Inter-strain competition and dispersal in aphids: evidence from a greenhouse study

1. Crops are often colonised by aphids having different life-history traits , and the aphids and their offspring compete for the shared resource. The i ntraspecific competition and dispersal characteristics of two strains (A an d B) of the cotton aphid Aphis gossypii on cucumber were examined in a gree nhouse study. Strain A normally feeds on cucumber, whereas strain B origina ted from melon but develops on cucumber. Both strains reproduce exclusively via parthenogenesis and can be discriminated using molecular markers. 2. The strains were released at the same time on different plants in a gree nhouse. Total density and their dispersion within and between plants were r ecorded through time, and the frequency of each strain was assessed using D NA fingerprinting. The Lotka-Volterra competition model was used to test fo r inter-strain competition and to estimate its intensity. 3. Strain A had the highest rate of increase and carrying capacity, and bec ame the most frequent strain, nearly displacing strain B. Strain B was the most common only on the plants it infested first. The competition was unbal anced as strain B was affected strongly by strain A, but strain A was only affected minimally by strain B. This asymmetrical relationship for strain B was not due only to competition for the shared resource. 4. Such competition may in part explain the reduction of genetic polymorphi sm observed by others in the field where colonising strains coming from dif ferent hosts compete and some are eliminated. Polymorphism in A. gossypii p opulations in nature is thought to be maintained by heterogeneous patches o f hosts differentially favourable for different clones.