In the UK, the non-native grey squirrel Sciurus carolinensis is regarded a pest species, damaging young trees by bark-stripping and impacting on other wildlife. The passing of the Grey Squirrel (Warfarin) Order, 1973, allowed the use of Warfarin in England and Wales for the control of grey squirrels. Grain treated with the poison could be dispensed via hoppers designed for access by grey squirrels only, in an attempt to control populations. However, concerns were raised over the bait in these hoppers being accessible to other animals.
From 1 May to l0 June 1975, a hopper designed for grey squirrel control (which complied fully with specifications laid down in the 1973 Order ) was trialled on the Wytham Estate, Oxford, southern England.
The hopper was sited (on the ground) as directed, ensuring that the hopper tunnel was horizontal and the bait container vertical. The methodology outlined in the Order was strictly followed, except that poison-free wheat grains were used. The use of the hopper (day and night) was recorded by an automatic infra-red sensitive camera.
Photographs showed that six species fed on the grain presented via the hopper: grey squirrel, wood mouse Apodemus sylvaticus, brown rat Rattus norvegicus, ring-necked pheasant Phasianus colchicus, robin Erithacus rubecula and jay Garrulus glandarius. It was also evident that much grain was scattered in the vicinity of the hopper (especially at night when grey squirrels are inactive) thus increasing accessibility to the potentially poisoned bait further; tracks indicated that fallow deer Dama dama and badgers Meles meles (not photographed) had been feeding on this.
It was estimated that 55-65% of the grain entered the environment via animals other than squirrels.
The results of this single-hopper trial indicate the lack of specificity of this method of control using the hopper design tested.
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