Efficacy of Trichoderma spp. spore suspensions against cotton-infesting spider mites (Acari: Tetranychidae)

Spider mites, particularly Tetranychus urticae Koch and T. turkestani Ugarov and Nikolskii (Acari: Tetranychidae), are major agricultural pests, and their increasing resistance to chemical acaricides underscores the need for alternative control strategies. This study explored the biocontrol potential of Trichoderma spp. against these mites. Specifically, we evaluated (1) the efficacy of spore suspensions from five isolates of four Trichoderma species (Trichoderma afroharzianum, T. guizhouense, T. harzianum and T. virens) and their combinations on different biological stages of T. urticae, (2) the efficacy of T. afroharzianum spore suspension on different populations of T. urticae (green and red forms) and T. turkestani in both Petri dish and pot experiments. Results showed that Trichoderma spore suspensions were ineffective against T. urticae eggs, with mortality rates ≤ 2.8% at 7 days post-application (dpa), which did not differ significantly from the control. However, the fungi caused significant mortality of 50-65% in the mobile stages of the mites (larvae, nymphs, and adult females) compared with the control. Based on the initial screening results, a single Trichoderma isolate (T. afroharzianum) was selected for detailed assessment across multiple spider mite populations. Petri dish assays at 7 dpa showed that T. afroharzianum was more effective against T. turkestani (up to 60%) and the green form of T. urticae (53%) than against the red form of T. urticae (35%). Pot experiments confirmed these results, showing that T. afroharzianum reduced egg and mobile stage populations by 36-39% in T. turkestani and T. urticae (green form and laboratory culture populations), whereas reductions in the red form of T. urticae were not statistically significant. These findings suggest that T. afroharzianum has potential as a biological control agent; however, its efficacy varies among spider mite populations, highlighting the need to integrate it with other biological or chemical strategies.