Greenbugs are a major pest of sorghum, wheat and other small grain crops in the Midwest. Organophosphates are one of the major classes of insecticides used to control greenbugs. But insects often become impervious to insecticides, and that has come true for the greenbugs and organophosphates.
Kun Yan Zhu, assistant professor of entomology at K-State, and former K-State graduate student Jian-Rong Gao discovered that some greenbugs developed their resistance in an unusual way. They are the first scientists to discover how this resistance works at the molecular level, Zhu said.
Zhu and Gao, who now works at the University of Massachusetts, were invited to present their work at the national meeting of the American Chemical Society April 7-11, in Orlando, Fla. Gao took first place in the 2002 Young Scientist Pre-and Postdoctoral Research award sponsored by the Agrochemicals Division and Dow Agrosciences for his work on greenbug resistance.
"There are millions of individuals of an insect pest species in a field. Some individuals may have certain genes that can confer resistance. Insects without such mechanisms will be killed by an insecticide. Those with the mechanisms survive and reproduce," Zhu said.
Greenbugs with the special immunity can be 13 times less sensitive for the enzyme known as acetylcholinesterase, or AChE, to certain organophosphates than normal greenbugs.
AChE is an enzyme that humans and insects alike need for their nervous systems to work correctly. Organophosphates kill insects by blocking the enzyme until their nervous systems do not function properly.
Genes are the DNA fragments directing the production of proteins or enzymes. Insects usually become protected against insecticides by increasing their metabolic rate or because the gene that makes the protein or enzyme mutates, so insecticides no longer work on them. Zhu and Gao compared the genes that produce the AChE enzyme in resistant greenbugs and normal greenbugs and found something surprising.
They were the same.
The gene hadn't mutated and there weren't extra copies of the gene in the resistant greenbugs. What they found instead is that the resistant greenbugs simply pump out the enzyme at a higher rate. Insecticides still block the AChE enzyme, but the increased amount of the enzyme requires more of the insecticide in order to kill the insect. Zhu calls it "overexpression" of the gene that manufactures the enzyme.
Their finding may lead to better methods of pest control.
"If we know the mechanism of resistance, we are in a better position to develop strategies to deal with resistance in insects," Zhu said.
Sometimes, one crop field will have resistant greenbugs and another won't. The techniques of Gao and Zhu can be used to find out if an individual field has greenbugs that are resistant to organophosphates.
"It makes a lot of sense to monitor resistance in the field," Zhu said.