The term “worm” is often used loosely by gardeners finding damage on their tomato plants, but it describes two very different groups of organisms. True earthworms are beneficial soil dwellers, while various insect larvae, frequently misidentified as worms, are the real culprits behind the destruction of tomato plants and fruit. Identifying the correct organism causing the problem is the first step toward protecting a tomato harvest.
The Role of Beneficial Earthworms
Earthworms, such as nightcrawlers or red wigglers, are detritivores whose diet consists primarily of decaying organic matter, bacteria, and fungi in the soil. They are not equipped to eat the healthy, living tissue of a tomato plant’s leaves or fruit. Their presence indicates healthy soil structure and fertility.
As they tunnel, earthworms create burrows that enhance soil aeration and water infiltration, benefiting the tomato plant’s root system. The material they excrete, known as castings, is rich in nutrients and acts as a slow-release, organic fertilizer. True earthworms are allies in the garden, constantly improving the conditions necessary for a productive tomato crop.
Identifying the True Tomato Pests
The organisms responsible for eating tomato plants are moth larvae, which are herbivores better described as caterpillars. The most destructive are the tomato hornworm and the cutworm. These pests are often mistaken for worms due to their soft, cylindrical bodies.
The Tomato Hornworm
The Tomato Hornworm (Manduca quinquemaculata) is a large caterpillar, growing up to five inches long. It is recognizable by its pale green color and a distinct black “horn” projecting from its rear segment. Though highly camouflaged, these voracious eaters can defoliate a tomato plant virtually overnight. After hatching from eggs laid on the underside of leaves, they feed for three to four weeks before burrowing into the soil to pupate.
Cutworms
Cutworms, the larvae of several different moth species, are a significant threat, particularly to young tomato transplants. These smooth-skinned, greasy-looking caterpillars range from one to two inches long and often curl into a tight “C” shape when disturbed. Cutworms are nocturnal feeders, hiding during the day and emerging at night to attack plants at the soil line.
How to Distinguish Different Types of Damage
Diagnosing the problem requires linking the type of damage to the pest’s feeding habit. Hornworm damage is dramatic, characterized by large sections of stripped leaves where only the central vein remains. A telltale sign of their presence is large, dark, pellet-like droppings, called frass, scattered beneath the plant.
Cutworm damage is distinct and usually appears suddenly on small plants. These pests get their name because they sever the seedling’s stem completely at or just above the soil level, causing the entire young plant to collapse or disappear. Later in the season, some cutworm species may also climb the plant to chew irregular holes into fruit that are touching the ground.
A third pest, the Tomato Fruitworm (Helicoverpa zea), attacks the fruit itself. The larvae bore a neat, round entry hole, often near the stem end. Once inside, the fruitworm feeds, creating a watery cavity contaminated with waste, which leads to premature ripening and decay.
Management Strategies for Tomato Pests
Manual intervention is one of the most effective and direct strategies for controlling hornworms, especially in a small garden setting. Daily inspection of the plants, looking for frass or signs of defoliation, allows for the handpicking of the large caterpillars. Once removed, they can be destroyed by dropping them into soapy water.
For cutworms, a physical barrier is the simplest defense for newly set transplants. Collars made from cardboard, toilet paper tubes, or plastic cups placed around the seedling’s stem and pushed an inch or two into the soil will prevent the nocturnal pest from wrapping around the base of the plant to feed. This protective measure is highly effective until the stem thickens and becomes less vulnerable.
Biological control methods focus on encouraging the pests’ natural enemies. Parasitic wasps, such as Cotesia congregata, are a powerful natural deterrent against hornworms. These wasps lay eggs inside the hornworm; the resulting white cocoons visible on the caterpillar’s back signal that the pest has been neutralized. The caterpillar should be left alone to allow the beneficial wasps to emerge and parasitize more hornworms.
Gardeners can apply organic treatments like Bacillus thuringiensis (Bt). This naturally occurring soil bacterium specifically targets and paralyzes the digestive system of caterpillars. The kurstaki strain (Btk) is effective against hornworms and fruitworms and is safe for use up to the day of harvest. Cultural control practices, such as tilling the garden soil in the late fall or early spring, help expose and destroy overwintering pupae, reducing the population before the next growing season.