Corn Earworm

2020 December 27

No, this is not about a catchy song about corn that you can’t get out of your head. Although it might be amusing if it was.

On August 25, 2019, we bought some ears of sweet corn from the grocery store. When we got them home and started peeling off the husks, I noticed that one of them had some insect damage at the tip:


This looked fresh, so I went through the corn silks[1] looking for the culprit, and found this:


OK, so since it was a caterpillar eating an ear of corn, and I know that the corn earworm (Helicoverpa zea) is a thing that exists, it is clearly the prime suspect.




When I started looking up images, though, at first I wondered whether I had misidentified it. The first few hits were either too green, or had a different pattern of spots. But as I kept looking, it became clear that this is a highly variable species (at least in the caterpillar form) and can look wildly different both as they age, and from caterpillar to caterpillar. We can see the range of variation clearly in this picture from the Purdue Extension Service:


So, yes, it looks like we do in fact have a corn earworm here.

It turns out that the only way we are likely to see corn earworms this far north is just like this, in sweet corn ears imported from further south. Their normal lifecycle is to feed on the developing corn ears, and then when the ear ripens they drop to the ground and burrow into the soil to pupate. The pupa is what overwinters, and apparently they aren’t cold-hardy enough to overwinter in Michigan. Their range seems to cut off right around southern Ohio and southern Indiana. Still, we didn’t want to take a chance on getting them established up here, so Sam fed it to her chickens when I was done.

Corn earworms are a serious pest of sweet corn[2], since people get squicky about eating caterpillars with their corn-on-the-cob. The earworms also eat the developing fruit and growing tips of tomatoes, cotton, beans, alfalfa, and tobacco, so they are kind of problematic across-the-board.

[1] The “silks” of an ear of sweet corn are essentially the female flowers. Corn produces pollen from the tassels at the top of the stalk, and wind blows the pollen onto the silks of the developing ears. Each strand of silk goes to a single kernel of corn, which it fertilizes. In general, the silks remain moist for a long time after the ear is pollenated, and don’t dry up until the kernels of corn fully mature and start to dry down. And, they make a convenient place for the earworms to hide out in the meantime.

[2] On the one hand, the inability of corn earworms to overwinter this far north would suggest that it would be easy to grow corn here, with one less pest to contend with. On the other hand, corn really requires a long, hot growing season to develop properly, and we have neither. There are some very short-season corn varieties that can just barely mature here, but in general corn just doesn’t thrive in this climate and I feel our gardening efforts are better applied elsewhere. In general, crops that list a required growing season over 75 days are likely to be a dead loss, and only the ones under 65 days are likely to do very well.

3 Responses
  1. Jenn Ridley permalink
    December 28, 2020

    We used to call them ‘protein supplements’, although we we very rarely ever actually ate them – they usually came out in the cooking water, or were well into the corncob. We didn’t have them every year, and now I know why — we got them after mild winters.

    Mum never bought corn-on-the-cob because we had about 120 linear feet of corn stalks every summer (a patch of 4-6 rows, planted over the course of a month so we’d have fresh corn for a month in the summer).

    Re: footnote #2. corn also requires better soil than you’ll usually find in the Copper Country. Dad rotated the garden every year, and the area that was going to get the corn planting got special treatment in the fall.

  2. December 31, 2020

    What a charming creature! I love the patterns on its hide. Shall we call it a hide? Or would skin be better? Oh well.

    Also, have a very happy New Year’s, Tim. Some day, we’ll have to meet in person and you can find out if hitting me on the head with a 2×4 improves my blogging.

    Until then, I remain your devoted online friend,

  3. January 6, 2021

    Thanks, KT, and a happy New Year to you as well. I’m probably not going to be in Southern California any time soon, but if I do we should look each other up.

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