Corn worms or corn earworms (Helicoverpa zea) are also called tomato fruitworms because they prey on both tomatoes (Lycopersicon esculentum) and corn (Zea mays). For this reason, don't plant corn and tomatoes close together to avoid possible cross-infestation. Corn earworms are pervasive throughout the Americas, from Argentina to Canada, and they also attack other fruits and vegetables. Their damage can be severe, so early identification and control are essential.
1. Physical Appearance
Corn earworm larvae, which do the damage to tomatoes, grow to a maximum length of 1 to 1 1/2 inches. Their main body color varies, ranging from green, brown and black to yellow and pink, with lengthwise stripes. The head is typically tan to orange. In addition, the corn worm's skin is covered with tiny spines or thorny projections, called tubercles. The corn worm larval stage lasts two to three weeks, after which corn worms enter the soil to pupate. Corn earworm pupae measure 1 inch or less, are dark brown and are in the top 2 to 4 inches of soil. After the pupal stage, corn worms emerge as adult moths, which also can be found on and around tomato plants in the evening. The moths have a 1 1/2-inch wingspan and yellow-olive forewings, each marked with a dark spot and three slanted, dark bands.
2. Plant Damage
Even if you don't see the corn worm larvae, you can still identify an infestation by the damage on tomato plants and fruit. A bore hole is often found at the base of a fruit stem, for example. Cutting open the tomato, you may find tunneling, frass -- feces -- and perhaps the corn worm still eating away. This pest prefers green, immature tomatoes, and will move to new fruits on the plant as they develop. Corn earworms may also feed on tomato plant leaves, so inspect your plants regularly for signs of chewing.
3. Eggs on Tomato Leaves
Another way to identify corn worms on tomatoes is to look for the eggs. The female lays them singly on leaves, but can lay 3,000 eggs in a season. Check tomato plant leaves regularly for eggs. The eggs are dome shaped, smaller than a pinhead, and white, cream or pale green immediately after being laid. Over the coming days, they change color to a yellow-gray with a reddish-brown band right before hatching. Eggs take two to 10 days to hatch, depending on the temperature.
4. Check the Calendar
In warmer climates, like in the southern U.S., corn worm pupae overwinter in the soil and emerge in early May. They can start laying eggs right away. In areas with cold winters, the pupae cannot survive the freezing weather. So, the corn moths must first travel from the South. You won't find larvae on your tomato plants in the Northeast until mid-July through September. So, if you live in Maine and see holes in your tomatoes in June, it is highly unlikely the culprit is a corn worm.