Impacts of long-term insecticide treatment regimes on skdr and kdr pyrethroid resistance alleles in horn fly field populations

Authors: DOMINGUES, LUISA - US Department Of Energy; Guerrero, Felicito; FOIL, LANE - Louisiana State University

Submitted to: Parasitology Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2019
Publication Date: 7/6/2019
Citation: Domingues, L.N., Guerrero, F., Foil, L.D. 2019. Impacts of long-term insecticide treatment regimes on skdr and kdr pyrethroid resistance alleles in horn fly field populations. Parasitology Research. 118:2485–2497. https://doi.org/10.1007/s00436-019-06386-8.
DOI: https://doi.org/10.1007/s00436-019-06386-8

Interpretive Summary: Development of insecticide resistance in field populations of the horn fly, Haematobia irritans, leads to the inability to control this pest and increases the economic damage it causes worldwide. In the present study, we evaluated the effect of different horn fly control strategies on the incidence of the pyrethroid target site resistance gene mutations, known as kdr and superkdr (skdr), at four field research stations in Louisiana, USA. The objective of the study was to test if pyrethroid susceptibility could be returned to a fly population once pyrethroid resistance had become established. The treatments were: 1) use of pyrethroid ear tags for six consecutive years, 2) use of pyrethroid ear tags for three consecutive years, followed by one year with no treatment, and followed by two years with organophosphate ear tags, 3) use of pyrethroid ear tags for three consecutive years, followed by one year with no treatment, and followed by two years with endosulfan ear tags, and 4) a mosaic. Our results showed that the continuous use of pyrethroid ear tags for six years (treatment 1) caused a significant increase in the resistance ratio to pyrethroids as well as the frequencies of both skdr and kdr resistance mutations (%R mutation) mainly by increasing the frequency of the skdr-kdr SS-RR and SR-RR genotypes. At the treatment 2 research station where pyrethroids were used for three years followed by one year of no treatment and two years of organophosphate ear tags, the resistance to pyrethroid was not significantly affected. At this research station, the %R-kdr allele remained relatively high and stable after the six years of the study while the %R-skdr significantly dropped. Similarly, at the treatment 3 research station where pyrethroids were used for three years followed by one year of no treatment and two years of endosulfan ear tags, the %R-kdr allele remained relatively high and stable after the six years of the study while the %R-skdr significantly dropped. However, unlike treatment 2, treatment 3 resulted in a slight increase in resistance to pyrethroids. In the mosaic treatment 4, the resistance ratio for pyrethroids showed a 2.5-fold increase over the six years of the study, but the skdr-kdr genetic profiles did not change as the %R mutations ( for both kdr and skdr) remained low and stable through the six years. In treatments 2 and 3, where pyrethroids ear tags were used for the first three years, the percentage of the SR-RR genotype (skdr-kdr) significantly dropped at both research stations. This is likely due to overwintering fitness costs associated with the skdr allele and the RR kdr genotype. These results show the difficulty in maintaining the efficacy of pyrethroids in a resistant horn fly population.

Technical Abstract: Development of insecticide resistance in field populations of the horn fly, Haematobia irritans, leads to the inability to control this pest and increases the economic damage it causes worldwide. In the present study, we evaluated the effect of different control strategies on the incidence of the pyrethroid target site resistance alleles, kdr and superkdr (skdr) at four field research stations in Louisiana, USA. The treatments were: 1) use of pyrethroid ear tags for six consecutive years, 2) use of pyrethroid ear tags for three consecutive years, followed by one year with no treatment, and followed by two years with organophosphate ear tags, 3) use of pyrethroid ear tags for three consecutive years, followed by one year with no treatment, and followed by two years with endosulfan ear tags, and 4) a mosaic. Our results showed that the continuous use of pyrethroid ear tags for six years (treatment 1) caused a significant increase in the resistance ratio to pyrethroids as well as the frequencies of both skdr and kdr resistance alleles (%R allele), mainly by increasing the frequency of the skdr-kdr SS-RR and SR-RR genotypes. At the treatment 2 research station where pyrethroids were used for three years followed by one year of no treatment and two years of organophosphate ear tags, the resistance ratio for pyrethroid was not significantly affected. At this research station, the %R-kdr allele remained relatively high and stable after the six years of the study while the %R-skdr significantly dropped. Similarly, at the treatment 3 research station where pyrethroids were used for three years followed by one year of no treatment and two years of endosulfan ear tags, the %R-kdr allele remained relatively high and stable after the six years of the study while the %R-skdr significantly dropped. However, unlike treatment 2, treatment 3 resulted in a slight increase in resistance ratio to pyrethroids. In the mosaic treatment 4, the resistance ratio for pyrethroids showed a 2.5-fold increase but the skdr-kdr genetic profiles did not change as the %R alleles (kdr and skdr) remained low and stable through the six years. In treatments 2 and 3, where pyrethroids ear tags were used for the first three years, the percentage of the SR-RR genotype (skdr-kdr) significantly dropped at both research stations. This is likely due to overwintering fitness costs associated with the skdr allele and the RR kdr genotype.