Down Regulation of a Gene for Cadherin, but Not Alkaline Phosphatase, Associated with Cry1Ab Resistance in the Sugarcane Borer Diatraea saccharalis

The sugarcane borer, Diatraea saccharalis, is a major target pest of transgenic corn expressing Bacillus thuringiensis (Bt) proteins (i.e., Cry1Ab) in South America and the mid-southern region of the United States. Evolution of insecticide resistance in such target pests is a major threat to the durability of transgenic Bt crops. Understanding the pests' resistance mechanisms will facilitate development of effective strategies for delaying or countering resistance. Alterations in expression of cadherin- and alkaline phosphatase (ALP) have been associated with Bt resistance in several species of pest insects. In this study, neither the activity nor gene regulation of ALP was associated with Cry1Ab resistance in D. saccharalis. Total ALP enzymatic activity was similar between Cry1Ab-susceptible (Cry1Ab-SS) and -resistant (Cry1Ab-RR) strains of D. saccharalis. In addition, expression levels of three ALP genes were also similar between Cry1Ab-SS and -RR, and cDNA sequences did not differ between susceptible and resistant larvae. In contrast, altered expression of a midgut cadherin (DsCAD1) was associated with the Cry1Ab resistance. Whereas cDNA sequences of DsCAD1 were identical between the two strains, the transcript abundance of DsCAD1 was significantly lower in Cry1Ab-RR. To verify the involvement of DsCAD1 in susceptibility to Cry1Ab, RNA interference (RNAi) was employed to knock-down DsCAD1 expression in the susceptible larvae. Down-regulation of DsCAD1 expression by RNAi was functionally correlated with a decrease in Cry1Ab susceptibility. These results suggest that down-regulation of DsCAD1 is associated with resistance to Cry1Ab in D. saccharalis

Longitudinal Measurements of Tarnished Plant Bug (Hemiptera: Miridae) Susceptibility to Insecticides in Arkansas, Louisiana, and Mississippi: Associations with Insecticide Use and Insect Control Recommendations

Concentration-response assays were conducted from 2008 through 2015 to measure the susceptibility of field populations of Lygus lineolaris (Palisot de Beauvois) from the Delta regions of Arkansas, Louisiana, and Mississippi to acephate, imidacloprid, thiamethoxam, permethrin, and sulfoxaflor. A total of 229 field populations were examined for susceptibility to acephate, 145 for susceptibility to imidacloprid, and 208 for susceptibility to thiamethoxam. Permethrin assays were conducted in 2014 and 2015 to measure levels of pyrethroid resistance in 44 field populations, and sulfoxaflor assays were conducted against 24 field populations in 2015. Resistance to acephate and permethrin is as high or higher than that previously reported, although some populations, especially those exposed to permethrin, appear to be susceptible. Variable assay responses were measured for populations exposed to imidacloprid and thiamethoxam. Average response metrics suggest that populations are generally susceptible to the neonicotinoids, but a few populations from cotton fields experiencing control problems exhibited elevated LC50s. Efforts to associate variability in LC50s with recorded use of insecticides and estimated cotton insect losses and control costs suggest that intensive use of insecticides over several decades may have elevated general detoxifying enzymes in L. lineolaris and some field populations may be exhibiting resistance to multiple classes of insecticide. These results suggest that efforts should be made to manage these pests more efficiently with a reduced use of insecticides and alternative controls

Genetic Variation Within and Between Strains of the Fall Armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae)

Limited information exists on molecular genetic variation and distribution of the corn and rice strains of the fall armyworm, Spodoptera frugiperda (J.E. Smith). This study was conducted to investigate the genetic structure of S. frugiperda across a part of its range in the United States. A 608-base-pair portion of the mitochondrial cytochrome oxidase I and II genes was sequenced from 71 individuals resulting in three corn and four rice strain haplotypes. Genetic divergence between the two strains ranged from 0.66 to 0.99%. A 562-base-pair region of the nuclear ITS-1 gene was also amplified and sequenced from 17 individuals representing both corn and rice strains. No variation was detected in any of the samples for the ITS-1 region. Analysis of molecular variance was conducted on the resulting mtDNA haplotypes from the Arkansas and Florida populations and as a hierarchical analysis between populations in the two states. Results indicate a significant overall ΦST for all populations with the hierarchical analysis revealing that this significant ΦST is due to structuring of the populations between states. The observed genetic structure is possibly due to the distribution of fall armyworm strains