INTERFERENCE AMONG COTTON NEIGHBORS AFTER DIFFERENTIAL REPRODUCTIVE DAMAGE

In indeterminate plant species, the rate of vegetative growth usually declines during the stage of active reproductive growth. Fruit sheddin g, as induced by insect herbivores, could counteract this decline. Due to the relative increase in vegetative growth, plants that have suffe red reproductive damage could be better able to intercept light and ac quire soil resources than undamaged plants. If so, plants with damaged neighbours might grow less than their counterparts with smaller, unda maged neighbours. This hypothesis was tested in high- and low-density cotton crops subjected to three treatments: (i) undamaged controls; (i i) uniformly damaged, in which all plants were damaged; (iii) nonunifo rmly damaged, in which every second plant was damaged. Damaged plants had their flowerbuds and young fruits manually removed at 85 days afte r sowing to simulate shedding as induced by Helicoverpa spp. (Lepidopt era) and mirid bugs (Hemiptera). As expected, damaged plants had great er leaf area and more vegetative dry matter than undamaged ones. This was most pronounced at high plant density. Neighbour status did not af fect vegetative growth but it had a substantial, asymmetric effect on the reproductive growth of target plants. Damaged targets recovered to the level of undamaged controls in terms of total fruit number but ha d a large reduction in the mass of mature fruit due to the limited tim e available for recovery. The effect of neighbour status, if any, on t he production of mature fruit in damaged targets was overridden by the limit imposed to recovery by the duration of the growing season. In c ontrast, neighbour status affected the production of mature fruit of u ndamaged targets: undamaged targets with damaged neighbours had 34% (l ow density) and 56% (high density) less mature fruit mass than their c ounterparts with undamaged neighbours. This was because (i) reproducti ve allocation and (ii) the propertion of total fruit that reached matu rity in target plants declined with increasing neighbour interference. Most studies dealing with changes in competitive relationships among plants subjected to differential herbivory have shown how undamaged pl ants may benefit from herbivores that feed on their neighbours. This s tudy shows that differential reproductive damage can cause the opposit e effect, as undamaged plants may have a significant reduction in prod uctivity due to the influence of neighbours whose vegetative growth wa s stimulated by the loss of reproductive organs.