Midsouth Multistate Evaluation of Treatment Thresholds for Tarnished Plant Bug in Flowering Cotton
The tarnished plant bug (TPB), Lygus lineolaris, has been considered a cotton pest in the Midsouth states for as long as we have kept records. For nearly 30 years, these states have reported cotton yield losses associated with TPB.
In recent years, the TPB has become a “key” pest of cotton. Before 1995, generally we controlled TPB with insecticides targeting other insect pests, such as tobacco budworm and boll weevil. Since the widespread adoption of Bt-cotton and eradication of the boll weevil, we use insecticides targeting these pests less often.
As a result, the TPB has become the primary insect pest of cotton in this region of the Cotton Belt. More recently, TPB has become resistant to several classes of insecticides, further compounding the problem, particularly in the Delta regions of the Midsouth states.
While TPB is a pest of cotton throughout the growing season, it is particularly damaging during the flowering period, when high levels of reproduction occur. Both adult and immature stages of TPB feed on cotton during the flowering period. Most feeding occurs on reproductive structures, where the pests insert their mouthparts into squares and small bolls. It is not uncommon for TPB to cause near-total crop loss in the absence of effective control strategies in some areas of the Delta.
Action threshold recommendations to initiate treatments for TPB control in flowering cotton are similar among all Midsouth Extension Services. The preferred action threshold in flowering cotton is three TPB per 5 row-feet using a black drop cloth. Other action thresholds are 12 to 15 TPB per 100 sweeps with a sweep net or 9 to 15 TPB per 100 plants for visual plant searches.
This multistate experiment was conducted in 2006 and 2007 to reevaluate the accuracy of TPB action thresholds on flowering cotton.
In 2006 and 2007, the experiments were conducted at 19 locations across the Midsouth. Experiments were designed as large replicated plots (24 rows x 100 feet). The locations and number of sites included Arkansas (7), Louisiana (3), Mississippi (3), Missouri (2), and Tennessee (4).
The treatments included the following:
- Automatic: Weekly insecticide applications initiated at first bloom and repeated every 7 days until plants matured to the NAWF 5 and 350 DD60s growth stage.
- Low: Insecticide application triggered when TPB density reached one TPB per 5 row-feet on a black drop cloth.
- Medium: Insecticide application triggered when TPB reached three TPB per 5 row-feet on a black drop cloth.
- High: Insecticide application when TPB density reached five TPB per 5 row-feet on a black drop cloth.
- Very High: Insecticide application triggered when TPB reached 10 TPB per 5 row-feet on a black drop cloth.
All action thresholds were adjusted for feeding habits of the other hemipteran species when present and were converted to TPB equivalents. Cotton fleahoppers represented one TPB, clouded plant bugs were counted as 1.5 TPB, and stink bugs represented three TPBs. All fields were planted with transgenic Bt cultivars to minimize the impact of caterpillar pests. Before first bloom, all plots were treated with a neonicotinoid insecticide when TPB infestations reached a prebloom action threshold. At first bloom, a black drop cloth (2.5 feet per side) was used to sample TPB densities. Insecticide applications were triggered when the average of the four replicates for each treatment was more than the designated action threshold.
Insecticide applications during the flowering period were made with organophosphate insecticides, Bidrin (dicrotophos at 8 ounces per acre) and Orthene (acephate at .83 pounds per acre). The insecticides were alternated for each application after that. At the end of the season, the tests were harvested, and lint yields were determined for each plot.
Average lint yields over all action thresholds ranged from 936 to 1,540 pounds per acre at six locations in 2006 and from 659 to 1,715 pounds per acre at 13 locations in 2007. Across all tests within a year, average lint yields were 1,272 and 1,087 pounds per acre in 2006 and 2007, respectively.
At all locations, TPB exceeded the low threshold at least once during both years. The current threshold of three TPB per 5 row-feet was reached in 79 percent (15 of 19) of the trials. The high and very high thresholds were reached in 68 percent and 26 percent of the trials, respectively.
At 12 locations in 2006 and 2007, at least the high threshold (five TPB per 5 row-feet) was reached. In those trials, lint yield declined as the action threshold increased but insecticide application frequency decreased. Approximately 1.5 percent yield loss was observed for every increase of one TPB per drop cloth sample.
Economic Injury Levels
To demonstrate how changes in insect control inputs influence the economic injury level, action thresholds were estimated using several cost-benefit scenarios. When cotton price is set at $0.65 per pound of lint, economic thresholds range from a low of 2.4 TPB per drop cloth sample if yield expectations are 1,200 pounds per acre to 3.8 TPB per drop cloth sample if yield expectations are only 600 pounds per acre.
When crop value increases to $0.85 per pound of lint, action thresholds are slightly lower. With this scenario, 1,200 pounds per acre of lint yield expectation lowers the action threshold to 1.7 insects per drop cloth sample compared to 3.3 insects per drop cloth sample when yield expectations are only 600 pounds of lint per acre.
When cotton prices are lower than those listed, the action threshold continues to increase and further reduce the actual number of sprays needed to provide a positive economic return.
In these trials, natural TPB populations feeding during the flowering period had a significant impact on cotton yield. Economic threshold levels for average conditions ($0.65 per pound) were estimated to be between 2.4 and 3.8 TPB per drop cloth sample. When higher prices are considered for the crop ($0.85 per pound), action thresholds were shifted to a slightly more conservative position, ranging between 1.7 and 3.3 TPB per drop cloth sample.
These results were shaped by two factors that may vary in some situations. The first factor was sampling frequency. All locations in these trials were sampled and sprayed (if needed) once per week. This frequency was longer than desirable for a pest such as TPB that can build populations rapidly through movement and reproduction. As a result, populations just under the action threshold during a sample for one week occasionally greatly exceeded the action threshold during the following week. A shorter scouting interval should raise the action threshold, since pests will not have time to reach very high densities.
The second factor that could not be controlled was insecticide effectiveness. Resistance to organophosphate insecticides has been reported for TPB in the Midsouth, particularly in the Delta. In a region without insecticide-resistant TPB populations, or if more effective insecticide were available, the frequency of sprays required to maintain the lower thresholds would likely be lower. This should increase the economic injury level, because insecticide costs would be lower.
Given the range of action thresholds derived from economic factors associated with cotton, the results generated in this project support a threshold “range” of approximately two to four TPB per drop cloth sample. Therefore, these results confirm that the existing drop cloth threshold of three TPB per drop is well within the range of the findings for this study.
The action threshold can be adjusted slightly and adapted to the particular situation depending on expected yield, commodity prices, insecticide cost, and insecticide resistance in an area. For instance, in Delta regions where TPB pressure is typically very high and significant insecticide resistance is likely, a producer may opt to treat at numbers as low as two to three TPB per drop cloth sample. But in areas where TPB pressure is low to moderate and insecticide resistance has not evolved, producers may opt to treat at numbers closer to four TPBs per drop cloth sample.
Angus Catchot, Fred Musser, Jeff Gore, Don Cook, and Chris Daves of Mississippi State University
Gus Lorenz, Scott Akin, and Glenn Studebaker of University of Arkansas
Kelly Tindall of University of Missouri
Scott Stewart of University of Tennessee
Ralph Bagwell and B. Rogers Leonard of LSU AgCenter
Ryan Jackson of USDA-ARS
The authors wish to thank Cotton Incorporated, University of Arkansas, LSU AgCenter, Mississippi State University, University of Missouri, U.S. Department of Agriculture ARS, and University of Tennessee for partial funding of this project.
Publication 2561 (reviewed July 2019)
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