Wednesday, July 15, 2009

Onion Thrips

The onion thrips (Thrips tabaci) has had a very productive season so far here in the Central Valley. Thousands - make that hundreds of thousands - of onions have little pieces of chlorophyl missing were the minute insects have chewed, sawed, and slurped the energy producing tissues from their stems. What makes the damage so disturbing is that the culprits (no bigger than a small comma) are hardly ever seen. In fact a farmer might get most of the way through a crop without even a hint that they have been guests in his field. When he does notice, it's often too late and the damage is done. The onions are smaller than they should be and often don't make the grade.

So here's a bit of advice for next year: start watching out for the litte critters when your onions are at the four or five leaf stage. That's the point when middle leaves are pushing up adjacent to one another. This is the hiding place that thrips prefer. It's also the place you can look to see if you have a developing problem. Just peel the leaves apart gently, look down into the fleshy tissue close to the developing bulb and look for tiny yellow bits of movement. If you don't see anything, try another plant. If you do find them, ask a local extension entomologist what product to use to control them. Make sure you add an adjuvant to spread the spray solution. Onions have a healthy wax layer and if you apply just a watery mix, it often fails to get down into the tight places where the thrips hide. You may need a couple of applications. They can be tough to control. If you see little black bugs around, leave them alone. They're pirate bugs eating the thrips.

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