Ask Your PCA: How important is residual and broad-spectrum worm control in tomatoes?

Issue Date: July 9, 2008

 

By Ray Stoll, Western Farm Service, CAPCA member

They're very important. Beet armyworm is one of the most important pests we have in processing tomatoes, but we also have yellow-stripe armyworm, looper, tomato fruit worm, and we can occasionally get tomato horn worm. So we definitely need a broad-spectrum control.

Residual is also important because these pests show up around mid-season and stick around through harvest. For resistance management, we almost never apply the same material twice in a season--and never back-to-back--so it's important that each insecticide last as long as possible. I like to see 21 days of control, if possible. If I can get that, I might need only one spray for the first tomatoes, and just two to four sprays for mid-season and late-season processing tomatoes.

In scouting for these pests, I look for egg masses and I get ready to treat as soon as I see them. There are distinct differences in the egg masses for each species. Beet armyworm eggs are about a quarter inch in diameter with about 20 to 50 eggs in each mass. There's also a tuft of white hair on the top. Yellow-stripe armyworms will lay 100 to 200 eggs in each mass and there's also a tuft of hair, but it's gray. Tomato fruit worms lay just one or two eggs and they're round. Loopers also lay just one or two eggs, but it's like someone sat on them. They're oblong, almost like a football.

I like to time my spray to the hatch, and that's another reason I need residual and spectrum. Sometimes, these might hatch all at once, but that's unusual. Usually, you'll have different species laying eggs at the same time and the same species laying eggs at different times.

I don't really have an economic threshold. It varies between species. For beet armyworm, my standard might be one egg mass per 30 feet of row. Loopers aren't as damaging to the fruit so I would hold off a bit with them. But yellow-stripe armyworms are very damaging, and they have those large egg masses of up to several hundred eggs in each. If I see even a couple of their egg masses in a single check of 200 yards or so, I might spray.

We have quite a few chemical controls. Each one has its strengths and weaknesses. One might be strong on beet armyworm, but weak on yellow-stripe. Another might be weak on loopers or tomato fruit worm. And a couple of them also get leafminers.

So we look for the specific pests in each field, then choose accordingly. And we always keep in mind resistance management and each chemistry's effect on beneficial insects. Processing tomatoes have several beneficial insects, including big-eyed bugs, lacewings, assassin bugs and several parasitic wasps. Most of the newer chemistries are easy on beneficials.

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item. Top