Temperature responses of a plant-insect system using a food-web performance approach

Evaluation of the performance of a plant-herbivore system as a whole is difficult due to the lack of fitness parameters that can be applied to both components. The individual use of traditional measures of performance (e.g., r(m), biomass) can provide useful, but incomplete information on the performance of insect herbivores and seldom incorporates plant performance. We propose the use of the net generational productivity (NGP) to evaluate the fitness of the herbivore, which can then be compared directly with the performance of the plant in biomass units, to obtain the food-web performance ratio (phi(H/P)). We compared three biotypes of the potato aphid, Macrosiphum euphorbiae Thomas (Hemiptera: Aphididae), when raised on three different host plants: potato (Solanum tuberosum L. cv. Norland) and two bell peppers (Capsicum annuum L. cv. Fascinato and cv. Crosby) (all Solanaceae) at temperatures ranging from 8 to 36 degrees C. The temperature profiles of the potato aphid biotypes suggest that this aphid is better suited to temperate climates, and its performance generally depends on the particular host-plant/biotype association. Plant growth performance showed that potato has a lower thermal tolerance, but has a faster growth rate than bell peppers, especially in the range of 16-24 degrees C. Temperature variation in the phi(H/P) ratio shows that aphids have a greater performance than plants, especially at lower temperatures, at which they can accumulate biomass up to 148 times faster. Because of the aphid's biological inability to withstand long exposures to temperatures above 28 degrees C, plants have a slight advantage over aphids. Nonetheless, as the performance of plants is extremely reduced at high temperatures, this advantage cannot withstand long-term exposures to extreme temperatures. This is the first attempt to obtain a parameter capable of determining the climatic profile and performance of a food web in an inclusive yet simple manner.