As global food demand continues to increase, effective pest control remains one of agriculture’s most pressing challenges.
Worldwide, farmers apply nearly 4 million tonnes of chemical pesticides annually to protect their crops, representing a $60bn industry.
While these compounds have significantly boosted agricultural productivity, their widespread use has raised concerns regarding environmental impact, health risks, and the long-term sustainability of modern farming.
To shed light on this, a research team led by Professor Gen-ichiro Arimura from the Department of Biological Science and Technology, Tokyo University of Science, closely examined the fine molecular interplay that occurs between spider mites and their host plants to test the theory of plant pest control.
Limitations in conventional pest control
The two-spotted spider mite, Tetranychus urticae, exemplifies the limitations of conventional pesticide-based pest control in agriculture and horticulture.
These microscopic arachnids infest a wide range of crops and fruit trees and can reproduce extremely quickly.
More importantly, unlike many other pests, they rapidly develop resistance to chemical pesticides, making control efforts increasingly challenging.
With pesticide resistance on the rise, farmers worldwide are urgently seeking alternative, sustainable pest control strategies.
The team focused on specific substances called elicitors secreted by T. urticae and examined their biological effects on various crops.
“An elicitor is a molecule that plants or pests possess that can enhance the defence response of plants,” explained Professor Arimura.
“In our previous research, we identified two tertrains, labelled Tet1 and Tet2, as elicitors in the salivary glands of two-spotted spider mites; these substances induce defence responses in the common bean and other commercially important crops.”
Why does pesticide resistance occur?
The research team investigated the effects of an additional 18 salivary gland proteins on the resistance of common bean leaves to T. urticae.
According to this initial screening, they identified two new tetranins—Tet3 and Tet4—that appear to reduce the reproduction of spider mites on the plants.
After a series of experiments involving genetic engineering and advanced molecular and biochemical methods, the team uncovered the roles of Tet3 and Tet4 in the complex interactions between T. urticae and its host plants.
Interestingly, they found that the expression of Tet3 and Tet4 varies greatly depending on which plant the mites fed on. Mites feeding on common beans, their preferred host, had significantly higher levels of Tet3 and Tet4 expression than those on cucumbers, a less preferred option.
Notably, plants exposed to mites with higher expression of Tet3 and Tet4 exhibited stronger defence responses, including increased calcium-ion influx, higher generation of reactive oxygen species, and elevated expression of a defensive gene named PR1. The individual application of Tet3 and Tet4 to plants had different effects on plant pest control defence responses, highlighting the specificity of each elicitor’s role.
“Taken together, our findings show that these tetranins respond to variable host cues that may optimise herbivore fitness by altering the anti-mite response of the host plant,” Arimura said.
Implications for sustainable farming and food security
The implications of these findings are twofold.
Firstly, understanding the molecular mechanisms that underlie interactions between organisms leads to a better understanding of evolution, ecosystems, and biodiversity. Elicitors such as tetranins act as crucial links in these complex systems, making their detailed study essential for uncovering broader biological insights.
From an agricultural perspective, tetranins and similar elicitors offer potential for crop improvement, as insights into the elicitor-sensing system can aid in breeding more sensitive and resilient crops.
Arimura concluded: “Elicitors may be useful as biostimulants that can increase the potential pest resistance of plants.
“The development of such organic farming techniques is extremely meaningful in today’s world, as the environmental and ecological impact of heavy pest control use grows more severe.
“Hopefully, identifying elicitors secreted by pests and elucidating their functions will lead to unprecedented spider mite countermeasures.”
Continued research on plant pest control could contribute to more sustainable agriculture and enhanced food safety.
