Insecticide and Bt Cotton Efficacy Bt Resistance Monitoring

Multiple replicated, small-plot trials are conducted annually on research stations across Alabama to evaluate current and future insect control strategies. This research helps to refine insecticide recommendations provided in Alabama Extension’s “Cotton Insect, Disease, Nematode, and Weed IPM Recommendations Guide” (IPM-0415). Plot sizes may be 4 to 8 rows wide and 25 to 30 feet long. Trials are initiated when the target pest population reaches threshold and are evaluated at intervals ranging from 3 to 14 days after application (DAA).

Efficacy of Thrips Management Options for Reducing Damage and Preserving Cotton Yields. Studies were conducted at the Prattville Agricultural Research Unit (PARU) in Prattville, Alabama and at the Tennessee Valley Research and Extension Center (TNVREC) in Belle Mina, Alabama, to evaluate selected seed and in-furrow treatments against thrips infesting seedling cotton. To evaluate treatments, whole plot thrips injury ratings were made on a 0 to 5 scale. On this scale, “0” means no injury is observed, “3” is considered the threshold when delays in maturity are likely and yield loss is possible, and “5” means thrips have killed all the plants in the plot. Although initial thrips injury ratings were higher at PARU, both locations had significant pressure at the final rating. Except for Cruiser and Orthene 97S at PARU, all treatments significantly reduced thrips injury ratings compared to the untreated controls (black seed, fungicide only) (table 2). As has been consistently reported across the mid-south and southeast, Cruiser performed poorly at both locations, while Orthene performance was different at each location (figure 3). Although no significant differences in yield were observed among treatments, using a recommended at-plant insecticide yielded an average of ≈117 pounds of lint per acre compared to the untreated control (table 3). These data show a consistent return on investment when using an at-plant insecticide to manage thrips in seedling cotton. Results from this study have also led to the removal of Cruiser for thrips management from Alabama Extension’s “Cotton Insect, Disease, Nematode, and Weed IPM Recommendations Guide” (IPM-0415).

Figure 3. Final thrips injury ratings (0 to 5 scale, 4th leaf) in at-plant seed and in-furrow insecticide trials in Prattville and Belle Mina, Alabama (2020). Letters indicate significant differences (P<0.05).

Efficacy of Selected Insecticides to Manage Tarnished Plant Bugs in Blooming Cotton. Trials were conducted at the Tennessee Valley Research and Extension Center (TNVREC) in Belle Mina, Alabama, to evaluate insecticides against tarnished plant bug (TPB) in cotton. Trials were initiated when TPB populations at least reach threshold (=3/5 rft) during peak bloom (August 8).

Trial One. Tarnished plant bug counts were done on 7 and 14 DAA by taking two black drop-cloth samples (=10 rft) in the center two rows of each plot. Although no significant differences were observed between treatments, Transform, Bidrin, and Bidrin + Diamond appeared to provide better control than other treatments and the untreated check (table 8). No significant differences were observed for yield; however, several treatments, such as Transform, Bidrin, and Bidrin + Diamond, provided substantial yield protection over the untreated check (table 6). These data support the decline in efficacy of pyrethroids for TPB management in north Alabama. The limited yield return from neonicotinoids suggests that these products should not be used to manage immature plant bugs; however, they are still effective against adult populations prior to bloom. Although Bidrin and Transform both provided good control, it is possible that Bidrin provided higher yield protection over Transform due to the added stink bug control.

Trial Two. A second trial was conducted to evaluate the secondary effects of stink bug treatments on TPB management. Plots were sprayed and the percentage of TPB damaged dirty blooms found on 30 row feet was recorded. As expected, the pyrethroids did not provide protection from TPB. However, the organophosphates provided significant reductions of dirty blooms (table 7). The initial goal of this study was to determine the efficacy of the products on stink bugs, but no differences were found between treatments and the untreated check (data not shown), likely due to inadequate plot size (four rows). Because of this, it can be assumed that any treatment trends for yield are likely related to plant bug injury.

Evaluation of Bt Technologies for Bollworm Management. A trial was done at the Prattville Agricultural Research Unit (PARU) in Prattville, Alabama, to monitor the efficacy of Bt cotton technologies against bollworms. Cotton was planted on May 7 to ensure that it would be blooming during the peak bollworm flight in late July. Scouting for eggs was done during the peak flight window to assess the potential bollworm pressure in the trial. When scouted for damage, 25 squares, 25 blooms, and 25 bolls were assessed in each plot for bollworm damage and/or larvae. A final damaged boll count per 60 row feet was made after defoliation and prior to harvest.

Damage evaluations were made after the peak bollworm flight date (August 10) to evaluate the level of protection provided by Bollgard 2 (Cry1Ac+Cry2Ab), Bollgard 3 (Cry1Ac+Cry2Ab+Vip3A), and Widestrike 3 (Cry1Ac+Cry1F+Vip3A). Overall, pressure was low in this trial; however, the Bt technologies provided significant protection in terms of damaged fruiting structures and yield compared to the non-Bt check (table 8, figure 5). Results in this study show that Bt technologies still provide good control of bollworms in Alabama, at least in low to moderate pressure situations.

Figure 5. Total number of stink bug injured bolls (out of 240 total) collected from a trial in Prattville, Alabama (2020).

 

Figure 6. Total number of damaged bolls (and % reduction compared to non-Bt) found just prior to harvest in Prattville, Alabama (2020).

Bt Resistance Monitoring

Monitoring Bollworm for Resistance to VIP Bt Technology. A block of non-Bt field corn was planted on research stations across Alabama to collect natural populations of bollworms to be screened for resistance to the VIP technology (Bollgard 3, WideStrike 3). Populations were collected and shipped to North Carolina State University to be screened against the VIP3Aa39 protein in the laboratory. The amount of VIP protein needed to kill 50 percent of the field population was compared to the amount needed to kill 50 percent of a lab colony that had never been exposed to the protein before testing. The tests showed that bollworm populations from Alabama in 2020 were as susceptible to the VIP protein as the lab colonies (table 10). This confirms that we do not have VIP resistance as of 2020.

Establishment of Sentinel Corn Plots to Monitor Bollworm Resistance to Bt Technologies. Four Bt corn varieties and one non-Bt variety were planted at the Tennessee Valley Research and Extension Center in Belle Mina, Alabama, to monitor in-season bollworm damage. Each variety was planted into nonreplicated strips four rows wide and 300 feet long at three planting dates, each two weeks apart. At the R3 growth stage, the number of live larvae found in 100 ears was recorded for each plot (table 11). At the R5 growth stage, the number of damaged kernels in 100 ears was recorded. Unlike in cotton, escaped bollworms are commonly found in 2-gene Bt corn. This is likely due to higher pressure in corn compared to cotton and differences in Bt expression between the crops.