Characterization of insecticide resistance across aquatic arthropods of the California Sacramento River watershed

Phenylpyrazole and pyrethroid insecticides are ubiquitous in waterways throughout the United States and impact aquatic invertebrate communities that are susceptible to their high concentrations common during rain events. In high exposure areas, the non-target crustacean Hyalella azteca, has evolved resistance to pyrethroid insecticides, protecting their populations from mortality. To better understand the extent of adaptive resistance within the invertebrate community of the Sacramento River watershed, CA, USA, aquatic arthropods were collected for evaluation of resistance mutations in two genes: resistance to dieldrin (rdl) and voltage-gated sodium channel (vgsc), the target genes of phenylpyrazole and pyrethroid insecticides. We targeted areas of the genes known to harbor resistance mutations and obtained partial sequences for rdl and vgsc across six arthropod orders. RDL protein sequences differed between crustaceans and hexapods at a critical amino acid (A301 in insects and S/G301 in crustaceans) associated with fipronil resistance in insects suggesting a genetic rationale for differences in fipronil sensitivity across arthropod orders. No resistance mutations were identified in the initial arthropods sequenced and a more extensive study of rdl and vgsc sequences in Chironomidae and Hyalellidae was performed across several sites in the Cache Slough. As seen previously, H. azteca populations showed evidence of resistance to pyrethroids, but no evidence of resistance was detected in Chironomidae to either insecticide class despite their high sensitivity to fipronil. To maintain populations in the Cache Slough, it is likely these chironomid populations are repopulated by flying adults from neighboring waterways following high episodic exposures of insecticides.