Field trials of anti-capsid insecticides on farmers' cocoa in Ghana, 1956-60. 1, Comparing the effects of treatments by assessing subsequent damage
1970
Johnson, C.G., Rothamsted Experimental Station, Harpenden, Herts., England, both previously of the | Burge, G.A., Rothamsted Experimental Station, Harpenden, Herts., England, both previously of the | Gibbs, D.G., Rothamsted Experimental Station, Harpenden, Herts., England, both previously of the
Two ways of assessing the amount of new damage occurring after spraying and three ways of assessing its trend are described and compared. Two assessments at fortnightly intervals of new damage to foliage adequately assessed the amount of damage, and the difference between the amount of new damage at 4 and 12 weeks after spraying adequately assessed trend. Canopy assessments every 6 months also gave a good indication of the effects of spraying and could probably replace assessments of trend of new damage by fortnightly assessments. The variability in the incidence of new damage on different plots after spraying was analysed and the number of replicates needed to detect differences between effects of different treatments was considered, both when data were untransformed and transformed to logs. The value of pre-treatment assessments of damage and subsequent grading and blocking of plots is also discussed in relation to inter-plot variability. The inclusion of untreated control plots in analyses of variance overestimated true errors for comparison of treated plots and hampered the detection of differences between them. Controls are therefore unnecessary for detecting differences in the incidence of new damage. But they are essential for comparisons of canopy regeneration because some plots can improve even without spraying.
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