It’s a crop-crossing threat that can quickly take over a field if control measures are late. The silverleaf whitefly caused cotton farmers more problems over a wider area in 2017 than in typical years.
Dr. Phillip Roberts is the cotton entomologist for University of Georgia Extension and located in Tifton, Ga., the heart of the state’s coastal plain. Most years, the SLWF is only a localized pest requiring management in a relatively small geographic area. But not so in 2017.
“High populations of SLWF were observed across the Coastal Plain during 2017 and significant acreage required treatment with insecticides,” he writes in the 2018 UGA Cotton Production Guide, which provides details on the pest’s biology, behavior and the best management practices in which to control it.
This Southeast Farm Press article provides an overview of the strategies to control SLWF in 2018 cotton.
SLWF is a major pest of vegetables, too, especially during the fall. This makes it an area-wide problem best managed with a community and state-wide effort.
One reason SLWF caused more-than-typical economic heartache in 2017 was due to an extremely mild winter prior to 2017 planting, and this weather pattern allowed the pest to establish and flourish early in the cotton-growing season. Many growers were playing catch up before they knew they were behind.
SLWF can reproduce on more than 600 plant species and survive winter months on cultivated and wild host plants, Roberts says. On cotton during the summer, SLWF completes a generation in about two weeks. Cooler temperatures slow development, and Georgia cotton growers this year received a natural leg up on the pest because the region received several days and extended periods of below-freezing temperatures this past winter, which the 2016/2017 winter season did not provide.
Both SLWF adults and immatures feed by sucking on plants. Damage ranges from:
- Reduced plant growth and vigor.
- General leaf decline.
- Honeydew deposits on leaves and open cotton. Honeydew accumulation on lint hurts fiber quality and may impact spinning efficiency at mills.
- Premature defoliation.
Yield losses in 2017 field trials in Tifton averaged 32 percent, ranging from 5 percent to 64 percent loses when comparing the mean yields of insecticide treatments for individual trials with the untreated, he says.
Helpful management points from the UGA guide include:
- Initiate control measures for SLWF just prior to the period of most rapid pest population development.
- Hairy leaf cotton varieties are more susceptible to SLWF compared with smooth-leaf cottons. Plant smooth leaf varieties to lower risk of SLWF if risk factors such as planting date or proximity to SLWF crops are high.
- The risk of SLWF problems increases as planting dates are delayed. SLWF stress the crop and potential yield loss is greater when infestations appear during squaring or early bloom compared with late bloom. April and early May planted cotton is at lower risk for SLWF problems compared with late May and June planted cotton.
- Location of cotton to other SLWF-infested crops matters. Spring vegetable and melon crops are a source of SLWF infesting cotton. In the fall, cotton is a source of SLWF infesting fall vegetables.
- Conserve beneficial insects and only use insecticides for other pests when thresholds are exceeded. In general, if growers see SLWF in cotton before the end of July, they have a potential problem.
- Hot and dry conditions favor SLWF development, survival and population buildup. Thunderstorms and extended rainfall events such as tropical systems negatively impact SLWF populations on an area-wide basis.
- Dryland or drought stressed cotton is at higher risk for SLWF compared with irrigated non-stressed cotton. Observations over the last several years show SLWF infestations are more severe in dry corners of pivot irrigated fields.
- Scout cotton for all insect pests on a regular basis. SLWF adults and immatures will be found on the underside of leaves. When thresholds are exceeded, especially for SLWF, intervene with the most appropriate insecticide in a very timely manner.
“It is critically important that initial insecticide applications are well timed. If you are late with the initial application control will be very difficult and expensive in the long run. It is nearly impossible to regain control once the population reaches outbreak proportions,” Roberts says.
Immature SLWF populations can go from threshold levels to very high populations in a week. Minimize or avoid insecticides such as organophosphates, which can flare SLWF when present.
The insect growth regulators, or IGRs, Knack and Courier have long residual activity and minimal impact on beneficial insects and perform well when applied timely. Knack is active on mature nymphs and eggs. SLWF adults exposed to Knack will lay sterile eggs. Courier is active on nymphs only. Neither IGR will directly control adults.
Assail and Strafer Max (acetamiprid), Sivanto, and Venom are active on immatures and adults. Control of adults is inconsistent due in part to reinfestation following application when populations are high. The goal, again, is to manage infield reproduction.
If an application to control comes too late, and this will happen from time to time even with the best efforts, growers are urged to use an insecticide active on all growth stages as the initial treatment. An IGR can be applied as a follow-up spray to extend residual once the population is under control.
And remember, a good community-wide practice is to destroy all SLWF host crops immediately after harvest. This includes vegetable and melon crops in the spring and cotton with timely defoliation and harvest in the fall.
For specific guidelines to control or prevent SLWF infestations in 2018, especially in Georgia, growers should contact their local Extension office.