A common caterpillar that bores into tomatoes is the tomato fruitworm (Helicoverpa zea); also known as the corn earworm or cotton bollworm. In fact one of the recommended ways to keep them away from your tomatoes is to keep your tomatoes away from your corn patch! They love fresh corn silk and tender young ears of corn and actually corn is the main host of this destructive caterpillar!
Tomato fruitworm caterpillars are the larval form of a rather nondescript tan colored moth. This moth lays solitary eggs on the host plant (e.g. tomato or corn) and can lay up to 35 eggs in a day. Small light colored caterpillars hatch out in a few days and begin feeding on the host plant. Older caterpillars vary in color from green to tan to brownish-red and most have stripes on the side of their body. The whole life cycle of these insects can be completed in about 30 days so there are usually several generations in a season.
The young caterpillars often begin by eating foliage but then gradually move to the fruit which is their preferred food. They bore into green tomatoes (usually at the top of the tomato near where the stem joins the fruit) making a hole about the size of a pea. Once inside, they feed on the meat and pulp eventually turning the inside of the fruit into a mass of nastiness and rendering the tomato inedible.
When they hatch on corn, they usually eat their way down through the silk and feast on the kernels at the tip of the ear. Sometimes they will bore through the husk on the side of the ear and consume kernels there, but they are most often found at the tips of the ear.
These caterpillars are pretty competitive so usually there is only one per fruit. Because of this, they can damage A LOT of vegetables if they are not controlled.
These caterpillars also attack green peppers, eggplant, and sometimes even beans. As far as corn as a host plant, once the silk dries and turns brown, it becomes less desirable and the fruitworm moth switches to tomatoes as a preferred host. This may be one of the reasons that tomato damage becomes more common later in the season.
I have noticed a few holes in my tomatoes lately but didn’t see any caterpillars – I guess because they were already inside destroying the fruit! These tomatoes were quite disgusting and I threw them out. Until I saw this question on the discussion board, I had just attributed the loss to fruit rot from excess rain. Now I’m betting that it was tomato fruitworm damage. We did find quite a few of these caterpillars in the corn when we harvested it a few weeks ago. Luckily, with the corn, you can just cut off the tip and salvage most of the crop – not so with the tomatoes!
The main problem with controlling these caterpillars is that once they bore inside the fruit, they are protected from insecticides. The trick is to control them before they find their way into the fruit. Monitor your plants for eggs and young caterpillars throughout the season and destroy any eggs you find on the foliage or simply remove any leaves with eggs.
Other than hand-picking, one of the best and safest (for beneficial insects like bees and other pollinators) ways to control the caterpillars is to apply Bonide Bt Thuricide or Captain Jack’s Deadbug Brew according to the label directions. Both of these products contain bacteria that kill the caterpillars after they ingest foliage or fruit that has been sprayed.
If your tomatoes are in bloom, be sure to spray in the evening or early morning when the bees and other pollinators are less active.
Once the caterpillars get inside the fruit, there is really no control.
In the fall, remove and destroy the old tomato vines. This will also help with disease control. Tilling the garden in the fall will help destroy many of the overwintering pupae or expose them to the elements and to predators.