By Brooke Hatfield
University of
Georgia

The people who eat Vidalia onions don’t need to worry 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,” said 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 said. “(Thrips) can cause reduced bulb
size, and they can also cause reduced yields.”

Pests carry pathogens

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 fungi they
carry can cause purple blotch.

The thrips ingest bacteria and defecate on the plants they
inhabit, potentially infecting the leaf tissue.

‘Not every year a thrips year’

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 said.

Cool, wet winters result in small thrips populations, and not
much insecticide is needed to stave off the insects. Warmer,
drier winters, though, have translated into more thrips in
onions, because the insects leave their 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 said.

Don’t overtreat

This can pose a threat to Vidalia onions going into the spring.
But the solution for farmers, Riley said, 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 overtreat.”

Riley tells farmers to use an economic threshold when treating
for thrips. “(Treat at) one thrips per plant initially,” he
said.
“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.