Temperature responses of a plant-herbivore-parasitoid system using the ‘Food-Web Performance’ approach

The evaluation of the performance of a tri-trophic food web as a whole is difficult due to the lack of parameters that can be applied to both plants and insects in an efficient manner. Traditional fitness parameters such as the intrinsic rate of increase (r_m), or body size as biomass, etc provide useful but limited information about insect performance, and cannot be utilised to compare insect and plant performance. 

For direct comparison with plant growth, we used the potential for insect biomass production in one generation time based on ‘Net Generational Productivity’ (NGP), which estimates the potential total amount of adult insect (herbivore, and parasitoid) live biomass that can be produced by one adult individual. We developed the ‘Food-Web Performance Ratio’ (φH/P) to uniformly scale the cumulative live biomass of the plant, herbivore and parasitoid that can be produced in a reference time unit, e.g. one generation-time of the herbivore. This approach allows us to directly relate the growth potential (in terms of biomass) of each trophic level under given conditions, such as different temperatures.

Nine tri-trophic food webs tested at temperatures ranging from 8 to 36°C were used to illustrate the approach. They were composed of: potato (Solanum tuberosum L. cv. “Norland”) and two bell peppers (Capsicum annuum L. cv. “Fascinato” and “Crosby”); three biotypes of the potato aphid (Macrosiphum euphorbiae Thomas); and the parasitoid wasp Aphidius ervi Haliday. The effect of temperature on the relative fitness of each of the components and performance of the food-web as a whole will be discussed in relation to potential climate change effects.