EFFECTS OF SIMULATED INSECT DAMAGE AND WEED INTERFERENCE ON COTTON GROWTH AND REPRODUCTION

Early-season insect pests of cotton (Gossypium hirsutum L.) crops, inc luding Lepidoptera larvae and mirids, feed on vegetative buds. The los s of vegetative buds transiently delays the plant's development and gr owth and has the potential to reduce its competitive ability. Yield re ductions due to weed interference and insect damage, therefore, could be greater than expected from the additive effects of weeds and damage acting separately. Three varieties, two levels of weed infestation, a nd two levels of simulated damage were combined in a factorial experim ent designed to assess the responses of cotton plants to the combined effects of damage and weeds. Weed treatments were: with (+W) and witho ut weeds (-W), and damage treatments included: undamaged control (-D) and damaged plants (+D) which had their active vegetative buds removed at 30 and 49 days after sowing. Variety and interactions between vari ety and other factors were normally nonsignificant for all the respons e variables measured in this experiment. Cotton height, width, product ion of flowerbuds and production of fruit were all affected by both da mage and weeds. While the effects of damage on these growth variables were transient, the effects of weeds normally increased with time thro ughout the season. Non-additive effects of weeds and damage were minor for plant height and width, and large for flowerbud and fruit product ion. At maturity, weed dry matter ranged from about 20 g m(-2) (-W tre atments) to 300 g m(-2)(+W) , cotton dry matter from 144 g m(-2) (+W+D ) to 945 g m(-2) (-W-D), and seed cotton production from 54 g m(-2) (W+D) to 417 g m(-2) (-W+D). Damage did not affect vegetative dry matte r and marginally increased seed production in -W plots. Non-additive e ffects of weeds and damage were negligible for vegetative dry matter b ut highly significant for seed production (P < 0.0001). These contrast ing responses of vegetative and reproductive growth are in agreement w ith neighbouring models of plant competitive interactions that emphasi se the effects of neighbour interference on the fecundity of target pl ants that are not mediated by changes in target-plant size.