Plant-mediated Competitive Displacement By Tetranychus Evansi And Host Adaptability Of Two Pests

Host plants provide food and space for herbivores and play an unparalleled role in competitive interactions among herbivores. After herbivory, resources reallocate to growth and defences and the composition of nutrient and secondary compounds are changed as well. Such induced changes not only affect the performance of herbivores but also the interactions between herbivores, as well as the interactions between herbivore and nature enemies, and even the composition of the community.The tomato red spider mite, Tetranychus evansi, an invasive pest of Solanaceae crops worldwide, has invaded Europe and displaced local spider mites, becoming the main pest. According to former studies, the southeast of China will be a potential invasion area of T. evansi. However, how the invasive pest displaced the local spider mites through competition has not been solved yet. Previous studies have shown that T. evansi can suppress tomato plant defences and benefit other herbivores locally, while the two-spotted spider mite, Tetranychus urticae, induces tomato defences and negatively affects other herbivores. How T. evansi interact with other herbivores through the induced changes of plants, the effect of mechanical damage(such as apex removal) on this invasive pest, as well as the preferences of T. evansi to different parts of host plants and mechanisms therein still are not clear. All these questions are important to unravel the mechanism of competitive displacement of T. urticae by T. evansi. Based on this, this study using plant defence inducing T. urticae and plant defence suppressing T. evansi firstly studied the susceptibility of the two species to mechanical damage, apex removal. Then, plant-mediated interactions between spatially separated T. evansi and T. urticae, as well as the induced Polyphenol oxidase activity, were studied. Afterwards, the competitive interaction between the two species feeding together and their colonization behavior were investigated. At the end, the host adaptability of spider mites and aphids were discussed, as well as the effect of temperature on host adaptation. Summarizing all the results we got following conclusions:a) Apex removal affected the two species differentially: the survival, fecundity and population growth of T. urticae were negatively affected; the individual performance of T. evansi was as well as on intact plants, while the accelerated growth of tomato leaves by apex removal promoted the population growth of T. evansi.b) The PPO induction by T. evansi was much less and later than by T. urticae, without influences of the simultaneous infestation by T. urticae. While, the infestation by T. evansi deceased the PPO activity of the leaf where T. urticae was feeding.c) The environmental carrying capacity of a tomato leaf for T. evansi was almost as three times as that for T. urticae and the population growth of T. evansi was also much higher. The preference of T. evansi was affected by the released number.d) The interaction between T. evansi and T. urticae was asymmetric: T. evansi could benefit the performance of co-infesting T. urticae through suppression of plant defences, but T. urticae had no effects on T. evansi. The colonization preference of both species was affected by the competitor: T. evansi chose the leaf where the population had a higher growth rate, while T. urticae chose the one where the population growth was slower.e) The competition results depended on the leaf where T. evansi and T. urticae feeding. When both species fed on old leaves or T. evansi fed on the old leaf and T. urticae fed on the young leaf, the population growth of T. evansi was much higher than T. urticae, thus T. evansi would outcompete T. urticae.f) An aggregation model with variable amplitude and cycle was constructed and could reasonably describe the aggregation-diffusion behavior of spider mites, aphids and other pests, indicating a universal significance.g) Acclimation to host plants could promote the fitness of herbivores, and the host adaptability of aphids was significantly affected by temperature.In summary, T. urticae was more susceptible to induced plant resistance and T. evansi was relatively more resistant. The environmental carrying capacity of tomato for T. evansi was much higher than for T. urticae. Tetranychus evansi also had a much higher population growth rate than T. urticae and could interfere the colonization choice of T. urticae. Meanwhile, Tetranychus evansi was a superior competitor on old leaves. To some extent, these results unraveled the mechanism of invasive pest T. evansi competitively displacing local pest T. urticae, especially under the plant mediated effects, providing experimental supports for the plant-mediated interactions between herbivores. The impacts of temperature on the adaptability of aphids provided theoretical basis for aphids’ seasonal migration and confirmed the effects of environmental changes on the selection pressure between host plants and herbivores.