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Interspecific Competition between Root‐Feeding and Leaf‐Galling Aphids Mediated by Host‐Plant Resistance

Moran, Nancy A., Whitham, Thomas G.
Ecology 1990 v.71 no.3 pp. 1050-1058
Chenopodium, Pemphigus betae, gardens, growth chambers, herbivores, host plants, interspecific competition, leaf galls, phloem, roots, sap, seed set
While the competitive interactions of herbivores are often debated, rarely have the resistance traits of their host plants been considered as an important factor that might determine the outcome of an interaction. In the study reported here, host—mediated interactions were explored for two aphid species that feed on plants of the genus Chenopodium. Pemphigus betae feeds underground on roots, while Hayhurstia atriplicis feeds aboveground where it forms leaf galls. Although they never encountered one another directly, these two aphid species shared a common resource, the phloem sap of their host. We examined the effects of each of these herbivore species on their host plants and on one another through several garden and growth chamber experiments. Four major results emerged. First, the root—feeding aphid P. betae had no significant effects on its hosts. In contrast, leaf galling by H. atriplicis reduced overall host mass by an average of 54% and seed set by an average of 60%. Second, competitive interactions between the herbivore species depended on the level of host resistance to leaf galling by H. atriplicis. On susceptible plants, leaf—galling colonies of H. atriplicis reduced P. betae numbers by an average of 91%, often eliminating the root feeders entirely. In contrast, on plants resistant to galling, H. atriplicis colonies were smaller and did not affect P. betae infesting roots of the same hosts. Third, the interaction was asymmetrical; although H. atriplicis had a strong negative effect on P. betae, the latter showed no measurable effects on H. atriplicis. Censuses indicated that the strong negative effects of H. atriplicis on P. betae, as found in the growth chamber and garden experiments, also take place naturally in the field. Fourth, although one might expect plants resistant to one aphid species to be resistant to other aphids also, resistance to leaf—galling aphids is not correlated with resistance to root—feeding aphids in this system.