RGR as a tool to evaluate the relative performance of a plant-herbivore system as affected by temperature

Comparing the relative performance of organisms that form a plant-herbivore system requires a ‘common currency’. We used the ratio of each organism’s RGR (relative growth rate in terms of biomass) as the ‘common currency’ for different aphid - host plant couples, to determine which of the two trophic levels would perform better with rising temperatures, using an experimental approach.

We evaluated the performance of nine bi-trophic systems formed by combining one variety of potato (Solanum tuberosum var. ‘Norland’), and two varieties of bell pepper (Capsicum annum var. ‘Crosby’© and ‘Fascinato’©), with three host plant-related biotypes of the potato aphid Macrosiphum euphorbiae at six constant temperatures (8, 12, 16, 20, 24, 28°C).

Results show that the RGR of the potato almost doubles that of the bell pepper, irrespective of the experimental temperature. The aphids do have different RGR values according to their biotype and host plant. Examination of aphid/plant RGR ratios at different temperatures show that there is no effect of temperature on relative performance for potato, with almost balanced 1:1 ratio of relative growth across the six temperatures. In contrast on bell peppers, the relative performance of the aphids suffered a mean proportional decrease of -.03197 for each increase of 1°C between 8 and 28°C, regardless of the pepper variety or aphid biotype. Aphids performed 7 times better than the plant at 8°C compared to approximately equal performance at 28˚C. We conclude that using RGR as a common currency of performance indicates that rising temperatures should decrease aphid performance relative to its host plant for pepper systems, while it should have no such effect for potato based systems.