By Brad Haire
University of Georgia
At the very least, the diamondback moth is resilient. At most, it can be unstoppable, said Alton "Stormy" Sparks, a vegetable entomologist with the University of Georgia Cooperative Extension.
Given enough time and opportunity, this insect pest can overcome anything. And it feeds exclusively on cabbage, leafy greens and other related crops, which are worth about $53 million each year in Georgia.
"It doesn't take a lot of damage to make leafy greens unmarketable," Sparks said. "Consumers tolerate little damage to the cabbage, collards, mustard or other leafy greens they buy in grocery stores."
The diamondback moth, or DBM, has a bundle of biological abilities that allow it to overcome insecticides, the main tool used to fight them. In fact, only one other insect in the world, the green peach aphid, can do it as well or better, he said. This aphid is in Georgia but doesn't cause much trouble.
Farmers have thrown dozens of insecticides across many chemical classes at the DBM over the past half-century, Sparks said. It has developed resistance to every one.
A DBM doesn't mutate to resist insecticides, he said. It overcomes them through a process known as resistance selection. Like a person whose genetic makeup may help him fight disease, a DBM in a population may have the one inherited DNA quirk that stops an insecticide from hurting it.
Take a cabbage field in south-central Georgia, where most leafy greens are grown. The grower notices too many DBM caterpillars beginning to feed on the field. He sprays the field with the latest, most effective spray to stop them. The spray handles almost all of the DBMs, but a few have an inherited-by-chance DNA quirk to not be hurt by that insecticide that day.
Those caterpillars develop, do what they do and make more caterpillars, passing on the quirk. In the heat of summer, a DBM can go from birth to giving birth in just two weeks. So the new, resistant population quickly begins to grow, live and thrive in the field.
"I've seen entire cabbage fields harrowed up because the diamondback moths got completely out of control," he said.
Growers can go from spraying a few times in a few months to spraying twice a week or more in an effort to stop DBMs. This control can cost millions of dollars each year in Georgia. An average insecticidal spray costs a grower $20 per acre. Georgia grows roughly 21,000 acres of leafy greens per year.
To overcome resistance selection, growers rotate several insecticides to try to kill all DBM caterpillars in a field. There are currently only three they can use.
Sparks said a host-free period, or a time when no leafy greens are grown in Georgia during the summer, would help. With no food, DBM populations would decrease drastically statewide.
But this would be hard to start, with economics playing a major role. Georgia growers have a market advantage and can grow leafy greens almost year-round.
DBMs sometimes completely disappear in Georgia, with no obvious reason. Although scientists around the world have studied them for decades, there's more to be learned.
"A lot of my time is spent evaluating chemistries to fight diamondback moths and developing programs to educate growers on best management strategies for them," Sparks said.
He's testing the effectiveness of new insecticides now that chemical companies want to sell growers to fight DBMs in Georgia. But he knows the moth will likely develop resistance to them, too, eventually.
"With proper resistance management," he said, "we hope to delay the development of resistance as long as possible."