The current study was conducted as a risk assessment of potential evolution of insecticide resistance phenotypes and genotypes in Asian citrus psyllid (ACP) populations to fenpropathrin as a result of laboratory selection. We also investigated cross resistance of ACP to fenpropathrin with to other insecticide modes of action. The obtained results should contribute to development improved resistance management strategies.
First, assays and selection were performed with adults from a field-collected strain from Wauchula, FL on July 15, 2018 (WF). The bottle bioassay technique was used to determine susceptibility levels of adults of WF strain ACP to fenpropathrin. The chemical residues were achieved by pipetting 200 µL of acetone into a bottle and by rotating the bottle until the acetone evaporated. Subsequently, 5- 10 ACP adults were aspirated and transferred to the treated bottle. The LC50 was determined. Surviving individuals were reared on plants for eight generations. For each generation, adults of the WF strain were exposed to the LC50 concentration. We determined the risk assessment of ACP phenotypical resistance to fenpropathrin. After eight selected generations, the realized heritability of resistance (h2) to fenpropathrin was determined. The estimated h2 to fenpropathrin was 0.10 by the end of selection. The h2 of fenpropathrin resistance was 0.17 and 0.44 during the first and the second rounds of selection, respectively. The h2 values obtained at second round of selection are very high and could indicated a high level of risk in the field population for development of resistance to fenpropathrin. The results also suggest that a brief selection experiment may be sufficient to detect the potential for the development of resistance.
Second, we investigated level of resistance to pyrethoids in laboratory and investigated the possible mechanism involved cross resistance to two relatively commonly used insecticides, dimethoate and imidacloprid. Results indicated that there was no evidence of high cross resistance to imidaclorpid (RR = 1.54) and dimethoate (RR = 4.36) for the WF fenpropathrin-resistant strain. At this point, rotation of fenpropathrin, dimethoate and imidacloprid should not increase insecticide resistance. These results are particular important for verifying the effectiveness of the rotation schedules we are putting into practice in the field.
Third, we investigated pyrethroid resistance levesl and the associated molecular mechanisms in fenpropathrin resistant strain of ACP. The relative gene expression of six cytochrome P450s (CYP6A1, CYP6A2-1, CYP6A13, CYP6A14, CYP6J1 and CYP6K1) and four glutathione S-transferases (GST1, GST2B, GST3 and GST4) were quantified in the selected population, and compared with the laboratory susceptible population. qRT-PCR analysis showed that expression of CYP6A2-1 had significantly increased in the selected population relative to the laboratory susceptible population. Our results indicated that increased target insensitivity and cytochrome P450 metabolic detoxification could be mechanisms responsible for the ACP resistance to the pyrethroid fenpropathrin.
The results further confirmed that the multiple resistance mechanism following artificial selection on the field strain did not confer significant cross resistance to insecticides with other modes of action. Thus, we are able to recommend, based on a large body of evidence, that fenpropathrin, imidacloprid and dimethoate can be effectively rotated in sequence as an effective resistance management protocol for ACP.