Crop monitoring to assess the risk of slug damage to winter wheat in the United Kingdom

The percentage of winter wheat seeds killed by slugs was measured, together with various risk factors, at 93 sites throughout the UK from 1987 to 1990. Estimates of slug populations at each site were obtained from traps baited with methiocarb pellets. The peak number of slugs trapped during the period from July until just before cultivation accounted for 26% of the variability in seed damage, more than any other single risk factor. A combined function of the number of slugs trapped at drilling and the percentage of fine soil aggregates in the seed-bed accounted for 21% of the variability in seed damage. Both (i) the peak number of slugs trapped before drilling, and (ii) the combined function of number of slugs trapped at drilling and the percentage of fine soil aggregates in the seed-bed, have potential for identifying fields with a negligible risk of slug damage to wheat seeds. However, further research is needed to improve the accuracy of forecasting slug damage. In particular, other forms of trap should be tested for their potential use in providing more accurate estimates of slug numbers and biomass at drilling, and techniques of assessing seed-bed conditions should be improved. The percentage of seedlings grazed by slugs was correlated with the numbers of slugs trapped before drilling, at drilling and after crop emergence.