Summary
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.
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.
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.
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.
Larvae of a Polyhymno species (Lepidoptera: Gelechiidae) feed on the ant-defended acacia, Acacia cornigera, in the tropical lowlands of Veracruz, Mexico. Polyhymno larvae construct sealed shelters by silking together the pinna or pinnules of acacia leaves. Although larval density and larval survival are higher on acacias not occupied by ants, shelters serve as a partial refuge from the ant Pseudomyrmex ferruginea (Hymenoptera: Formicidae), which defends A. cornigera plants; thus, shelters provide Polyhymno larvae access to an ant-defended host plant. P. ferruginea ants act as the primary antiherbivore defense of A. cornigera plants, which lack the chemical and mechanical defenses of non-ant-defended acacias. Thus, defeating the ant defense of A. cornigera provides Polyhymno larvae access to an otherwise poorly defended host plant. Damage caused by Polyhymno larval feeding reaches levels which can kill A. cornigera plants.
The relative importance of resource‐mediated competition versus apparent competition was studied in two native aphids, Monellia caryella, and Melanocallis caryaefoliae, on pecan, Carya illinoensis. We compared reproduction and body size of the two aphid species when they were caged on pecan leaves as single or mixed species, and when green lacewing larvae (Chrysoperla comanche or Chyrsopa nigricornis) were present or absent. The experiment was repeated two times on mature compound leaves in the field in the autumn of 1997 and summer of 1998 and once in a greenhouse, on young seedlings with simple leaves.
Our results suggest that interspecific competition was variable and asymmetric; in both the Summer 98 experiment and the greenhouse experiment, reproduction of M. caryaefoliae was significantly reduced in the mixed species treatments relative to the single species treatment, while M. caryella reproduction was significantly reduced only in the Summer 98 experiment. No evidence of an effect of competition on reproduction was found in the Autumn 97 experiment. M. caryella body size was reduced in one of the mixed species treatments in the Autumn 97 experiment. Body size of both aphid species was reduced in the mixed species treatments of the greenhouse experiment.
The introduction of green lacewing larvae reduced the reproduction of aphids in two of the three experiments in comparison to controls. However, a significant interaction between aphid and predator treatment (M. caryella in the greenhouse experiment) was found in only one experiment. Variation in the outcome of competition was more likely to be due to aspects of plant quality, including leaf age and previous aphid feeding.
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