Excerpts from Jim Conrad's
from the June 20, 2010 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO
SQUASH VINE BORER
During the organic garden's first weeks the prize plant was a lusty zucchini seeming to promise all the harvest we needed, just from it. But then a couple of weeks ago it stopped growing, the edges of its expanding leaves turned brown and crisp, the whole plant grew anemic looking, and clearly something was wrong. Finally this week I couldn't stand looking at its wretchedness any longer, pulled it up and, suspecting what the problem was, twisted its brittle stem until it split. The problem is shown below:
It was the dreaded Squash Vine Borer, MELITTIA CUCURBITAE. I've run into this critter before but when I got the above picture on the screen I was surprised by some of the grub's features.
First, the grub has the rudiments of a regular caterpillar's tiny feet. Second, the head at the top looks like it's equipped with two beady, brown little eyes. Similar brown dots appear along the grub's sides and I assume that they are spiracles -- breathing holes. But those "eyes" on the head left me wondering. For, regular caterpillars have large compound eyes consisting of hundreds or thousands of little windows. They just don't have beady, brown eyes like these. And even if they were beady, brown eyes, of what use would they be inside the stem?
On the very day I took that picture, Eric in New York sent me an article from The New York Times. That article is about caterpillars with "false eyes" that cause the caterpillar to look like it's staring at any potential predator. So maybe that's what's going on here.
But, still the question remains as to what use such false eyes might be in the darkness inside a squash stem. Do some predators break open squash stems looking for grubs? If they do, would the face displayed on our zucchini grub scare them away? At least to me, the eyes just make the grub look goofy, but maybe a stem-slashing bird would feel different.
It's hard to control stem borers in an organic garden, basically because once they're inside the stem they're pretty well protected. If you can figure out the grub's location in the stem, maybe by spotting frass (poop) at little holes in the stem, the stem can be cut open lengthwise and the grub picked out. Where borer outbreaks are really bad, nylon stockings or aluminum foil can be wrapped over a vine's lower stems. Another trick is to cover the squash vine's stem at various points with soil, encouraging rooting, so that if a lower part of the stem gets destroyed, roots at the upper part can take over.
But, in practice, these measures are a bit awkward to use. Stem borers are just a real problem. My main technique against them is to put out many more plants than I expect to need, and scatter them, hoping that if some get zapped in one place, others may survive in others.