Weeds resistant to PPO-inhibiting herbicides threaten the profitability of crop producers relying on this chemistry. In Amaranthus palmeri, mutations at G210 (∆G210) and R128 (R128G/M) of the PPX2 gene were reported to confer PPO-inhibitor resistance. Here, A. palmeri samples from nine states in America, having survived a field application of a PPO-inhibitor, were genotyped to determine the prevalence of these mutations. Less than 5% of the 1828 A. palmeri plants screened contained the ∆G210 mutation. Of the plants lacking ∆G210, a R128 substitution was only found in a single plant. An A. palmeri population from Alabama without mutations at G210 or R128 had a resistance ratio of 3.1 to 3.5 for fomesafen. Of the candidate PPX2 mutations identified in this population, only V361A conferred resistance to lactofen and fomesafen in a transformed bacterial strain. This is the first report of the V361A substitution of PPX2 conferred PPO-inhibiting herbicide resistance in any plant species. Future molecular screens of PPO-inhibitor resistance in A. palmeri and other species should encompass the V361A mutation of PPX2 to avoid false-negative results.

Limnocharis flava (L.) Buchenau is a problematic weed in rice fields and water canals of Southeast Asia, and in Malaysia this invasive aquatic weed species has evolved multiple resistance to synthetic auxin herbicide and acetohydroxyacid synthase (AHAS) inhibitors. In this study, it was revealed that, a single nucleotide polymorphism (SNP) at amino acid position 376, where C was substituted to G at the third base of the same codon (GAC to GAG), resulting in Aspartate (Asp) substitution by Glutamate (Glu) was the contributing resistance mechanism in the L. flava population to AHAS inhibitors. In vitro assay further proved that, all the L. flava individuals carrying AHAS resistance mutation exhibited decreased-sensitivity to AHAS inhibitors at the enzyme level. In the bensulfuron-methyl whole-plant bioassay, high resistance indices (RI) of 328- and 437-fold were recorded in the absence and presence of malathion (the P450 inhibitor), respectively. Similarly, translocation and absorption of bensulfuron-methyl in both resistant and susceptible L. flava populations showed no remarkable differences, hence eliminated the possible co-existence of non-target-site resistance mechanism in the resistant L. flava. This study has confirmed another new case of a target-site resistant weed species to AHAS-inhibitors.

UNASSIGNED: Insecticide resistance is one of the most important problems associated with the control of Musca domestica, due to the potential of the rapid development of resistance to different chemical insecticides. The present study was carried out to evaluate dichlorvos resistance in the house fly populations collected from central regions of Iran, Isfahan Province and Chaharmahal and Bakhtiari Province, during 2017 to 2019.
UNASSIGNED: Bioassays were carried out using a standard topical application method as well as a fumigation method. The Koohrang population (susceptible) with the lowest LD50 values to dichlorvos was chosen to calculate the resistance ratios (RR). Altered sensitivity of acetylcholinesterase (AChE), a target enzyme for dichlorvos, was investigated.
UNASSIGNED: According to the results, very high levels of dichlorvos resistance were observed in the Mobarake population (RR= 80.25-fold by topical application and 33-fold by fumigation bioassay), and Isfahan population (RR= 107.30-fold by topical application and 43-fold by fumigation bioassay) compared to the Koohrang population. Acetylcholinesterase of the Koohrang population was the most sensitive to inhibition by dichlorvos based on the determination of median inhibitory concentration (IC50), but AChE of Mobarake and Isfahan populations were 741.93- and 343.94- fold less sensitive to inhibition.
UNASSIGNED: The insensitivity of AChE was possibly involved in dichlorvos resistance in the house fly populations.

BACKGROUND: Anisantha and Bromus spp. are widespread and difficult to control, potentially due to the evolution of herbicide resistance. In this study, UK populations of four brome species have been tested for the early development of resistance to acetolactate synthase (ALS)-inhibiting herbicides commonly used in their control.
RESULTS: Glasshouse assays confirmed reduced sensitivity to ALS-inhibiting herbicides in single populations of A. diandra, B. commutatus and B. secalinus, and in three populations of A. sterilis. By contrast, all 60 brome populations tested were sensitive to the ACCase-inhibiting herbicide propaquizafop and glyphosate. Dose-response with two ALS herbicides showed broad-ranging resistance in the A. diandra, A. sterilis and B. commutatus populations. In the B. commutatus population, this was associated with a point mutation in the ALS enzyme conferring target site resistance (TSR). Additionally, resistant populations of A. sterilis and B. commutatus populations contained enhanced amounts of an orthologue of the glutathione transferase phi (F) class 1 (GSTF1) protein, a functional biomarker of nontarget site resistance (NTSR) in Alopecurus myosuroides. There was further evidence of NTSR as these plants also demonstrated an enhanced capacity to detoxify herbicides.
CONCLUSIONS: This study confirms the evolution of resistance to ALS inhibiting herbicides in brome species in the UK by mechanisms consistent with the evolution of both TSR and NTSR. These findings point to the need for increased vigilance in detecting and mitigating against the evolution of herbicide resistance in brome species in Northern Europe. © 2020 Society of Chemical Industry.

