Outcome of indirect competition between two aphid species mediated by responses in their common host plant

1. This study evaluates the potential for indirect competition between two phloem-feeding aphids as mediated by a shared host, pecan Carya illinoensis. In a greenhouse experiment, one of two aphid species, Monellia caryella and Melanocallis caryaefoliae, were introduced to pecan seedlings, removed for a period, and then introduced for a second time. Aphid performance and food quality, i.e. phloem amino acid concentration and composition, were measured in leaves after the first and second exposure to aphids. After the second exposure, leaves had been fed upon previously by either conspecifics or heterospecifics, and both direct and delayed effects were evaluated on adjacent leaves. 2. The performance of M. caryaefoliae was reduced by previous aphid feeding of both conspecifics and heterospecifics. The performance of M. caryella was unaffected by prior aphid feeding. 3. Feeding by M. caryaefoliae induced changes in amino acid content of the phloem. This alteration occurred within infested leaves and did not cause any changes in the phloem of adjacent leaves. 4. Feeding by M. caryella did not induce changes in phloem amino acid content, but seemed to inhibit M. caryaefoliae's ability to alter the phloem. The inhibition caused by M. caryella was local and might be the cause of the indirect competition observed.