Jonathan D. Siebert, B. Rogers Leonard, Alexander M. Stewart and Karla D. Emfinger
Cotton is a perennial plant capable of recovering from many stresses during Louisiana’s long growing season, including insect damage. However, as plants near maturity, their capacity to recover from stresses is reduced. Management of late-season defoliating insects in Mid-South and Southeastern cotton-producing states has changed dramatically with the introduction of genetically engineered, insect-resistant cottons (Bt cotton) with a foreign gene from Bacillus thuringiensis (Bt), the use of target-selective insecticides and boll weevil eradication programs. These technologies have reduced the number of broad spectrum insecticide applications used in integrated pest management programs for cotton.
The occurrence of damaging caterpillar levels during the post-bloom period of cotton development remains a problem both in conventional and Bt (Bollgard) cotton. Their damage appears as holes in leaf tissue or entire leaves missing in the crop canopy. Premature defoliation delays overall crop maturity, makes the timing of harvest-aid application (chemical defoliators) difficult and ultimately reduces cotton yield and lint quality.
LSU AgCenter entomologists have previously evaluated the effect of simulated insect defoliation on cotton plants to determine the amount of injury that the crop could tolerate at physiological “cutout” without significant yield loss. Cutout, in practical terms, defines the end of the effective fruiting period. Bolls produced after cutout may not have enough time to mature before harvest and contribute to yield. Cutout, in fact, is a moving target, but it typically is defined as the plant stage when flowering occurs on a fruiting branch five mainstem nodes below the terminal leaf (NAWF5). In these studies, cutout was defined as NAWF5, and harvest-aid applications were based on developmental stages designated by accumulated heat units (HU) beyond cutout. Cotton growth and development are closely related to HU accumulation, using 60 degrees F as the base temperature for cotton growth. Heat units are calculated by subtracting 60 from the sum of the daily high and low temperatures divided by 2.
Establishing Defoliation Thresholds
In the first study, plots of cotton plants were defoliated at four levels based on plant height. These treatments were applied when the cotton crop accumulated 350 HU beyond cutout. Considerable data have shown that harvestable bolls are not susceptible to insect injury after those bolls have accumulated 350 HU; however, no such information is available to support yield tolerances to defoliation by leaffeeding insects.
Treatments corresponded to no defoliation, removal of all leaves from the lower one-third of plants (33 percent), removal of all leaves from the lower two-thirds of plants (66 percent) and nearly complete defoliation (>99 percent).
Results indicated that as defoliation levels increased from 33 percent to nearly complete defoliation, cotton yield consistently declined. Cotton yields of plots defoliated at 66 percent or greater were significantly lower than non-defoliated plots (Figure 1). These studies established the threshold of simulated insect defoliation at 66 percent, which is the point where yield is significantly reduced at 350 HU beyond cutout.
Timing Crop Termination Strategies
A second study at the Dean Lee and Macon Ridge Research Stations during 2003 and 2004 used the 66-percent threshold to evaluate the effect of insect defoliation timing on cotton yield and quality. Varieties planted at Macon Ridge were Delta and Pine Land Delta Pearl and Stoneville ST 5599 BR and at Dean Lee were Delta and Pine Land DP 451 BG/RR and Stoneville ST 4892 BR. Manual defoliation (simulated insect injury that resulted in removal of all leaves from the lower two-thirds of the plant canopy) and chemical defoliation treatments (the application of harvest-aids) were applied at 450, 550, 650, 750 and 850 accumulated HU beyond cutout. A standard chemical defoliation treatment applied at 1,050 HU beyond cutout was also included for comparison. Percent open harvestable bolls and nodes above cracked boll (NACB) were recorded at each application. NACB refers to the number of mainstem nodes between the uppermost open first position boll and the last harvestable boll.
There was no significant reduction in yield when simulated insect defoliation occurred after 550 or more HU were accumulated beyond cutout. Lint yield, averaged across four experiments, was only 82 percent of the standard treatment (chemical defoliation at 1,050 HU beyond cutout) when simulated insect defoliation occurred at 450 HU beyond cutout (Figure 2). Chemical defoliation at 450, 550 and 650 HU beyond cutout reduced lint yield 38 percent, 37 percent and 15 percent, respectively, below that of the standard treatment (Figure 3). Simulated insect defoliation did not affect fiber properties. Chemical defoliation of plants that accumulated 550 or fewer HU beyond cutout significantly lowered fiber micronaire (a measure of fineness determined by resistance of air flow through a specified weight of fibers under a specific degree of compression) but did not affect fiber strength, length, elongation or uniformity.
Terminating insecticide management strategies for lateseason defoliating insects may begin at 550 HU beyond cutout, which usually corresponds to approximately 10 percent open bolls or NACB 7. Chemical defoliation should not be initiated until plant development exceeds 750 HU beyond cutout, approximately 40 percent open bolls or NACB 5.6. Maximum lint yields were obtained with chemical defoliation once the crop had accumulated 1,050 HU beyond cutout, 80 percent open bolls or NACB 2.9.
Low levels (less than 33 percent after cutout) of lateseason insect-induced defoliation may be beneficial in reducing the incidence of boll rot pathogens by increasing air movement and light penetration within the cotton canopy. Minor levels of defoliation in the lower crop canopy can occur without significantly reducing yield; however, insect management strategies must be applied to prevent excessive defoliation levels from progressing up the mainstem.
Other research has documented lower fiber micronaire associated with premature harvest-aid application compared to proper application timing. These data also support current AgCenter harvest-aid recommendations for cotton defoliation. Maximum lint yields and optimum fiber quality can be obtained with harvest-aid application at 42 percent to 81 percent open bolls and NACB of 3 to 4 depending on variety, boll distribution and environmental conditions in Louisiana.
Jonathan D. Siebert, Research Assistant, Department of Agronomy and Environmental Management, LSU AgCenter, Baton Rouge, La.; B. Rogers Leonard, Professor, Macon Ridge Research Station, Winnsboro, La.; Alexander M. Stewart, Assistant Professor, Dean Lee Research Station, Alexandria, La.; Karla D. Emfinger, Research Associate, Macon Ridge Research Station.
(This article appeared in the summer 2005 issue of Louisiana Agriculture.)