Earthworms, for instance, are essential for soil health, but other worm-like organisms, such as cutworms, grubs, and nematodes, can quickly ruin plants. The first step in addressing a worm problem is accurately identifying the creature causing the damage, as removal techniques vary drastically depending on the species.
Identifying the Worm Causing Damage
Garden pests often mistaken for earthworms are typically the larval stage of insects. Cutworms are plump, soft-bodied caterpillars, usually gray, brown, or black, that can grow up to two inches long. When disturbed, they characteristically curl into a tight “C” shape and are infamous for severing the stems of young plants at the soil line, especially in the spring.
Wireworms are the larvae of click beetles, recognized by their slender, hard, shiny, and segmented bodies. These pests feed on seeds, roots, and underground stems, leading to poor germination and tunneling damage in root crops like potatoes and carrots. Grubs, such as the larvae of the Japanese beetle, are white, C-shaped, fleshy organisms with a distinct brown head. They feed on plant roots just below the soil surface, causing grass to die in patches that can be easily rolled back like a carpet.
Cabbage worms, the larvae of the cabbage white butterfly, are velvety green caterpillars that create large, irregular holes in plants of the cabbage family. They are often detected not by sight, but by the large amounts of dark green fecal pellets, known as frass, they leave behind on the leaves. Invisible to the naked eye are plant-parasitic nematodes, which damage plants by feeding on roots. Visible damage includes stunted growth, yellowing foliage, and the formation of galls or swellings on the roots, which can impair the plant’s ability to absorb water and nutrients.
Managing Beneficial Earthworms
Earthworms are beneficial organisms that should be encouraged because they are nature’s soil engineers. Their burrowing activity creates tunnels that significantly improve soil aeration, drainage, and water infiltration. They also enhance soil fertility by consuming decaying organic matter and excreting nutrient-rich “castings,” which are concentrated sources of plant-available nutrients.
In rare cases, such as in potted plants where a dense population can damage delicate roots or compact soil, their numbers may need reduction. A non-lethal method involves soaking the plant’s pot in a bucket of water for about 20 minutes to encourage the worms to rise to the surface. They can then be manually collected and relocated to a compost bin or a garden bed where their activity is welcome. Another temporary measure is to allow the soil to dry out between waterings, as earthworms thrive in consistently moist conditions.
Targeted Control Methods for Pest Worms
Immediate control of pest worms often involves a combination of mechanical, biological, and, if necessary, targeted chemical treatments. Mechanical methods include hand-picking visible surface feeders like cutworms and cabbage worms, which is most effective in the evening when they are active. For cutworms, physical barriers like cardboard or plastic collars placed around the base of young seedlings can prevent them from severing the stem.
Soil-dwelling pests like grubs, wireworms, and nematodes can be managed using soil solarization, a process that involves covering moist soil with clear plastic sheeting during the hottest months. This technique traps solar energy and raises the soil temperature to levels that can kill pests and their eggs in the top 6 to 12 inches of soil over four to six weeks. Deep tillage can also physically expose soil-dwelling pests like grubs and wireworms to predatory birds and desiccation.
Biological control offers a highly specific and low-impact approach using natural enemies. Applications of Bacillus thuringiensis (Bt), a naturally occurring soil bacterium, are effective against caterpillars like cabbage worms and cutworms when ingested. Different strains of Bt, such as Bt kurstaki (Bt-k) for leaf-feeding caterpillars and Bt galleriae (Bt-g) for Japanese beetle grubs, are highly selective and do not harm beneficial insects or humans.
Another biological tactic is the application of beneficial nematodes, microscopic roundworms that hunt and kill pest larvae in the soil. Species like Steinernema carpocapsae and Heterorhabditis bacteriophora are commonly used to target grubs, cutworms, and root weevils. These beneficial organisms are applied by watering them into moist soil during the early morning or evening to protect them from UV light, and they kill the host pest within 48 hours. Chemical control should be a last resort, using targeted, low-impact products applied precisely to the affected area, and always following label instructions.
Future Strategies for Pest Prevention
Long-term pest management relies on cultural practices that make the garden environment less hospitable to harmful worms. Crop rotation is a powerful tool against pests with limited mobility or host ranges and many plant-parasitic nematodes. By rotating plant families in a three-to-four-year cycle, gardeners disrupt the pest’s life cycle by removing its preferred food source, which causes the population to decline.
Physical exclusion with floating row covers is highly effective for preventing moths and butterflies from laying eggs that hatch into damaging caterpillars, like cabbage worms. These fabric barriers are installed immediately after planting and must be secured around the edges to block entry. For crops that require insect pollination, such as squash, the covers must be removed during the flowering period.
Specific planting choices can also deter certain pests, such as French marigolds, which release a biochemical compound called alpha-terthienyl from their roots. This compound is toxic to plant-parasitic nematodes, particularly the root-knot and lesion species. Maintaining balanced soil moisture is also a preventative measure, as pests like wireworms and cutworms often thrive in overly wet or poorly drained soil.