Insecticide use strategies in mid-south cotton fields for tarnished plant bug
1Louisiana State University AgCenter, Macon Ridge Research Station, Winnsboro, LA, USA, USA
2Mississippi Agricultural and Forestry Experiment Station, Verona, MS, USA
Correspondence: rleonard@agcenter.lsu.edu
The tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), is a common pest of cotton throughout the southern and southeastern areas of the U. S. Cotton Belt. Yield losses in cotton due to this pest vary temporally and spatially. The primary factors for these variations in population levels and economic injury include: (1) environmental conditions favorable for tarnished plant bug population growth and development, (2) prolonged weather events that result in preferred wild hosts becoming less attractive, (3) environmental conditions that stress cotton plants and results in greater sensitivity to tarnished plant bug injury, and (4) efficacy of insecticides registered for tarnished plant bug control. Across the mid-south states of Arkansas, Louisiana, and Mississippi during 1991–2005, tarnished plant bug has infested 77–99 percent of cotton acreage. During those years, 44–93 percent of the infested acres were treated with one or more insecticide applications. The frequency of insecticide applications has varied during the past 15 years, but a general increase has occurred across the entire period. During 1991–1993, the annual number of insecticide applications targeting tarnished plant bugs was less than one. During 2003–2005, producers applied 2.3–4.4 insecticide treatments per year for this pest. The costs of these control strategies have increased 10-fold, from $5 million to greater than $50 million during the previous 15 years. In spite of inputs for insecticide use strategies, estimates of the average cotton yield losses attributed to this pest during the five year period of 2001–2005 are higher than that observed for any year in recent history. Historically, the tarnished plant bug has been considered an early season pest of cotton and has been of little concern after the initiation of flowering. The cotton plant is most susceptible to yield losses from tarnished plant bug injury between the four to six true leaf stages through early squaring. Tarnished plant bug injury generally results from feeding on small flower buds (squares), which abscise from the cotton plant. Tarnished plant bug feeding on bolls was considered to be a relatively non-significant source of yield loss. However, this insect has become a primary pest of cotton plants during the flowering and boll maturation periods. This pest can induce abscission until bolls accumulate greater than or equal to 245 heat units after anthesis. Seedcotton yields may be significantly reduced by tarnished plant bugs until bolls accumulate at least 330 heat units after anthesis. Given the opportunity, tarnished plant bugs are capable of significant injury to cotton during the flowering period. Historically during the flowering and boll maturation periods, tarnished plant bugs were inadvertently controlled by organochlorine, organophosphate, carbamate, and pyrethroid insecticides targeting boll weevil, Anthonomus grandis grandis (Boheman) and heliothines. The success of the boll weevil eradication programs and wide-scale adoption of Bacillus thuringiensis Berliner var. kurstaki cotton cultivars eliminated many of those applications. In addition, the use of more selective insecticides that target Lepidopteran pests (i.e. spinosad, indoxacarb, and emamectin benzoate) have increased survival of tarnished plant bugs. Finally, tarnished plant bug resistance to a variety of insecticides (organophosphates, carbamates, pyrethroids) in the mid-south states has contributed to the elevated status of this insect pest. The action thresholds for tarnished plant bug in pre-flowering cotton have been well-established for several years, and recent research efforts are generating data to further support those recommendations. However, with considerable changes in cotton production, IPM practices, and the pest status of tarnished plant bug during the last decade, the action thresholds in flowering cotton are a topic of considerable research interest. The current recommendations are mostly qualitative and based on experience of the pest manager rather than quantitative scientific data. This problem is compounded by the difficulty in sampling for tarnished plant bugs in flowering cotton. In general, sweep nets are the preferred method for sampling tarnished plant bugs in pre-flowering cotton. However, sweep net samples do not provide an accurate estimate of tarnished plant bug population densities during the flowering period of cotton plant development. The most accurate method for estimating tarnished plant bug densities in flowering cotton appears to be the drop cloth. Drop cloth samples provide an adequate method for estimating populations, but are considered to be time efficient for practical application by agricultural consultants and farm managers. Federal and state entomologists in the mid-south have several ongoing projects to refine sampling protocols and action thresholds for tarnished plant bug. A series of trials compared and calibrated all recommended sampling methods including visual observation, drop cloth, and sweep net based upon the present action thresholds. Another study validated flower bud (square) injury as an effective measurement of insect damage. The results indicate that an actual treatment threshold to manage this pest should be in the range of 10–20 percent injured squares. A third group of experiments is examining a series of action thresholds during the flowering and boll maturation stages under a range of environmental conditions throughout the region. Satisfactory control of tarnished plant bug in mid-south (Arkansas, Louisiana, Mississippi) cotton fields with insecticides has become difficult in recent years, especially during the mid-to-late season. Insecticides recommended for tarnished plant bug include compounds in the organophosphate, carbamate, neonicotinoid, pyridine carboxamide, and IGR chemical classes. Of all those listed, acephate and dicrotophos are the most frequent compounds selected by producers due to their concurrent efficacy against other Heteropteran pests of cotton. Mid-south populations of tarnished plant bug have been shown to express varying levels of resistance to several of these insecticides and instances of unsatisfactory control have become common. The agrochemical industries are aggressively pursuing research and development of novel insecticides, but there are limited alternatives to currently recommended products. To prolong the efficacy of available products, insecticide use strategies recommended by mid-south entomologists are based upon temporal restrictions and a rotating schedule. During the pre-bloom stages of cotton development, the organophosphate insecticides are not recommended, and if tarnished plant bug populations exceed action levels for treatment, other products should be applied. During the flowering and boll maturation stages, organophosphates are the primary insecticides. Producers are discouraged from using subsequent applications of the same product and should rotate with compounds in other classes before returning to an organophosphate treatment. The IGR, novaluron (Diamond), also is recommended in co-applications with other products to improve residual efficacy against immature stages of tarnished plant bugs. The tarnished plant bug has become the new key pest of cotton in the mid-south region of the U.S. Considerable research efforts are underway to validate and improve sampling protocols and action thresholds during the early-, mid-, and late-seasons of cotton development. Many of the recommended insecticides are not providing consistent control; and novel compounds are desperately needed. The evolving insecticide use strategies that are in place promote careful selection of products for use during specific periods of plant development.
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