Wednesday, June 13, 2012

Bt to the rescue!

We've added a new pest control measure to our repertoire.

Here is why:

These guys like to eat tomatoes.

And scallions.

And all kinds of leaves, like this eggplant:

They like to eat the flowers too.
They also like pepper plants, chard, lettuce, and beet greens. We've only found a couple in the mustard greens. 

It's pretty unusual for a pest, particularly a caterpillar, to attack so many different plant families with such voracity. Especially scallions. Who ever heard of a caterpillar eating a green onion?

We first noticed them in the greenhouse, around the time we started setting plants out for the season. We didn't think much of them because 1. we've never had any kind of bad caterpillar issues before this year 2. we've seen these before but their population has never reached a problem level 3. we were kind of busy. 

I thought we'd see a little damage, then they would disperse when the weather warmed and they had access to other vegetation outside. But suddenly they are all over the place. They're in the field, in the hoophouse, and still on the few starts remaining in the greenhouse.

Apparently we have a caterpillar problem.

So far, we can easily bounce back from the damage. But if the population increases there will be some significant losses.

So we started a bit of research. It turns out our caterpillars are a Helicoverpa sp. As you might a imagine, they have about a zillion common names - one for every crop they attack. Some of the more popular are corn earworm, tomato fruit worm, and cotton bollworm.

They are very common agricultural pests, but they don't over winter this far north, which is why we have never seen them in large numbers before. Apparently the moths can fly quite a distance in the spring, spreading north. Our best guess is that this spring was warm enough that they arrived in time to squeeze in an extra early generation of northern caterpillars, resulting in the population boom we are currently witnessing.

We've decided to combat them with an equally common biological control method. Bacillus thuringiensis, usually called Bt. Basically, Bt is a naturally occurring bacterium that, when ingested by caterpillars (and only caterpillars) creates a lethal toxin. If you would like to learn a little more about it, please click here. Normally it lives in the soil. We've simply bought some in a concentrated liquid form. We're spraying the bacteria on our plants so the offending caterpillars are sure to ingest some. This control method is allowed under organic certification.

For some of you, Bt may sound familiar - and a little scary - because of its association with genetically modified crops. I want to be clear that we are not using anything genetically modified, nor are we using a synthetic pesticide, nor something toxic to anything other than caterpillars. Also, if there is a caterpillar nearby that isn't eating our sprayed crops it will not be affected by the Bt because caterpillars have to actually eat the bacteria before they will produce the toxin.

As for the GM connection. You may have heard of Bt corn. Bt corn has been modified so that all parts of the plant produce the toxic proteins that Bt bacterium normally produce when ingested by a caterpillar. Farmers grow this corn to kill European corn borers, another caterpillar pest of corn.

As a result, Helicoverpa caterpillars in general have been exposed to a lot of Bt toxins, in the form of corn plants, and some have developed a resistance to Bt.

Let's hope the caterpillars currently munching our tomatoes have not.    


  1. Thanks for the post Andrea -- informative and clearly written. Fingers crossed for success!

  2. They've eaten our rhubarb as well as mint and hostas. They're not picky.