Witten By: Emily Cabrera, UGA IPM Communications Coordinator
Expert Sources: Alton Sparks, Extension Vegetable Entomologist, Bhabesh Dutta, Extension Vegetable Disease Specialist
Vegetable growers are cautioned to be on the lookout now as silverleaf whitefly populations are already exploding in the Tifton-Colquitt area. “This is typically where we see this pest most years because so much of the state’s vegetable and cotton production happens around here,” explained Alton “Stormy” Sparks, University of Georgia Cooperative Extension vegetable entomology specialist. “That said, we are receiving word that SLWF are being seen elsewhere, so go ahead and begin scouting if you have vegetables planted.”
Sparks has been checking fields in Tifton for the past few weeks and reports that counts on squash where he is not currently spraying are 150 adults per leaf. “That is an overwhelming amount, so if you are in this area of Georgia, you need to go ahead and plan on spraying insecticides because the threshold for SLWF has already been far exceeded,” cautioned Sparks.
Extension vegetable disease specialist Bhabesh Dutta explained how severe this insect pest can be on the vegetable industry. “Based on the 2017 assessments; we had 40% yield loss in snap beans (70% of the fall grown snap beans were lost due to whitefly transmitted virus complex), yellow squash (35% loss; 60% fall grown ), zucchini squash (15% loss; 25% fall grown), tomato (20% loss; 60% of fall grown tomato),” Dutta reported.
Scouting for SLWF in vegetable production entails turning young leaves over to visually examine the bottom of the leaf where adults congregate. Evaluating whether insecticides are performing involves closer examination of older foliage under magnification as most of the insecticides for whitefly primarily control immature stages. If a grower only looks at the adult population, they would conclude that nothing is working very well simply because many of the insecticides do not kill adults.
“A lot of growers are trying to figure out what to do this fall, and are preparing themselves with the idea that it’s going to be a bad season,” said Sparks.
There are a few things that can be done to help reduce the impact of the outbreak this year. Cotton growers should apply insecticides for whiteflies as soon as they are needed as it is very difficult and expensive to try and catch up to a whitefly outbreak after it is well established. As with all crops, cotton growers should harvest on time and plan on immediately removing debris from fields following defoliation and harvest. This will help reduce the population size as we enter vegetable production season.
Another measure for reducing the impact of this pest is for vegetable producers who have not yet planted crops to be sure to look for virus resistant varieties. For tomato growers, SLWF transmits a virus called tomato yellow leaf curl virus (TYLCV). Although it can infect a relatively wide range of plant species, it is best adapted to tomato. Susceptable tomato plants infected early in their development will not provide any marketable fruit. There are many TYLCV resistant varieties available that produce well in the southeast.
The two major viruses that whiteflies transmit to cucurbits are cucurbit leaf crumple virus (CuLCrv) and sida mosaic virus. Both viruses also effect snap beans. “The problem in cucurbits is that we don’t have anything that is truly resistant,” explained Sparks. “And in snap beans, we have some varieties that are more tolerant to the various viruses out there, but again, none that are truly resistant.”
Ultimately, for fall vegetable production, Sparks recommends using preventative sprays to buffer the impact from the increased SLWF population we’re observing this year. “Most insecticides against whitefly are primarily active on nymphs or eggs, so evaluation of efficacy requires scouting these stages,” Sparks said. “We only have two or three products out there with significant efficacy on adults. The best products for use on adults are Sivanto Prime, Venom, Assail and PQZ. PQZ is a newer product that some growers may not be familiar with but has had excellent activity on adults.”
Sparks mentions that we may receive a little help knocking back SLWF populations if we end up receiving a tropical storm system with heavy rains for a few days. “The spotty showers we have been getting so far don’t really knock back SLWF the way a large system does,” said Sparks. “Intermittent showers will only give us a few days of local relief, but those populations will build back again within a few days.”
Though the numbers aren’t as bad as they were in 2017, 2020 is shaping up to be far worse than the last two years. Sparks urges vegetable growers to stay vigilant. “This is definitely an agro-ecosystems pest, what cotton growers are doing now will impact vegetables, and what vegetable growers do will impact next year’s production. We learned a lot from 2017, that was a devastating year, so we are all familiar with how we have to work together to help minimize the damage this pest will have on Georgia agriculture this year.”
To learn more about cross-commodity management of silverleaf whitefly, check out Extension Circular 1141.