Do Nematodes Really Work for Garden Pest Control?

Nematodes are microscopic roundworms that parasitise specific pest species, killing them from the inside while leaving plants, earthworms, mammals, and beneficial insects completely unharmed. When applied correctly, they are one of the most effective and targeted biological controls available to home gardeners — with results comparable to chemical treatments for certain pests, particularly vine weevil and slugs.

The key phrase is "applied correctly." Nematodes have specific temperature and moisture requirements, and ignoring them is the most common reason applications fail.

Which Pests Do Nematodes Target?

Different nematode species target different pests. The most widely available for home gardens include:

• Steinernema kraussei — targets vine weevil larvae in the soil. Apply in late summer or early autumn when grubs are small and soil is still warm.
• Phasmarhabditis hermaphrodita — targets slugs and their eggs in the soil. Apply in spring and autumn when soil is moist and above 5°C.
• Heterorhabditis bacteriophora — targets chafer grub, leatherjackets, and some other soil-dwelling larvae. Needs warmer soil (above 12°C) than other species.
• Steinernema feltiae — targets fungus gnat larvae in compost and growing media, making it ideal for greenhouse and houseplant use at cool temperatures (down to 5°C).

How Nematodes Work

Nematodes enter the pest through natural body openings or directly through the cuticle, then release bacteria that kill the host within 24–48 hours. The nematodes reproduce inside the cadaver and the next generation moves through the soil seeking new hosts. The cycle continues until host populations are depleted, after which nematode numbers naturally decline.

Application Conditions

Nematodes need moist soil to move and find hosts — they cannot swim through dry soil particles. Water the area thoroughly before application, apply the nematode solution in the evening or on a cloudy day to reduce UV exposure, and water in immediately afterwards. Keep the soil moist for at least two weeks after application. Avoid applying to frozen, waterlogged, or baked-dry soil.

Most species need soil temperatures above 5°C; check the specific requirement for the species you are using. Applying too early in spring or too late in autumn is wasteful.

Storage and Shelf Life

Nematodes are living organisms and must be refrigerated on arrival. Most packs are viable for four to six weeks when refrigerated. Do not freeze. Use as soon as practical after purchase, mixing the powder or liquid concentrate according to instructions immediately before application. Never leave mixed solution standing in direct sunlight.