Studies of Crop Loss following Insect Attack on Cotton in East Africa

Abstract

The study of crop loss following insect attack on cotton in Uganda, described in Part I of this work, was continued for two further seasons (1954–56), using the same experimental procedure, except that half the DDT in the spray formulation was replaced by toxaphene, only three periods of protective treatment were used for each date of sowing, and the late-season ones were continued until boll-opening was complete, so as to give maximum control of boll worm.Methods are described of estimating an index of leaf damage caused by Lygus spp., based on comparisons with standard grades. At Serere (Eastern Province), the index of leaf damage was closely related to Lygus populations in the crop, and at Namulonge (Buganda) it was markedly reduced on plots protected from Lygus attack, but in the 1954–56 experiments no relationship could be found between the mean seasonal index and the loss of fruiting bodies attributable to Lygus attack, or the final yield.Analysis of the cause and incidence of shedding of developing fruit bodies (flower buds or young bolls, the latter predominating among those recovered) showed that the total amount of shedding was roughly the same whatever the treatment, that due to insect damage tending to be balanced by that due to other causes, which were probably physiological in nature. There was thus some over-riding factor, other than pest attack, that governed the crop finally held by the plant.Damage caused by insects to older bolls, which do not shed, constitutes a real loss that is reflected in the final harvest. The proportion of clean seed cotton was higher in the treated plots, particularly in the early-sown cotton, in which analysis of the harvested locules showed that the damage was predominantly due to bollworm, but there were no significant differences in yield between the various treatments in either date of sowing or in either season.It is concluded that under such conditions as those experienced at the Research Station in the three seasons of the experiments, insect damage to cotton is small in relation to other environmental factors governing crop production.