Submitted to: Southwestern Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 14, 2008
Publication Date: September 1, 2008
Citation: Snodgrass, G.L., Abel, C.A., Jackson, R.E., Gore, J. 2008. A Bioassay for Determining Resistance Levels in Tarnished Plant Bug Populations to Neonicotinoid Insecticides. Southwestern Entomologist.33:173-180 Interpretive Summary: The tarnished plant bug is the main pest of cotton in the mid-South and is controlled with insecticides. In 1995, plant bug populations highly resistant to the pyrethroid class of insecticides were found in MS, and this resistance quickly spread throughout the delta of AR, LA, and MS. Pyrethroid insecticides are no longer recommended for the control of plant bugs in this area. Plant bug populations with high levels of resistance to the organophosphate insecticide acephate (Orthene) were found in the hill and delta regions of MS in 2005. Acephate previously was the most widely used and effective insecticide for control of plant bugs in cotton. By the end of 2006, resistance to acephate was found in plant bug populations throughout the hills and delta in MS and in AR and LA. In addition, some populations were found that were highly resistant to carbamate, organophosphate, and pyrethroid insecticides which are three of the four main classes of insecticides available for plant bug control in cotton. To deal with this resistance, entomologists in the mid-South strongly recommended to growers that they alternate insecticide classes when controlling plant bugs in cotton beginning in 2007. The only widely used class of insecticides to which plant bugs are not resistant are the neonicitinoid insecticides imidacloprid (Trimax) and thiamethoxam (Centric). This makes them critical to plant bug control by alternation of insecticide classes. A resistance monitoring program to detect resistance development to the neonicitinoid insecticides was needed. This manuscript presents how a bioassay for measuring resistance to this class of insecticides was developed and used over a period of two years to meet this need. Current results showed that no resistance to thiamethoxam had developed and only a small amount of resistance to imidicloprid was present. The manuscript also presents baseline data for comparison to detect resistance changes in future years.
Technical Abstract: A laboratory bioassay was developed and used to test field populations of the tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), for resistance development to the neonicitinoid insecticides imidacloprid (Trimax®) and thiamethoxam (Centric®). The bioassay determined LC50 values by feeding known doses of the insecticides to adult tarnished plant bugs in a 10% solution (by weight) of honey in water. Field populations from 19 (imidacloprid) and 9 (thiamethoxam) locations in the mid-South were tested in 2006 and their LC50s were compared to LC50 values determined in tarnished plant bug populations at the same locations in 2007. The LC50 values were also compared to LC50 values determined with imidacloprid and thiamethoxam using a susceptible population from Crossett, AR. Results of the comparisons showed that no resistance to thiamethoxam had developed in the 9 populations tested. Some resistance to imidacloprid was found in most of the test populations, since their LC50 values were significantly higher than the LC50 value for the susceptible population from Crossett. Resistance to imidacloprid did not increase in populations at 10 locations from 2006 to 2007 and their LC50 values were not significantly different in the two years they were compared. The LC50 for imidacloprid found in 2007 was significantly higher than the LC50 found in 2006 in populations at only 4 of the 19 test locations. These results showed that while some resistance to imidacloprid was present, the overall tendency was for the resistance to remain the same or decrease in the two years it was studied. The bioassay developed and used in the study is the only rapid method available for monitoring resistance in tarnished plant bug populations to the neonicitinoid insecticides. The baseline data for both imidicloprid and thiamethoxam provides a basis for comparison to determine changes in resistance in future monitoring studies.