a solution of Bacillus thuringiensis (also called Bt). This is a completely safe and effective biological control that works by giving the caterpillars a terminal stomachache. It works only on caterpillars and has no residual effect on the edibility of the plants. You should be able to find Bt at any nursery or garden center; Dipel and Thuricide are two brands, but there are others as well. As with all pesticides – even organic ones – always read and follow the label exactly.

Because the Bt needs to be ingested to work, don't be surprised if the damage continues for several days.

I hope this helps,


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