The appearance of holes in developing or ripening tomatoes is a common and frustrating experience for home gardeners. A dark entry point signals that a pest has breached the skin and begun to feed inside. Determining the exact culprit is the necessary first step in protecting your harvest. This requires inspecting both the damaged fruit and the surrounding foliage to correctly diagnose the problem.
Identifying the Primary Hole-Boring Culprits
The most common culprits for deep holes in tomato fruit are the larval stages of moths: the Tomato Fruitworm and the Tomato Hornworm. The Tomato Fruitworm (Helicoverpa zea), also known as the corn earworm, is a destructive pest that targets the fruit itself. The larvae are caterpillars that vary widely in color, ranging from pale green to nearly black, often marked with fine light stripes.
The Fruitworm typically feeds on foliage first, but once fruit sets, the young larvae tunnel directly into the tomato, often near the stem end or the calyx. A single larva can damage multiple fruits before it is fully grown. The Tomato Hornworm (Manduca quinquemaculata) is the larva of the large Sphinx Moth and is recognizable by its impressive size, reaching up to four inches in length.
This large caterpillar is bright green with distinctive V-shaped white markings and a noticeable black or red “horn” at its rear end. While the Hornworm is known for its voracious appetite for leaves, it will also chew large, ragged holes into the fruit. Slugs and snails can also create holes in fruit, though they usually target young seedlings and tender foliage.
Distinguishing Damage Patterns and Clues
Identifying the pest depends on observing the characteristics of the damage and any residue left behind. Fruitworm damage is characterized by a small, neat entry hole in the fruit, which often becomes dark. Once inside, the caterpillar consumes the internal tissue, resulting in a messy, watery cavity frequently filled with dark, moist excrement, known as frass.
In contrast, the presence of the Tomato Hornworm is usually first indicated by significant, rapid defoliation, with entire leaves stripped bare. When this pest attacks the fruit, the damage tends to be a larger, more ragged hole compared to the Fruitworm’s entry point. The most tell-tale sign of a Hornworm is the large, dark, pellet-like frass scattered on the leaves and the ground beneath the feeding site.
Slugs and snails leave behind a silvery, shiny mucus trail as they move, which is an unmistakable sign of their activity. Their feeding leaves irregular, ragged holes in the leaves and fruit. Fruitworms are often hidden inside the fruit, while Hornworms, though camouflaged, can be found clinging to the stems near defoliated areas.
Targeted Solutions for Pest Control
Once the pest is identified, applying a targeted control measure is essential. For both the Tomato Fruitworm and the Tomato Hornworm, the most direct method for small gardens is handpicking the larvae off the plant. Since the Hornworm is large and harmless, it can be easily removed and dropped into a container of soapy water.
A highly effective organic control for both caterpillar pests is the application of Bacillus thuringiensis (Bt), specifically the kurstaki strain (Btk). This naturally occurring bacterium is ingested by the feeding larvae, paralyzing their digestive system and killing them. For the Hornworm, encouraging natural predators, such as the parasitic Braconid wasp, is beneficial. If a Hornworm is seen covered in small white cocoons, leave it alone to allow the wasps to emerge.
Controlling slugs and snails requires managing moisture and creating physical barriers or traps. Since these mollusks thrive in damp conditions, reducing excess moisture and removing debris where they hide is helpful. Simple, non-chemical traps, such as shallow dishes filled with beer sunk into the soil, will attract and drown them. Applying iron phosphate pellets, which are safe for use around pets and wildlife, provides an effective bait.