Methods of assessing population increase in aphids and the effect of growth stage of the host plant on population growth rates

Realistic values of population growth rates are needed when used in forecasting programmes, e.g., in a programme of integrated control. Therefore, comparisons were made in a chrysanthemum – aphid system between different methods of assessing population growth rates. The reproductive performances of the aphid species Aphis gossypii and Myzus persicae were measured on two chrysanthemum cultivars using three plant growth stages (young vegetative, budding and flowering). In the first set of experiments, development time and reproduction were used to estimate the population growth rate r_m. The mean relative growth rates (MRGR) were also assessed. It was shown for the first time that the relationship between r_m and MRGR was influenced by aphid species. In a second experiment, the aphid population increase on a whole plant was measured and r_m was estimated by calculating the slope of the (ln transformed) population increase. It is shown that population growth rate is affected by the growth stage of the plant, and that cultivar and aphid species interact with plant growth stage in influencing population growth rate. Thus, no single growth stage of chrysanthemum for maximal aphid population growth can be assigned, but the budding and flowering stage are the most suitable in three out of four aphid × cultivar combinations. Comparison between the results from both experiments demonstrates clearly that more realistic values for r_m are obtained when measured on whole plants.