Effects of plant availability and habitat size on the coexistence of two competing parasitoids in a tri-trophic food web of canola, diamondback moth and parasitic wasps

Using experiments and spatial modeling, we here show the conditions for two competing parasitoids to coexist under two levels of plant availability in a tri-trophic food web of canola, diamondback moth (Plutella xylostella) and two parasitic wasps (Diadegma semiclausum and Cotesia vestalis) that target the same life stage of the herbivore diamondback moth. Plant availability had a significant effect on both the abundances of parasitoids and the herbivore. However, parasitoid abundances were not different with or without the occurrence of other parasitoid species, suggesting no effect of parasitoid coexistence on their abundances. Using a three-species Lotka–Volterra model, we confirmed the competitive exclusion of species with low consumption rate (=initial population size × per capita consumption rate) in a wellmixed habitat patch. We further built a cellular automaton (CA) to explore the minimum habitat size that ensures the coexistence of these two parasitoids, assuming different searching efficiencies (i.e. different radiuses of searching area and movement abilities for locating the host). With the in-patch dynamics portrayed by the three-species Lotka–Volterra model, the CA model demonstrated the possibility for the two parasitoids to coexist when the number of habitat patches reached a certain number. This research thus highlights the importance of both plant availability and habitat size to the dynamics and stability of the tri-trophic food web.We are grateful to the DST-NRF Centre of Excellence for Invasion Biology (CIB) at Stellenbosch University for logistic help. We wish to thank Myron P. Zalucki for his useful comments on early draft of the manuscript. This work was funded by Tarbiat Modares University and partly by Intergrated Pest and Disease Management of Oil Crops of Iran