PLANT COMPENSATION, NATURAL BIOLOGICAL-CONTROL, AND HERBIVORY BY APHIS-GOSSYPII ON PRE-REPRODUCTIVE COTTON - THE ANATOMY OF A NON-PEST

Crop plant compensation for herbivory and the population dynamics of h erbivores are two key elements in defining an herbivore's pest status. We studied the dynamics of natural, unmanipulated populations of the aphid Aphis gossypii on seedling plantings of cotton, Gossypium hirsut um and Gossypium barbadense, over a 4-year period in California's Cent ral Valley. Aphid populations colonized all plantings, but reached den sities in excess of 0.5 aphids/leaf during only one year (1991), when outbreaks occurred. Outbreak populations were, however, ephemeral; pre dation and parasitism suppressed aphid populations prior to the initia tion of flower bud production, when cotton plant growth may become pho tosynthate-limited. Effective natural biological control was observed despite the action of hyperparasitoids and the heavy mortality of imma ture parasitoids that occurred when predators consumed parasitized aph ids. We conducted manipulative experiments during 1991 and 1992 to qua ntify the ability of pre-reproductive G. hirsutum to compensate for ap hid herbivory. In 1991 aphid populations in the high-damage treatment reached densities as high as any observed naturally during thp past 37 years. Damage symptoms were severe: leaf area was reduced by up to 58 % and total above-ground plant biomass was reduced by 45%. By the time of crop harvest, however, plants had compensated fully for the early damage in each of the three traits that define cotton's economic value : the timing of crop maturation, the yield of cotton fiber, and the qu ality of cotton fiber. Aphid feeding damage did, however, produce some changes in plant architecture that persisted throughout the growing s eason, including a decrease in the number of vegetative branches. In 1 992 aphid populations and associated damage were much lighter, but the qualitative responses to herbivory were consistent with those observe d in 1991. Plant compensation for early damage was complete for econom ically significant measures, and vegetative branch production was agai n suppressed in mature cotton plants. There was no evidence for a chan ge in the suitability of G. hirsutum as a host plant for A. gossypii a s a result of early damage ('induced resistance'), We conclude that pr e-reproductive G. hirsutum, which has not yet begun strong allocations to reproductive structures nr established architectural complexity, h ac retained effective means nf compensating for herbivory Tn contrast to other systems exhibiting strong compensation, G. hirsutum appears t o compensate in part by enhancing apical dominance. The recognition of early-season A. gossypii as non-pests is critical to the sustainabili ty of cotton production, because it will allow growers to forego pesti cide applications that accelerate the evolution of pesticide-resistanc e and disrupt natural communities of predators and parasitoids.