Chemical insecticides are used extensively to kill pests and thereby limit the harm they cause. However, overreliance on insecticides can promote rapid evolution of insecticide resistance in insect populations.
“One silver lining is that, in the absence of insecticides, resistant insects often have a disadvantage relative to insects that are not resistant – at least initially,” said Bruce Tabashnik, head of the Department of Entomology at the University of Arizona. “This penalty associated with insecticide resistance is called a fitness cost, which can include reduced growth rate, survival, and fertility.”
In a new study published in PLoS Biology, Xianchun Li, an insect molecular biologist in the College of Agriculture and Life Sciences, and his colleagues Wenqing Zhang and Rui Pang discovered how one insect beats the cost of resistance.
The paper focuses on the brown planthopper, a tiny hemipteran insect that is the world’s most destructive pest of rice.
“The hopper damages rice directly by sucking plant juice and by transmitting two specific rice viruses at the base of the tillers, making plants turn yellow and dry up, a condition known as ‘hopper burn,’” Li said. “If left unmanaged, it can cause up to 60% yield loss.”
Li and his co-authors studied the planthopper’s resistance to imidacloprid, an insecticide that targets the nervous system. They found that the increased detoxification of imidacloprid that confers the planthopper’s resistance is linked with elevated production of reactive oxygen species (ROS). ROS are oxygen-containing reactive chemicals that are naturally produced by most organisms during respiration and can be overproduced in response to stress, such as infection or exposure to toxic compounds.
“Overproduced ROS must be removed from the body. Otherwise, they will cause irreversible damage,” Li said. “Thus, we hypothesized that the increased production of ROS associated with resistance to imidacloprid causes the observed fitness cost.”
In the second stage of resistance evolution in the planthopper, selection favored a mutation in a peroxiredoxin gene, which enhances removal of ROS, and thus overcomes the fitness cost associated with overproduced ROS, explained Li.
“We’ve identified the peroxiredoxin as a fitness modifier gene where the mutation increases its expression and thus ROS-scavenging capacity in imidacloprid-resistant planthoppers.”
“Because of the widespread occurrence of peroxiredoxins, these new results could have broad implications for understanding how resistant insects overcome fitness costs,” said Tabashnik. “This discovery might even provide insights useful for boosting removal of ROS in people and thereby improving our health.”
In continued efforts to understand evolved pest resistance to insecticides, the brown planthopper case study underscores the importance of altering pest control tactics when resistance is observed.
“It is best not to continue to use an insecticide once the pest has evolved resistance to it,” Li said. “This study may prompt an important shift from focusing on the first stage of resistance to both stages of resistance evolution and further research into other fitness modifier genes.”
*Photos sourced from the image collection of the International Rice Research Institute (IRRI).