A study of the effects of ground cover on overwintering slug populations and effect of coulter design on slug incidence in direct drilling : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science at Massey University
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A two stage study involving the effect of vegetation cover on overwintering slug populations, and the effect of coulter design on slug incidence and damage in a direct drilled cereal was carried out during the 1980/81 growring season. The first stage of the study showed that ground cover affected slug activity on the soil surface, but only in the most adverse environment did any actual decrease in slug populations occur. Differences occurred in the effectiveness of the trapping techniques depending on the density of the ground cover. Pitfall traps appeared to be more effective in dense ground covers, while brick or shelter traps appeared to be more effective in low density ground covers and especially with bare ground. Rainfall, soil temperature and soil moisture were measured and it appeared that slug numbers recorded in the traps were correlated to different environmental parameters depending on the ground cover. In dense covers the slug number recorded was correlated to temperature, in medium density ground covers the numbers had a slight correlation to soil moisture, and in low density ground covers they were correlated to rainfall. The second stage of the study involved two dates of drilling, using three coulter types (triple disc, hoe, chisel coulter) and measuring slug numbers occurring in the seed grooves and slug damage to seeds and seedlings. It was found that coulter design had no effect on slug ingression into the seed groove, or on slug damage to the direct drilled crop. There was however a strong correlation between slug numbers in the seed groove and seed and seedling damage (r=0.78, r=0.93 respectively). Pre drilling conditions affected the number of slugs entering the seed grooves (the denser the vegetation the greater the slug number occurring in the seed groove), and slug damage to the seedlings. Moisture levels also affected the number of slugs entering the seed grooves and seed and seedling damage by slugs. Moister conditions produced the greater number of slugs in the seed grooves and the highest seed and seedling damage.