Interactions between fertilizer use and leatherjacket control in grassland

A coordinated experiment was set up in eight fields of established grass; four In Northern Ireland and four in the West of Scotland. At each site one of three different fertilizer rates (0, 75 and 150 kg N ha^-1) were applied to plots that had previously either received a 0.72 kg ha^-1 chlorpyrifos treatment to control leatherjackets or no spray. Both fertilizer and insecticide applications increased herbage yields. Yield increases in response to chlorpyrifos treatments were equivalent to that obtained from 75 kg N but were not linearly related to leatherjacket numbers. A curvilinear function relating loss per leatherjacket to population size was derived and its validity is discussed. It is concluded that herbage yield increases resulting from chlorpyrifos and nitrogen applications were independent and cumulative.