What are eelworms, well they also called nematodes and are mostly microscopic worm like animals and some are significant plant pests. They are very species specific so you find a certain type of eelworm on one particular species, but those that are considered a pest have one thing in common they feed on the host plant. They can feed on any part of the plant and some are specific to root hairs while others focus on roots themselves or other parts of the plant. They are also able to transmit diseases via viral infections and at present there is no control for soil dwelling eelworms and a large concern for specific industries like fruit production and potatoes and tomatoes.
Nematodes come in all types and above is an illustration of the impact of a nematode specific to pines causing pine wilt disease. It is therefore important to maintain your trees in a healthy condition so they can naturally fend of diseases but also to reduce the potential for disease transmission by ensuring your trees are properly sourced initially.
Therefore be careful where you buy or collect trees for bonsai or for any other use for that matter as it is important to know where they have come from and what potential pests they may harbour that you may not be aware of in the early stages.
Check which species currently are at risk and for more general information read the Forestry Commission Biosecurity guidance.
There is good news though in that not all eelworms are pests, some are positive in the soil and feed on other bacteria, fungi and other microorganisms plus larvae and can be an effected control for vine weevil.