Both bollworm and tobacco budworm cause similar injury to cotton by feeding on squares, flowers, and bolls. In the field, it is difficult or impossible to distinguish between the caterpillars (larvae) of these two species. Damaged fruiting structures typically shed or large bolls may rot. The bollworm continues to be a serious threat in Tennessee despite the use of Bt cotton on most acres. Tobacco budworm typically causes little damage because of the wide adoption of Bt cotton, to which it is highly susceptible. However, infestation of tobacco budworm on non-Bt cotton can cause substantial yield loss, and they are highly resistant to insecticides from several classes of chemistry (e.g., pyrethroids). For more information visit the Cotton Insects Bollworm (W030) and Tobacco Budworm (W031) Fact Sheets.
Larvae feed on squares, flowers, and bolls. Holes and frass on these structures are a sign of infestation. Treatment is based on the average number (and size) or larvae found or the percentage of damaged fruiting structures. Examine a group of 5 plants at a minimum of 10 locations in a field. Look for larvae and signs of injury in the top 5 nodes and also examine at least one white or pink bloom and one additional boll in the mid canopy on each plant. Record the average number and size of larvae found per plant. A supplemental or alternative method is to examine 25 squares and 25 bolls in at least 4 locations in a field and record the number of squares and bolls with injury.
Non-Bt Cotton. Insecticides are recommended on the basis of knowing which species (bollworm vs. tobacco budworm) and how many are present in the field. Prior to bloom, treat when eight or more small larvae are present per 100 plants (or when populations threaten to reduce square retention below 80 percent). After first bloom, treat when four or more small larvae per 100 plants are present (or when 5% or more of squares and bolls are damaged and larvae are present). Insecticide application will often be needed when 10-20 percent or more of plants are infested with eggs.
Bt Cotton. Recent data indicates bollworm are developing resistance to some Bt toxins. Thus, insecticide applications for bollworm in Bt cotton are more likely and especially for WideStrike, TwinLink, and Bollgard II. Treatment is less likely for WideStrike 3, TwinLink Plus, and Bollgard III varieties. Prior to bloom, treat when eight or more surviving larvae (> 1/4 inch or longer) are present per 100 plants, or when populations threaten to reduce square retention below 80 percent. After first bloom, treat when four or more surviving larvae (> 1/4 inch or longer) per 100 plants are present. Treatment should also be made if a combination of square and boll sampling shows 6 percent or more injury (e.g., 3 percent square injury and 3 percent boll injury, 4 percent square injury and 2 percent boll injury, etc.). Scouting fields twice per week may be necessary once blooming has begun, especially if heavy egg lays are occurring. Treatments based on eggs alone is not usually recommended, but applications to Bollgard II, TwinLink, and WideStrike technologies should be considered when high numbers of eggs are present (> 20-30% plants with eggs). Whole plant examination may be necessary to find eggs and/or surviving larvae within the plant canopy.
In both Bt and non-Bt cotton, the treatment threshold should gradually increase after cotton reaches cutout (NAWF5) until NAWF5 + 350 – 450 DD60’s at which time insecticide applications for bollworm and budworm are no longer necessary.
Pyrethroid insecticides are NOT recommended against tobacco budworm infestations because of insecticide resistance. Time applications to control newly hatched larvae (< 1/4 inch length). Multiple applications on a 4- to 5-day interval may be needed. Tank-mixing pyrethroids with other insecticides may improve control of pyrethroid-resistant tobacco budworms but are only recommended when the budworm ratio is no more than 25 percent and populations are less than 8-10 larvae per 100 plants. Change insecticide chemistry if a control failure occurs.
|Insecticide (Trade Names)|
|Lb Active Ingredient|
|bifenthrin (Brigade 2, Discipline 2, Fanfare 2)||0.078 - 0.1||5 - 6.4 oz||7|
|cypermethrin 2.5||0.078 - 0.1||4 - 5 oz||7|
|deltamethrin (Delta Gold 1.5)||0.023 - 0.03||2 - 2.56 oz||7|
|esfenvalerate (Asana XL 0.66E)||0.036 - 0.05||7 - 9.6 oz||7|
|β-cyfluthrin (Baythroid XL 1)||0.0156 - 0.020||2 - 2.6 oz||7|
|γ-cyhalothrin (Declare 1.25)||0.0146 - 0.02||1.5 - 2.05 oz||7|
|λ-cyhalothrin (Warrior II 2.08)||0.03 - 0.04||1.92 - 2.56 oz||7|
|Z-cypermethrin (Mustang Max 0.8)||0.0188 - 0.0225||3 - 3.6 oz||7|
|BOLLWORM AND TOBACCO BUDWORM|
|acephate 90 (Orthene 90S)||0.9||1 lb||5|
|chlorantraniliprole (Prevathon 0.43 SC)||0.047 - 0.09||14 - 27 oz||9|
|chlorantraniliprole, λ-cyhalothrin (Besiege)||See label||7 - 12.5 oz||9|
|emamectin benzoate (Denim 0.16)||0.01 - 0.015||8 - 12 oz||7|
|indoxacarb (Steward 1.25)||0.11||11.3 oz||8|
|methomyl (Lannate LV 2.4)||0.45||24 oz||4|
|spinetoram, methoxyfenozide (Intrepid Edge)||See label||6 - 8 oz||8|
|spinetoram (Radiant SC 1)||0.033 - 0.0625||4.25 - 8 oz||8|
|spinosad (Blackhawk 36% WDG)||0.056 - 0.072||2.0 - 3.2 oz||8|
* Pyrethroids have often been used when the population is exclusively bollworm, such as would be expected on Bt cotton varieties, but the efficacy of pyrethroid insecticides for the control of bollworm has declined. Thus, alternative chemistries or tank mixes with alternative chemistries may be needed for adequate control.
- Plant early maturing varieties and avoid unnecessary insecticide applications that may disrupt populations of natural enemies.
- Bt cotton varieties provide excellent control of tobacco budworm.
- Bt cotton varieties provide good but variable levels of control of bollworm. Insecticide applications may be needed in some cases, particularly in flowering cotton.
- Insecticide applications can be terminated when cotton has accumulated 350-400 DD60s past NAWF5 (NAWF5 = average of 5 or fewer nodes above a first position white flower).