Control of parasite burdens in wild red grouse Lagopus lagopus scoticus through the indirect application of anthelmintics

Abstract

Summary Red grouse Lagopus lagopus scoticus populations in Britain exhibit cyclic fluctuations in abundance with periodic population crashes. Substantial evidence exists to suppose that parasitic worms are the major factor causing these population crashes. This experimental study was undertaken between 1996 and 2000 on two areas of moorland in the north Pennines, UK, to evaluate the efficacy of the indirect application of anthelmintic drugs in the control of the caecal nematode Trichostrongylus tenuis. On one area, grouse were given access to anthelmintic drugs incorporated into grit (medicated grit) to control T. tenuis numbers, while on the second control area the birds had access to grit without anthelmintic drugs incorporated (plain grit). In the winter of 1998–99 the treatment was reversed between the two areas. Adult grouse from the experimentally treated area had significantly lower infections of T. tenuis than grouse from the control area. Hen grouse, which had access to anthelmintic drugs, did not produce significantly more eggs or have a greater hatching success than hens with access only to plain grit. Chick survival was significantly greater on the medicated grit areas, with a significant difference in breeding success. Hens that had access to anthelmintic drugs reared more than twice as many chicks as control hens. No significant difference was found in red grouse survival between grit treatments. The increase in grouse numbers arising from the use of medicated grit enhances the viability of red grouse shooting because the cost of treatment appears to be low relative to the return from an increased harvest.