Help! Caterpillars Are Eating My Plants

Pests are guests you don't want to invite to your garden table.

Pieris Brassicae Caterpillar

Pieris Brassicae Caterpillar

Pieris brassicae is also called cabbage butterfly, cabbage white or cabbage moth.

Photo by: Shutterstock/Alexandre Le Bourdais

Shutterstock/Alexandre Le Bourdais

Q: Do you have any suggestions on how to keep caterpillars from eating all my herbs and tomatoes? They have attacked my sage especially.

ANSWER:

I have to admit that I was initially surprised by your question. Not about the tomatoes, as they are a popular food source of tomato hornworm (Manduca quinquemaculata) and Colorado potato beetle (Leptinotarsa decemblineata) larvae. What shocked me is that the greatest damage is to your sage plant(s). Common garden sage has a chemical composition that makes it unpalatable to most garden pests. As a result my sage plants have never suffered insect damage, even when their neighbors are overrun and decimated.

However, in pondering your dilemma, I recalled reading about caterpillars munching on alkaloid-laden plants that they would not normally touch as a way to self-medicate against parasites. I wonder if that’s what is happening in your garden? Another possibility is that the weather has brought an infestation of a specific insect whose population was so insignificant in the past that its damage went previously undetected. 

Or perhaps an atypical pest has found its way to your garden, one that doesn’t mind the taste of sage. I have also found that slugs and earwigs have run amok in my own garden this year and are chomping on plants that are normally immune, simply because there are so many of them and competition for food is fierce. I liken the garden this year to a popular restaurant that is overbooked — the yummiest treats at the buffet table are dwindling. It’s hard out there for a pest!

Regardless of the cause, you want answers and I want to help you. Fortunately, there are solutions.

Organic Control

  • My first suggestion when it comes to pest control in an organic garden is to deploy the five-finger solution: aka handpick and squish ‘em. If you’re squeamish about squishing you can always wear gloves and drop them into a bucket of soapy water. A few short squishing sessions over the course of a week may be enough to eradicate the problem. Follow that up with random spot checks. Look underneath the leaves for eggs and caterpillars as well as their frass (aka caterpillar poop) as proof of their existence, even if you don’t see them.
  • You can’t squish larvae that you can’t see, in which case you might try simply cutting back the damaged plant and destroying the foliage (burn it). Fortunately, there is enough time left in the season for your sage to recover and offer up a second harvest.
  • Plant umbelliferous flowers (i.e. dill, fennel, parsley to name a few) that will attract parasitic wasps to your garden. Interplant them around and among the plants that have sustained the most damage. These wasps will lay their eggs inside the caterpillar’s body, eventually killing the host.
  • When all else fails you can try applying Bt aka Btk (Bacillus thuringiensis) a bacterium that is considered harmless to humans but acts as a poison against caterpillars. I mention this option because I know it will come up but do not recommend it for a few reasons. Not all caterpillars are harmful in the garden and while Bt is selective in that it only kills caterpillars, it is not selective in the caterpillars that it kills. Anything that eats the leaves of infected plants is fair game. I would also argue that no spray is completely benign. Since you are spraying directly onto the leaves of an herb, I’d consider whether this is something you are comfortable in ingesting: safe or not.

Garden authority Gayla Trail is the creator of YouGrowGirl.com.

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