The silverleaf whitefly (SLWF) is a devastating economic pest of cotton. A multi-county area surrounding Tifton, GA was heavily infested in 2016. In 2017, this infested area has spread over much of the 1.3 million acres of cotton planted in Georgia. In early to mid August of 2017, these SLWF infestations have spread throughout the Wiregrass area of southeastern Alabama.
The SLWF was first observed on cotton in Alabama in Mobile county in 1997. This pest has historically been associated with the more arid regions of cotton production such as California, Arizona, and the Rio Grande Valley of southern Texas. Major outbreaks occurred in those areas in 1992 and 1993.
SLWF has sucking mouthparts and their feeding is similar to aphids. This feeding will stunt plants and reduce their vigor. However, a more serious problem is their secretion of honeydew, which falls in lower parts of the plant. This would be lower leaves during the growing season but open bolls as the plants mature. A sooty mold grows in this honeydew, which will reduce the quality of the lint once cotton begins to open.
Heavy infestations of SLWF can cause premature defoliation. SLWF are not known to die off from a naturally occuring fungus like aphids. SLWF populations continue to increase until cotton is defoliated or until the leaves drop from SLWF feeding.
The first sign of a SLWF infestation will be the presence of whiteflies clustering around the plant terminal or underneath the terminal leaves. The adults will fly when disturbed, Population increases will be observed about two plus weeks after the first presence of adults. SLWF adults deposit eggs underneath the leaves. These eggs hatch into a crawler stage which finds a place under the leaf to begin feeding. This immature stage is then immobile until it develops into an adult. The total life cycle of the SLWF is 15-18 days, depending on the temperature.
Treatment decisions for SLWF can be made by examining for the presence or absence of immatures on the 5th main stem leaf below the terminal. Controls are recommended when 50% of the plants have immatures on the lower surface of this leaf. Immatures will appear oval, flattened, and yellowish in color. They can be separated from aphids by their flattened shape and the absence of appendages and movement.
Points to remember about SLWF as expressed by Dr. Phillip Roberts, Extension entomologist, UGA: Do not overreact but be careful to not underreact to this pest. You cannot get behind with controls and ever catch up. Try not to do anything to make the situation worse. Do not treat for other pests unless necessary. When controlling other pests, use the most selective insecticides on beneficial insects. Cotton is not safe from SLWD damage until the day it is defoliated for harvest. Growers should try to get to defoliation time with green leaves in the plant terminal with no honeydew present.
Rainfall may reduce the number of adults but will have no effect on the immature stage underneath leaves. The damage potential from the SLWF is greatest on late planted cotton (late May-June). This pest prefers hairy leaf or semi-hairy leaf varieties over smooth leaf ones. Hot and dry conditions are favorable for more rapid SLWF reproduction.
Controlling SLWF in 2017 will be very expensive and challenging due to the unavailability or short supply of most recommended controls. The most effective control can be achieved with the insect growth regulator (IGR) type insecticides Knack or Courier. Both work on the immature stage. Their activity is slow but they have long residual. It is advisable to wait 10-14 days after treatment before making opinions about benefits.
Other products that have activity on the SLWF are acetamiprid (Assail/Intruder), Venom, and Sivanto. Centric at high label rates will suppress the adult stage.