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Last week, when I dug over my small vegetable patch, I failed to see a single earthworm. In previous years, and certainly several years ago, the soil seemed to be full of them. The vegetable patch is used to grow runner beans each year and receives a ready supply of organic material from composted kitchen waste and pigeon litter. No chemicals are used on the soil apart from slug pellets – containing metaldehyde – for a few weeks in the spring and I remove them once the bean plants are established. Have the slug pellets killed off the earthworms or are there other forces at work?

• I have a wormery and notice differing amounts of activity throughout the year, as a result of numerous effects. When the winter is cold, worms tend to burrow deep into the soil, where it is warmer. They come to the surface when it rains, so if the weather has been dry it could be that the worms just haven’t come up to the surface yet.

Worms dislike acidic conditions, which pigeon litter tends to produce, so it could be that the soil has become too acidic. Additionally, the fact that you grow the same crop in the same place every year will not help, because this encourages parasites and soil disease.

There are several things you could try in order to bring the worms back. These include sprinkling crushed eggshells, which are alkaline, into your compost to redress any pH problem, rotating your crops and replacing slug pellets with beer traps or bran. Both are very effective and much less harmful to other wildlife.

Liz Barratt, Leeds, West Yorkshire, UK

• All that may be true, but the most popular suggestion in the replies we have received is that the worms are being eaten – Ed

• A sinister explanation may be related to the location of the garden in question – west Dunbartonshire. The New Zealand flatworm Arthurdendyus triangulatus, a voracious earthworm predator, was accidentally introduced to Edinburgh in 1965 and has spread across the UK. The alien flatworm is especially widespread on the warmer and wetter west coast of Scotland, particularly in the central belt, and one of its favoured habitats is gardens.

Anecdotal evidence suggests that the flatworm can reduce earthworm populations. It feeds at night, wrapping itself around any prey and digesting it externally. During the day, it prefers undisturbed sites and can often be found under large plant pots and stones. It is recognisable by its liver-coloured appearance and a covering of mucus. The mucus can be allergenic to humans so handle flatworms with gloves.

Phil Hulme, Lumphanan, Kincardineshire, UK

• I suspect that you have one or more New Zealand flatworms living in the garden. They are now widely distributed throughout the UK, and are becoming a serious problem in British gardens because they seem to live exclusively on earthworms.

Losing earthworms from your garden will mean that the soil is more likely to become waterlogged because the worms will no longer be aerating it during their burrowing activity. Many native bird and mammal species will also be affected because earthworms form an important part of their diet.

New Zealand flatworms have been found to be susceptible to high-temperature treatments which do not affect plant health. According to the Henry Doubleday Research Association, immersing whole plants or plant root balls in water at 34 °C for 10 minutes or standing the plants in a warm room kept at a minimum of 26.5 °C for 24 hours or at 30 °C for 12 hours will be sufficient to kill any flatworms present in the container. No chemical controls are available that don’t also harm earthworms.

For more infomation see:

John Penny, Lecturer in Ecology, Bell College, Hamilton, Lanarkshire, UK