According to a recent whitefly resistance update provided by David Schuster, entomologist at the Gulf Coast Research and Education Center in Balm, the silverleaf whitefly (also known as biotype B of the sweetpotato whitefly) and the disease it spreads — tomato yellow leaf curl virus (TYLCV) — remain key pests plaguing Florida tomato growers.
The neonicotinoid insecticides remain an integral part of controlling the whitefly and reducing potential spread of TYLCV. Because of the risk of whiteflies developing resistance to the insecticides, researchers have been monitoring susceptibility of field populations to applications of neonicotinoids. The monitoring has shown that resistance is possible and requires growers take proactive action to avoid problems in the field.
In addition, biotype Q of the sweetpotato whitefly has been found in greenhouses and nurseries in 22 states, including Florida. Although this biotype has not been detected in the field, it represents a new threat to vegetables and other crops in the state because it is resistant to many insecticides commonly used against whiteflies.
In 2003, a Resistance Management Working Group was formed to promote resistance management activities. The working group consists of University of Florida research and Extension personnel, companies marketing neonicotinoid insecticides, commodity groups, and commercial scouts.
Because of cases of resistance and the threat posed by biotype Q, the Working Group met again in 2006 to revise whitefly resistance management recommendations. These recommendations include field hygiene and other cultural practices, which should be considered a high priority in overall whitefly management strategies.
Keep It Clean
Field hygiene should be a high priority in managing whiteflies, incidences of TYLCV, and insecticide resistance. Growers first should establish a minimum two-month crop-free period during the summer, preferably from mid-June to mid-August. Next, growers should disrupt the virus/whitefly cycle in winter by creating a break in time or space between fall and spring crops, especially in tomatoes.
Destroy the crop within five days of final harvest to decrease whitefly numbers and sources for TYLCV. Final crop destruction can be achieved with a burndown herbicide tank-mixed with a heavy application of oil (not less than 3% emulsion) and non-ionic adjuvant to destroy crop plants and kill whiteflies quickly.
Rogue tomato plants with symptoms of TYLCV and certain weeds can act as hosts for whiteflies, making weed control all the more important. Cull tomatoes should be disposed of as far away from the production fields as possible. If deposited in pastures, fruit should be spread (instead of dumped) in a pile to encourage cattle to eat the culls. Fields should be monitored for germination of tomato seedlings, which should be controlled by mowing or with herbicides.
How growers handle transplants also influences the likelihood of whitefly woes. The obvious first step is planting transplants that are whitefly and virus-free. It is also important to grow and isolate vegetable transplants away from ornamental plants, which host the pests.
Other tips growers can use range from planting vegetable varieties that show resistance to TYLCV to even avoiding yellow clothing, tools and, equipment, as they attract adult whiteflies. When it is time to plant a new crop, avoid planting near an old, infested crop. In fields with a history of whitefly infestations and TYLCV, plant on ultraviolet light reflective (aluminum) mulch to deter the pests.
It is an often preached sermon that is critical — Always Follow The Label. Chemical label instructions will help growers avoid insecticide resistance when followed properly. Selective use of neonicotinoids is important to preserve their effectiveness throughout the season. In the nursery, do not use neonicotinoids if biotype Q is present. If biotype B is present, make only one application seven to 10 days before shipping. In the field, use neonicotinoids only during the first six weeks to leave a neonic-free period at the end of the crop.
As the saying goes: “Do unto thy neighbor as…” These sage words apply to whitefly management, because if full-blown resistance to insecticides (especially neonicotinoids) occurs, all growers will feel the pain. Everybody should do their part to manage against resistance, but growers should keep abreast of operations in upwind fields, especially during harvest and crop destruction, just to be sure “thy neighbor” is managing whiteflies properly.