When a tomato plant’s leaves start disappearing, the damage is typically caused by one of three categories of pests, each requiring a specific control strategy. Identifying the exact pattern of consumption is the first step toward successful pest management. Understanding the culprit’s feeding habits can guide you to an effective solution and prevent crop loss.
Large-Scale Defoliators
The most immediate leaf loss is often caused by the tomato hornworm, a larva that can consume entire leaves and small branches quickly. This caterpillar, which can grow to be four to five inches long, is a master of camouflage due to its pale green color and V-shaped markings. The most reliable sign of its presence is the dark green or black pellet-like droppings, known as frass, which accumulate on leaves below where the caterpillar is feeding.
Hornworms are the larvae of the five-spotted hawk moth, and their large appetite requires prompt action. The most direct control method for a home garden is handpicking the caterpillar and dropping it into soapy water. Look for hornworms covered in small, white, rice-like cocoons; these are the pupae of beneficial parasitic wasps and should be left alone. For widespread infestation, an organic insecticide containing Bacillus thuringiensis (Bt) can be applied. This insecticide only harms caterpillars that ingest it and is most effective on smaller larvae.
Extensive defoliation can also be the work of larger vertebrate pests like rabbits or deer. Rabbits typically chew young tomato plants off near the base, while deer strip foliage from the upper parts of the plant. Unlike insect damage, which is localized, vertebrate feeding results in a large portion of the plant being cleanly removed. Control involves physical barriers, such as fencing. Fencing for rabbits should be at least two feet high and buried six inches deep to prevent digging underneath.
Puncturing and Skeletonizing Pests
Damage that appears as numerous small holes, speckles, or transparent patches is the signature of smaller, sap-feeding insects. Flea beetles, tiny beetles that jump when disturbed, create a characteristic pattern called “shot-holing.” This leaves behind a dense scattering of small, round holes less than one-eighth of an inch in diameter. This damage is most harmful to young seedlings, which can be stunted or killed if foliage loss is extensive.
Spider mites, which thrive in hot, dry conditions, cause damage by piercing plant cells to extract the contents. This feeding leaves behind a fine speckling, or stippling, of tiny white or yellow spots, which eventually makes the leaves look bronzed or dusty. In severe infestations, fine webbing may be visible on the undersides of the leaves, where the mites primarily colonize.
Management for these pests relies on topical and barrier controls that differ from manual removal. For flea beetles, using reflective silver plastic mulch is effective, as the reflected ultraviolet light disorients the insects. For both flea beetles and spider mites, insecticidal soap or neem oil can be applied directly to the foliage. Ensure coverage of the leaf undersides where pests hide. A strong jet of water can also physically dislodge spider mites in the early stages of an infestation.
Ground-Level and Nighttime Feeders
Damage from nighttime feeders can be confusing, as the pests are gone during the day. Slugs and snails are common nighttime feeders that leave behind large, ragged holes in the lower leaves and a telltale silvery, slimy trail. These mollusks hide during the day in cool, moist places, such as under mulch, rocks, or garden debris.
Another pest active at the soil line is the cutworm, the larva of a night-flying moth. Cutworms often sever the stem of a young transplant right at or just below the soil surface, making it appear as if the plant was neatly cut down. When disturbed, these caterpillars will curl into a tight C-shape.
Control for these ground-level pests involves unique trapping and barrier methods. For slugs and snails, shallow containers of beer buried at ground level act as a drowning trap, attracting the mollusks with the yeast scent. Copper barriers, such as copper tape around a raised bed, can also repel them, as the metal reacts with their slime to cause a mild electric shock. To prevent cutworm damage, a simple cutworm collar can be placed around the stem of each transplant. This collar, made from a cardboard tube or plastic cup, should be pushed an inch or two into the soil, creating a fence the cutworm cannot climb.