Nematodes or "eelworms" are one of the most abundant creatures in the living soil. They are an essential part of the soil food web. Some of them feed on plants, some feed on dead organic matter, and others are parasites of other living organisms. Just as there are "bad bacteria" and "good bacteria" from the human perspective, so there are "bad nematodes" and "good nematodes" in the soil. The nematodes that feed on living plant material can be considered to be "bad nematodes" - eg the potato eelworm. However nematodes that kill other plant pests are considered "good nematodes" - eg the nematode that kills slugs.
Parasitic nematodes seek out suitable hosts by swimming in the thin film of water on soil particles, locating hosts by detecting carbon dioxide and other waste products. Once they find a host, they enter the body cavity through the mouth, anus or spiracles, and release bacteria (Xenorhabdus). The bacteria kills the host within hours, and the nematodes grow and reproduce within the 'broth', which they produce. The next generation of infective juveniles leaves the dead host, and moves in search of fresh hosts. Infected hosts become brown and flaccid, but once dead can be difficult to find in soil.
When to use them
Temperature and humidity are important to the success of these biological controls. Nematodes travel by slithering around the soil particles so the soil needs to be kept moist. If it is hot and dry they will either shrivel and die or hide away deep underground. On the other hand they don't survive very waterlogged ground - they will actually drown within about 2 hours if placed in water. Likewise if the soil is freezing the nematodes will perish. Most beneficial nematodes will not survive a British winter outside and will need to be reapplied each year.
However in answering the question "When do I apply" it is also important to focus on the pest you are trying to kill. For example slugs can be found outside in every season apart from winter, and in glasshouses all year round. However if you are aiming to kill chafer bugs, for example, then you need to hit them when the larvae are young in August and September.
We supply SEVEN different nematode packs
The beneficial nematodes (eel worms) are supplied in a paste or a gel in sealed packs which can be just popped in the refrigerator for a few days if you can't use them straight away. Simply mix them with water to the ratio described on the packet and water them into pots or open ground with a watering can or hose. The average temperature should be 5°C or above - and the nematodes will be more active at 10°C. Order just the quantity you need at the time you need it - these are living creatures and the pack should be used while it is fresh.