Further studies of crop loss following insect attack on cotton in Uganda

Abstract

Earlier studies of crop loss following insect attack on cotton in the elephant-grass zone of Uganda were followed up for two more seasons (1965–67), using large plots and routine spray schedules with different insecticides. The spray schedules were four or six fortnightly applications of insecticide, beginning at week 8 from emergence in the 1965–66 season, and four fortnightly or eight weekly sprays in the 1966–67 season. The insecticides used were DDT, endrin and dicrotophos in the first season and DDT, endrin and endosulfan in the second.Estimates of Taylorilygus vosseleri (Popp.), Heliothis armigera (Hb.), Earias biplaga Wlk., Cryptophlebia leucotreta (Meyr.), Pectinophora gossypiella (Saund.), Dysdercus spp., Aphis gossypii Glov., Tetranychus sp. and Eotetranychus sp. showed that, although one or other of the insecticides reduced the different pests to very low levels, yield increases were not obtained. In the 1966–67 trial, when the more important pests were present in very low numbers, DDT and endosulfan significantly reduced yields of clean seed cotton and it is suggested that these formulations are slightly phytotoxic to Uganda cottons.The relationship between the shedding of fruiting bodies and the harvesting of the crop showed that sprays applied after week 14 did not affect the total crop in any way. Increased soil fertility caused the plants to form more fruiting bodies and, although many of these were shed, a higher proportion were retained with or without protection from pest damage.It is concluded that the pest damage is too light to affect yields of early-sown cotton where varieties of indefinite growth habit can compensate for early loss of fruiting bodies in the extended growing season. Routine spraying gives no benefit and, in some seasons, can be harmful.

Insect infestations and their effects on the growth and yield of field crops: a review

“…Wheat also shows competition between organs of the same plant, for at normal spacing, not all the shoots which are produced can survive to produce ears (Bunting & Drennan, 1966). Another example of competition between organs of the same plant is the " physiological shedding " of reproductive organs (Coaker, 1957;McKinlay, 1965;McKinlay & Geering, 1957;Ingram, 1969). Where there is competition between individual plants or between plant organs mere will probably be a threshold value below which populations of pests have little effect on yield, as opposed to quality.…”

“…This is the reverse of (b) and occurs when the sink is larger than the source, and is commonly seen in crops of indefinite growth such as many varieties of cotton. An insect attack merely decreases the amount of " physiological shedding " and delays harvest (Coaker, 1957;McKinlay & Geering, 1957;Ingram, 1969). (d) Attacked organs are not essential for yield formation.…”