“Black clocks” call time on invasive flatworm

Entomologists in Belfast may finally have found a way of limiting the spread of the New Zealand flatworm, which invaded the British Isles in the 1960s. Speaking at the Royal Entomological Society’s national meeting Entomology 2002, which will take place at Cardiff University on 12–13 September 2002, Dr Archie Murchie of the Department of Agriculture and Rural Development (Northern Ireland) will announce that certain British beetles could help repel the invader by preying on it. Finding a natural predator for the flatworm is important because it is decimating native earthworm populations. Earthworms play a vital role in soil fertility and are an important source of food for birds and small mammals.

Preliminary results from field experiments show that ground beetles (known locally as “black clocks”) can significantly reduce flatworm numbers. While earlier important discoveries by Edinburgh researchers had found that the beetles ate flatworms in the laboratory, these new field-scale trials indicate that the beetles could help reduce flatworm numbers in the wild. “We now have evidence that natural predation plays a role in limiting flatworm populations. We knew that beetles ate flatworms, but there is a difference between a predator eating prey and being useful in control. I am sure that if I was stuck in a petri dish, I might eat a flatworm, but it would not be my choice otherwise!” Dr Muchie says.

Dr Murchie divided a flatworm-infested field into several plots, separated by plastic barriers. Some plots were then sprayed with an insecticide to reduce the number of beetles. Dr Murchie then measured flatworm numbers in each plot and found that they were more numerous in the sprayed, beetle-depleted, plots.

New Zealand flatworms have spread across Britain and Ireland after they were accidentally introduced in 1963 in the soil around imported plants. Since then, they have established large populations in Northern Ireland, Scotland and around Manchester where the mild, moist weather suits them best. Flatworms are easy to identify because of their distinctive colouration. According to Dr Murchie: “Almost everybody considers it a horrible creature: flat, liver-coloured and covered in slime. It feeds on its prey by wrapping itself boa constrictor-like around earthworms, digesting them externally, and mopping up the resultant earthworm soup with its spongy mouthparts.”

Apart from being unattractive, this invasive flatworm is having a serious impact on our native earthworms, particularly the large, long-lived species such as Lumbricus terrestris. Dr Murchie is currently trying to quantify this impact by moving flatworms around a field, thereby artificially creating areas of high and low population density. He then measures earthworm numbers to assess the impact of flatworms. “Preliminary results suggest that flatworms are having a significant impact on long-lived earthworms. The bigger earthworm species have deep, vertical and semi-permanent burrows, which make it relatively easy for a flatworm to find and eat them. In addition, they breed slowly so they cannot absorb the flatworm predation pressure as well as smaller species can,” Dr Murchie says.

Earthworms help keep soils fertile and support birds and mammals higher up the food chain. According to Dr Murchie: “Earthworms are tremendously important in many soil processes. Their burrowing helps drain and aerate the soil, and their feeding is a major component in the decomposition process, so lower earthworm numbers would have an economic impact on agriculture. Farmers in Northern Ireland alone spend £60–70 million a year on fertilisers. If all earthworms were suddenly to disappear from Northern Ireland agriculture, that bill could easily double within a few years.”

Dr Murchie will present his full findings at 12:10 on Friday 13 September 2002.