Can Plant Defence Mechanisms Provide New Approaches for the Sustainable Control of the Two-Spotted Spider Mite Tetranychus urticae?

Abstract

Tetranychus urticae (T. urticae) Koch is a cosmopolitan, polyphagous mite which causes economic losses in both agricultural and ornamental plants. Some traits of T. urticae hamper its management, including a short life cycle, arrhenotokous parthenogenesis, its haplodiploid sex determination system, and its extraordinary ability to adapt to different hosts and environmental conditions. Currently, the use of chemical and biological control are the major control methods used against this mite. In recent years, some studies have focused on plant defence mechanisms against herbivores. Various families of plant compounds (such as flavonoids, glucosinolates, or acyl sugars) have been shown to behave as acaricides. Plants can be induced upon appropriate stimuli to increase their resistance against spider mites. This knowledge, together with the understanding of mechanisms by which T. urticae detoxifies and adapts to pesticides, may complement the control of this pest. Herein, we describe plant volatile compounds (VOCs) with repellent activity, and new findings about defence priming against spider mites, which interfere with the T. urticae performance. The use of VOCs and defence priming can be integrated into current management practices and reduce the damage caused by T. urticae in the field by implementing new, more sustainable crop management tools.

Keywords: Tetranychus urticae; herbivore-induced plant volatiles; indirect defences; plant defence; spider mites.

Figure 1

Plant resistance to adapted and non-adapted lines of T. urticae. Two different citrus genotypes, C. reshni and C. aurantium, are susceptible and resistant, respectively, against T. urticae when this mite was originally reared on citrus (Clementine) leaves for more than 30 generations. However, both citrus genotypes are strongly resistant to the infestation of mite lines grown in F. arundinacea leaves for more than 30 generations. Three-month-old plants were infested with five adult females mites per plant, and the number of adults was scored 14 days after infestation.

Figure 2

Interplay between host-adapted and non-adapted mites and susceptible and resistant plants. Different lines of T. urticae can be found in the field. Some lines can modify the defence mechanisms of the plant by injecting effectors into the host. Mite effectors can interfere in the activation of defence pathways. When the effector is absent from the mite, the plant responds to the infestation by activating effective defences such as the JA pathway. This JA activation gives rise the production and accumulation of terpenoids, glucosinolates, flavonoids, acyl sugars, and other defensive compounds with detrimental effects on the mite. Additionally, green-leaf volatiles (GLVs) and terpenoids can also contribute to resistance and to volatile compounds (VOCs)-IR in distal plant tissues or neighbouring plants. From the mite side, adapted lines of T. urticae are extremely efficient detoxifying host chemicals, and may manipulate host responses. This would remove the need for the detoxification of phytoalexins from the host. In contrast, non-adapted strains are recognised by the host, which responds massively by activating defensive compounds and proteins. Finally, the host itself can also be conditioned by defence priming: upon appropriate stimuli, the host can prepare to react faster and stronger to the mite attack, even upon attack by adapted lines of T. urticae, and therefore be resistant. Green arrows indicate plant responses, red arrows indicate mite responses, and blue arrows indicate the behaviour of mite populations.