Cotton Pest Management Newsletter #13
August 23, 2001

Cotton Situation: The Georgia Weekly Weather and Crops Report for the week ending August 19^th listed the crop as 8% with open bolls. Many areas could use a good rain.

Insect Situation: Tobacco budworm and corn earworm populations are a mixed bag and vary significantly from area to areas and field to field. Stink bugs are also variable but are at threshold in some fields which have not been treated. A few areas are observing spider mites, loopers, and fall armyworms. Whitefly numbers are building in the Tift County area.

Silverleaf Whitefly: Silverleaf whitefly (SLWF) numbers are building in some fields in the Tift County area. Reproduction (the presence of immatures) has been observed in some fields. Correct identification of whiteflies is important. Many of us are familiar with the banded winged whitefly which has faint but visible grayish bands on the wings whereas the SLWF is solid white. Banded winged whiteflies are much easier to control compared with SLWF. During 2000, SLWF infestations were observed in many southwest Georgia counties, but most economic damage occurred in Tift and Colquitt Counties, especially on late planted cotton. The adult SLWF is about 1/20 inch in length, solid white, and holds its wings roof-like over its body. Eggs are oblong, pointed, yellowish brown in color, and laid on the underside of leaves. The first immature stage is known as the "crawler" and moves about on the underside of the leaf searching for a suitable site to feed. The crawler attaches itself to the leaf and completes three more molts as a flattened oval nymph which is yellowish in color. These scale-like nymphs remain stationary on the underside of the leaf feeding on plant sap. As the nymphs develop, red eye spots will become visible. The red eye spots are easily seen during the pupal stage. It requires as little as 18 days to develop from egg to adult.

Both adult and immature SLWF feed on the underside of leaves by sucking plant sap with their piercing sucking mouthparts. Whiteflies produce honeydew similar to aphids which can serve as media for sooty mold development. Honeydew accumulation on open bolls may also cause problems. Excessive feeding also results in a general decline of leaf health which may cause premature defoliation. It is likely that April and early May planted cotton will avoid problems in many areas. However if SLWF populations continue to build, late planted cotton is at risk for economic yield loss. Our best management programs are avoidance through the use of cultural practices such as avoiding late planting, using smooth leaf varieties, conserving natural controls, and destroying crop residues which may harbor SLWF. Management of SLWF with insecticides can be difficult and expensive.

We basically have three options when faced with economic infestations of SLWF: 1) Do Nothing, 2) Knack Insect Growth Regulator, and 3) Contact and Systemic Insecticides. During 2000 we demonstrated yield losses of 252 and 60 lbs. lint per acre when heavy SLWF infestations were not controlled. If immature SLWF are found in fields which have are not near maturity, failing to control SLWF could be yield limiting and thus some type of intervention will be needed to reduce populations.

Knack is an insect growth regulator and has been the most consistent and efficacious treatment for management of SLWF. Knack is an expensive treatment and must be used correctly to receive full benefit. Knack has a long residual (several weeks) and is slow acting in general. When female SLWFs feed on treated foliage, her eggs will be sterile. Knack will also control immatures when they pupate (red-eye stage), thus nymphs present at application will continue to feed for several days until mortality occurs when pupating. Another consideration with the use of Knack is that any new plant growth occurring after application will be unprotected. Based on field demos in recent years we have established a threshold for use of Knack when 5 immatures can be found in a square inch somewhere on the underside of the fifth mainstem leaf below the terminal. Knack must be used early. Knack will not provide timely control of SLWF in fields which are severely infested.

Contact and systemic insecticide tank mixes can provide temporary relief from SLWF. However multiple applications will be needed if cotton needs to be protected for an extended period of time as reinfestation would be expected in severely infested areas. In a small plot trial conducted during 2000, Danitol plus Orthene provided good control of adults. Use of contact and systemic insecticides would be good options on cotton that does not need to be protected for an extended period of time or if other pests are also in the field which need to be controlled. Several other options for control or suppression are listed in the Pest Control Handbook. A starting point for a threshold using contact and systemic insecticides would be 5-10 adult SLWF on the fifth mainstem leaf below the terminal.

Stink Bugs: Scouting for stink bugs can be frustrating. Making accurate counts of bugs is difficult due to their elusiveness and that is one reason we have established a boll damage threshold. Additionally, other pests such as tarnished plant bugs, leaf footed bugs, etc. can also damage developing bolls and that damage is indistinguishable from stink bug damage. However, when making damage counts we must also be observant for stink bugs when doing other procedures. Our threshold is 20% internal damage to quarter sized bolls and stink bugs observed in the field. Recommended treatments include several pyrethroids, Bidrin or methyl parathion. Vydate was also recently labeled for control of southern green and green stink bugs and would also be a good option.

Tobacco Budworm and Corn Earworm: Mixed populations of TBW and CEW are present in many areas of the state. Pressure ranges from very low to high in isolated fields or areas. It appears to me that populations have varied from field to field more this year than in years pasts. Mixed populations should be treated as a TBW population, especially in areas where pyrethroid resistant TBW is suspected.

Insect Updates: Check the Cotton Insect Hotline (1/800-851-2847) for updates on current insect conditions. The Cotton Pest Management Newsletter is also posted on the Internet at www.gaipm.org and then click on "cotton."

Sincerely,

Phillip Roberts
Extension Entomologist