Josh T. Copes, Gordon L. Snodgrass, Jonathan Sharp, B. Rogers Leonard, Dennis Ring and David C. Blouin
The tarnished plant bug is the most yield-limiting and costly arthropod pest attacking Louisiana cotton. Integrated pest management tactics are limited for tarnished plant bug, and infestations are controlled almost exclusively with chemicals. This insect is a persistent, season-long pest and typically requires multiple insecticide applications to maintain adequate control and reduce economic losses. The organophosphate insecticide acephate has been one of the products recommended to control this pest. However, in recent years, the actual amount of acephate applied to Midsouth cotton acreage has approached the maximum frequencies of sprays and total active ingredient allowed by the label in a season. Despite the increased use, reports of poor field performance with acephate against this pest have become common. Acephate resistance in tarnished plant bugs could potentially limit cotton production. The purpose of this study was to survey acephate susceptibility among Louisiana populations of tarnished plant bug using field trials and laboratory bioassays.
Twenty field trials were conducted in six parishes – Franklin
and West Feliciana
– during 2007-2009 against native infestations of tarnished plant bugs (Figure 1). Five treatments, including a nontreated control and four rates of acephate (0.5, 0.75, 1.0 and 1.25 pounds active ingredient per acre of Orthene 97 SP) were included in each trial. The trial sites included areas on commercial fields and LSU AgCenter research stations. The effects of each insecticide treatment were estimated by counting the numbers of tarnished plant bugs that remained in each plot at five to seven days after the application. The general procedure used to sample field plots required the use of a black shake sheet (2.5 feet by 2.5 feet) placed between two rows of cotton. The plants adjacent to each side of the sheet were vigorously shaken to dislodge all insects and have them fall onto the cloth. All tarnished plant bugs were recorded on the sheet. The process was repeated in each plot for a total sample of 10 row feet.
In 14 of the field trials, acephate applied at 0.5 pounds active ingredient per acre reduced tarnished plant bug below the levels in the nontreated plots. However, this rate provided acceptable field control only during the 2007 trials, based upon reducing infestation levels below the action threshold of six insects per 10 row feet (Figure 2). During 2008 and 2009, acephate rates of 0.75-1.25 pounds active ingredient per acre were needed to adequately control infestations. These results indicate that acephate efficacy at a given rate appears to have decreased during 2007-2009.
Laboratory bioassays were used to estimate acephate dose mortality responses for 20 populations of tarnished plant bugs collected during the 2007- 2009 period. Insects were collected in nine cotton-producing parishes including those mentioned above plus Caddo, Catahoula, Rapides and Concordia. Insects were also collected in one noncotton- producing parish (East Baton Rouge) from March to September of each year (Figure 1). A standard sweep-net (15 inches diameter) was used for collecting insects from native host plants and cotton.
A resistance ratio of 3.0 was used to estimate the likelihood of field control failures with acephate (Figure 3). The ratios ranged from 0.53 to 10.4 among Louisiana populations sampled during 2007-2009. All populations surveyed during the study demonstrated resistant ratios greater than 3.0, except for the 2007 Wisner population. Only five other populations – 2007 Monroe A (3.14), 2007 Monroe B (3.13), 2008 Wisner (3.4), 2008 Newlight (3.08) and 2009 Winnsboro (3.10) – expressed resistance ratios similar to the 3.0 critical level.
The results show that Louisiana tarnished plant bug populations are maintaining a high frequency of insecticide resistant individuals even prior to acephate exposure on cotton. Field efficacy trials and laboratory bioassays indicate that resistance in Louisiana tarnished plant bugs is partially responsible for decreasing acephate performance. Acephate resistance levels are variable, but widespread across the state. Field efficacy trials indicate that to successfully control persistent tarnished plant bug infestations, acephate rates of at least 0.75 pounds active ingredient per acre would be required.
To maintain the value of acephate in cotton production for control of tarnished plant bug, producers and pest managers should limit the frequency of acephate sprays per season, rotate with other recommended insecticides and co-apply acephate with other products that demonstrate different modes of action. In addition, acephate applications that target other pest species in adjacent crops may be exposing populations of tarnished plant bugs to selection pressure prior to their migration into cotton. Therefore, chemical control strategies for pest management across the farmscape should consider the impact of acephate selection on tarnished plant bugs and include the use of alternative chemistry whenever appropriate. Furthermore, complete reliance on the alternative recommended products will likely not solve this problem.
Josh T. Copes, Graduate Research Assistant, Department of Entomology, LSU AgCenter, Baton Rouge, La.; Gordon L. Snodgrass, Research Scientist, Jamie Whitten Delta States ARS-SFCIML Research Center, Stoneville, Miss.; Jonathan Sharp, Extension Associate, Scott Research & Extension Center, Winnsboro, La.; B. Rogers Leonard, Professor and Jack Hamilton Chair in Cotton Production, Macon Ridge Research Station, Winnsboro, La.; Dennis Ring, Professor, Department of Entomology; and David C. Blouin, Professor, Department of Experimental Statistics, LSU AgCenter, Baton Rouge, La.
(This article was published in the summer 2010 issue of Louisiana Agriculture.)