By Frank Giles and Clint Thompson
Whiteflies have grown into one of the top concerns for specialty crop growers in the Southeast. The memories of the 2017 outbreak are still fresh in the minds of many who lost entire crops to the swarming clouds of the pest.
According to “The 2017 Georgia Plant Disease Loss Estimates” report compiled by Elizabeth Little, an Extension plant pathologist for the University of Georgia (UGA), plant disease losses, including control costs, were $936 million. While this figure encompasses all crops, the report does show the impact of whiteflies on vegetables. For instance, whitefly-transmitted viruses (WTVs) caused between 30% and 50% crop loss in squash and cucumber. There was nearly an 80% crop loss in snap beans that year. WTV diseases such as cucurbit leaf crumple virus and cucurbit yellow stunting disorder virus were particularly severe. There are about five WTVs that affect vegetables in the Southeast.
In Florida, the 2017 spring crop was affected as well. Melon crops were impacted with three WTV diseases, and tomatoes suffered from tomato yellow leaf curl virus in some areas. While 2017 was especially bad, University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) entomologist Hugh Smith was quoted at the time as saying there had not been seasons with light whitefly pressure in recent years.
Weather Factor
The weather is a contributing factor to the severity of the problem. In 2016 and 2017, it was hot and dry, which are conducive conditions for whiteflies to get out of control.
Bill Snyder, UGA entomologist, has been studying the impact of climate on the pest. He and colleagues have used statistical models to look for what causes whitefly outbreaks at particular times and places.
“Those first bad outbreak years in 2016 and 2017 were some of the hottest, driest years ever recorded in Georgia. This was our first clue that extreme weather might be triggering whitefly outbreaks,” Snyder said. “We have found that, by far, drought is the most important factor. Summer droughts correlate very tightly with high numbers of whiteflies, which then can spread into fall vegetables. We think this is because drought-stressed plants aren’t as good at defending themselves against pests, and because hot and dry conditions are also very hard on the predatory insects that attack whiteflies. Our next step will be to develop predictive models that will allow us to project into the future, so we can tell growers when they might expect whitefly problems on their farm so they can plan accordingly.”
Georgia and North Florida experienced some very cold temperatures this winter, even snow. According to Chris Zelman, a meteorologist with WALB in Albany, Georgia, the amount of snowfall recorded throughout Southwest Georgia on Jan. 21–22 ranged from 2 inches to 9 inches. According to the Pensacola News Journal, there were 8.9 inches in Pensacola, Florida, and 10 inches in nearby Milton, Florida. Will the cold spell help keep whitefly numbers lower this year?
“Our modeling work suggests that drought is the most important driver of whitefly outbreaks, and that under those conditions, whiteflies can increase very rapidly even if they aren’t so common at the beginning of the growing season,” said Snyder. “So, my guess would be that the recent cold won’t make a huge difference, but that’s really nothing more than my best guess.”
In early February, there were pockets of drought in several spots in the Southeast, and conditions had warmed up significantly, which are ripe for the pest. Right on cue, whitefly numbers started to increase. Scouts reported increased population levels in Homestead, Florida. There also was tomato yellow leaf curl virus in tomatoes and increasing mosaic and silver leaf symptoms in cucurbits.
Andre da Silva, Extension specialist and associate professor of horticulture at Auburn University, said he’d expect numbers to increase and that the pest has been showing up earlier in Alabama.
“Every year from 2022 to 2024, we have started seeing more whiteflies going north. Early in the year, we’re already seeing whiteflies getting to Clanton, Alabama, which is north of Montgomery. In 2023, I found whiteflies all the way to Montgomery,” da Silva said. “Now we’re seeing them a little bit north of Montgomery. That’s probably because of the weather patterns.
“The damage (from whiteflies) is as big as in Georgia, but because we don’t have as large of a production area as Georgia, the impact is lower. Our biggest problem is whiteflies chew on tomatoes, pumpkins and cucurbits in general. We have a large greenhouse production in the south as well with transplants. We are seeing a lot of whiteflies in greenhouses.”
Management Recommendations
While whiteflies can be very difficult to control when the numbers overwhelm, there are management recommendations to consider. Ted McAvoy, UGA vegetable Extension specialist, said an integrated approach is best. This includes planting cultivars tolerant or resistant to WTV (see sidebar).
Reflective plastic mulches have been shown to help ward off the pest. According to McAvoy, weed control is another important factor.
“We know weeds, such as prickly sida, are a host for sida golden mosaic virus, based on research by my UGA colleague Bhabesh Dutta,” McAvoy said. “Other important weeds to control are morning glory, wild mustard and lantana based on work by UGA plant pathologist Sudeep Bag. UGA’s Ty Torrance and Taylor Singleton have found less whiteflies on cucurbit crops when row-middle grass cover crops were implemented.”
Common chemical controls include Sivanto Prime, PQZ, Exirel and Knack. UF/IFAS recommendations include using soil applications of neonicotinoids at planting for longer-season crops, such as watermelon, so that there is less chance of affecting bees pollinating the crop. During the rest of the season, avoid spraying neonicotinoids; those can be replaced by diamides or Sivanto Prime. On transplants in the production facility, apply a neonicotinoid one time not more than seven days before planting.
See edis.ifas.ufl.edu/publication/IN871 for more on whitefly scouting and control.
Click here to hear more about whiteflies from Bill Snyder.