Efficacy of common protection devices in preventing night-time damage of cabbage crops by Spot-billed Ducks in Japan

Abstract

Data from the Japanese Ministry of Agriculture, Forestry and Fisheries indicate wildfowl damage to vegetable crops is common in Japan, but specific details on the problem and the effectiveness of alleviation methods do not exist. A flock of up to 99 Spot-billed Ducks ( Anas poeciloryhncha ) roosted on a river during the day at Kawagoe City, central Japan. At night some or all of this flock fed on cabbage ( Brassica oleracea Capitata group) crops nearby. Farmers deployed numerous scaring devices in the area to prevent crop damage. We grouped these into four categories: (1) monofilament lines; (2) monofilament lines in conjunction with either scarecrows, flags, and illegal mist nets; (3) flags, and flags in combination with scarecrows, and mist nets; and (4) exclusion nets, and tested the relative efficacy of these methods over 73 fields. Considering only presence or absence in fields of damaged plants, the relative effectiveness of the four categories were ranked from most to least effective as: nets > monofilament lines and others monofilament lines = flags and flags plus others. The proportions of cabbage plants damaged per field were lowest in fields protected by nets. The amount of material removed from damaged plants did not differ significantly between the categories. Crop value depends on a combination of factors, notably yield (number of plants per 10 kg), monthly changes in sale price, and plant quality. In 1996 a poor yield, slow growth rate, and poor quality plants as a result of duck depredation could have caused economic losses of 68.2% or more in some cases. Deployment of nets over the crop using metal frame supports is a costeffective protection method, but nets must be adequately supported and maintained. Although effective, nets are not favoured by farmers because they are difficult to maintain. They are also hazardous to nontarget birds in the area such as Gray Starling ( Sturnus cineraceus ), Dusky Thrush ( Turdus naumanni ), Rufous Turtle Dove ( Streptopelia orientalis ) and Skylark ( Alauda arvensis ) which die if accidentally entangled in the mesh. We therefore discuss alternative crop protection strategies which can be considered.