Stink bugs cause spots on tomatoes

By Marianne C. Ophardt, Special to the Tri-City HeraldAugust 3, 2012 

KENNEWICK, Wash. -- The brown marmorated stink bug has been plaguing the eastern part of the U.S., damaging crops and invading buildings, but it's a green stink bug that is a problem for some area gardeners.

The feeding of this green stink bug is causing yellowish to whitish cloudy spots on the fruit of infested tomatoes. Just beneath the skin of a cloudy spot are spongy clusters of hardened white cells. While still edible, this makes the fruit difficult to peel and impairs their eating quality.

This is the time of year that stink bugs residing in nearby weeds move to the garden to pierce ripening tomatoes for their juice. Scientists disagree about exactly what causes the spots, but it's generally believed that they inject a toxin into the fruit when they pierce it.

There are different green and brown stink bugs that can cause this damage, but the one gardeners are bringing to me are bright green "Say" stink bugs. These shield-shaped bugs are about a half-inch in length with orange spots at the base of their triangular back. The nymphs, or young stink bugs, are blackish in color.

According to University of Utah bug experts, one stink bug per plant can cause 5 percent to 10 percent damage. That means that it doesn't take excessive amounts of stink bugs to cause significant injury to garden tomatoes.

If you suspect that you have a stink bug problem, the Utah expert indicates that you can find the culprits "by vigorously shaking the plant and examining the dirt beneath for fallen insects." Another hint that stink bugs are present is the brown liquid frass (poop) that they leave behind as dried spots on the leaves and fruit where they've been feeding.

If you have a stink bug problem, control will not be easy. That's because stink bugs don't reside in the garden, but in nearby weedy areas, such as fence rows and ditches.

Control involves eliminating weeds to prevent their overwintering and keeping weeds down during the gardening season. Potential overwintering hosts include Russia thistle, plantain, mullein, mustards, mallow, dock, blackberries, legumes, as well as other weeds.

In addition to good weed management, pesticides can be used for control. You should also treat weedy areas that may be serving as stink bug havens.

If you're gardening organically, try using insecticidal soap sprays. Keep in mind that a soap spray only affects the bugs to which it's directly applied, so you'll need to reapply the soap spray regularly.

There are also garden insecticides containing bifenthrin, permethrin, cyfluthrin, imidacloprid, or carbaryl that are effective against stink bugs. Be sure to find a product labeled for use in the vegetable garden, specifically on tomatoes and other crops you plan to treat.

Some of these garden insecticides are toxic to bees. Be sure to follow all label directions and take the precautions needed to protect bees, wildlife and yourself. Always check the label to find out how many days after spraying you must wait before you can safely harvest and consume (or preserve) the fruit. This may be labeled as "days to harvest" or "pre-harvest interval."

Finally, to answer another question that might be on your mind, yes, stink bugs do really stink.

As a defense mechanism, they have small glands on their body that emit a foul smelling liquid when threatened or mistreated. Eat one in a raspberry or hand squash them in the garden, and you'll find out that they come by their name honestly.

* Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

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