The people who eat Vidalia onions may not know about thrips, but these tiny insects can cause severe problems for the farmers who grow them. “Thrips in onions have traditionally been looked at as a lesser problem over the years,” says David Riley, an associate professor of entomology with the University of Georgia.
“But recent data suggests that thrips can be involved with increasing disease problems, which are the major problems in Vidalia onions,” Riley says. “Thrips can cause reduced bulb size, and they can also cause reduced yields.”
These aren't the only problems the insects can cause. The main hazard thrips pose is as carriers of harmful pathogens such as bacteria and fungi. Bacteria that thrips spread can cause center rot, and the fungi they carry can cause purple blotch.
The thrips ingest bacteria, and defecate on the plants they inhabit, potentially infecting the leaf tissue.
So why not use insecticides to rid farmers of the pests?
“It's not that simple,” Riley says. “Thrips are masters of resistance to insecticide.” Spraying too much or too often can result in thrips becoming immune to the chemicals used.
Thrips can cause problems directly to plants, too, by feeding on and scarring leaf tissue. The leaf scars can become entrances for disease.
“Even at low numbers of thrips, they may be increasing the incidence of disease,” Riley said.
Farmers haven't always needed to be on the alert. “The interesting thing about thrips and onions is that not every year is a thrips year,” Riley says.
Cool, wet winters result in small thrips populations, and not much insecticide is needed to stave off the insects. Warmer, dryer winters, though, have translated into more thrips in onions, because the insects leave the fall host plants earlier.
“If the temps are higher, they'll reproduce, and you'll start getting development of thrips even as early as the end of December or January,” Riley says.
This can pose a threat to Vidalia onions going into the spring. But the solution for farmers, Riley says, isn't to bombard thrips with insecticides.
“If you take the attitude that ‘the only good thrips is a dead thrips,’ then what you're going to wind up doing is spraying when you don't need to, and then you're going to start causing these problems of insecticide resistance,” he said. “I would say it's almost better not to treat than to over-treat.”
Riley tells farmers to use an economic threshold when treating for thrips. “(Treat at) one thrips per plant initially,” he says. “And then you wait until thrips reach a level of five thrips per plant.”
This not only reduces the chance of pesticide resistance but also helps the environment and can reduce the amount of money spent on pesticides.