For many years, the emphasis of industry discovery programs has been on finding new target sites of pesticides and finding pesticides that inhibit single targets. There had been an emphasis on genomics in finding single targets for potential pesticides. There is also the claim that registration of single target inhibiting pesticides is simpler if the mode of action is known. Conversely, if one looks at the evolution of resistance from an epidemiological perspective to ascertain which pesticides have been the most recalcitrant to evolutionary forces, it is those that have multiple target sites of action. Non-target-site resistances can evolve to multi-target-site inhibitors, but these resistances can often be overcome by structural modification of the pesticide. Industry has looked at pest-toxic natural products as pesticide leads, but seems to have abandoned those where they can find no single target of action. Perhaps nature has been intelligent and evolved many natural products that are synergistic multi-target-site inhibitors, and that is why natural compounds have been active for millennia? We should be learning from nature while combining new chemistry technologies with vast accrued databases and computer aided design allowing fragment-based discovery and scaffold hopping to produce multi-target site inhibitors instead of single target pesticides. © 2019 Society of Chemical Industry.

We evaluated the effects of four different 6-year duration control strategies on the resistance levels and frequency of the pyrethroid target site resistance alleles, superkdr (skdr) and kdr, at four field populations of Haematobia irritans irritans (Linnaeus, 1758) (Diptera: Muscidae) in Louisiana, USA. Consecutive use of pyrethroid ear tags for 6 years caused a significant increase in the resistance ratio to pyrethroids as well as the frequencies of both skdr and kdr resistance alleles. After 3 years of consecutive use of pyrethroid ear tags, followed by 1 year with no treatment, and followed by 2 years with organophosphate ear tags, the resistance ratio for pyrethroid was not significantly affected, the %R-skdr significantly dropped while the %R-kdr allele remained relatively high and stable. Similar results were observed when pyrethroid ear tags were used for three consecutive years, followed by 1 year with no treatment, and followed by 2 years with endosulfan ear tags; however, this treatment resulted in a slight increase in the resistance ratio for pyrethroids. In a mosaic, the resistance ratio for pyrethroids showed a 2.5-fold increase but the skdr-kdr genetic profiles did not change, as the %R alleles (skdr and kdr) remained low and stable through the 6 years. Lack of exposure to pyrethroid insecticides for 3 years significantly affected the skdr mutation but not the kdr mutation, preventing re-establishment of susceptibility to pyrethroids. SS-SR (skdr-kdr) individuals were responsible for the maintenance of the kdr mutation in two of the populations studied, and fitness cost seems to strongly affect the SR-RR genotype. None of the four treatment regimens evaluated in the study had satisfactory results for the management of kdr resistance alleles.

BACKGROUND: Bidens subalternans (greater beggarticks) is a tetraploid and troublesome weed infesting annual crops in most tropical regions of the world. A glyphosate-resistant (GR) B. subalternans biotype was detected in a soybean field from Paraguay. A series of physiological and molecular analyses were conducted to elucidate its resistance mechanisms.
RESULTS: The GR biotype had a high level of resistance (> 15-fold LD50 ), relative to a glyphosate-susceptible (GS) biotype. Shikimate accumulation was up to ten-fold greater for GS compared with GR. We found no differences in sensitivity when plants were treated and kept under lower (10/4 °C) or higher temperatures (25/20 °C). GS and GR had the same relative EPSPS gene copy number, and similar glyphosate absorption and translocation rates. Neither biotype metabolized glyphosate. A double amino acid substitution (TIPT - Thr102Ile and Pro106Thr) was found in only one EPSPS allele from one of the two EPSPS homoeologs present in tetraploid GR B. subalternans.
CONCLUSIONS: This is the first report of a TIPT double mutation conferring high levels of glyphosate resistance in a weed species. The presence of both wild-type and TIPT mutant EPSPS on the polyploid genome of GR B. subalternans may offset a potential fitness cost, requiring additional research to confirm the absence of deleterious effects. © 2019 Society of Chemical Industry.

The introduction of glyphosate-resistant (GR) crops revolutionized weed management; however, the improper use of this technology has selected for a wide range of weeds resistant to glyphosate, referred to as superweeds. We characterized the high glyphosate resistance level of an Amaranthus hybridus population (GRH)-a superweed collected in a GR-soybean field from Cordoba, Argentina-as well as the resistance mechanisms that govern it in comparison to a susceptible population (GSH). The GRH population was 100.6 times more resistant than the GSH population. Reduced absorption and metabolism of glyphosate, as well as gene duplication of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) or its overexpression did not contribute to this resistance. However, GSH plants translocated at least 10% more 14C-glyphosate to the rest of the plant and roots than GRH plants at 9 h after treatment. In addition, a novel triple amino acid substitution from TAP (wild type, GSH) to IVS (triple mutant, GRH) was identified in the EPSPS gene of the GRH. The nucleotide substitutions consisted of ATA102, GTC103 and TCA106 instead of ACA102, GCG103, and CCA106, respectively. The hydrogen bond distances between Gly-101 and Arg-105 positions increased from 2.89 Å (wild type) to 2.93 Å (triple-mutant) according to the EPSPS structural modeling. These results support that the high level of glyphosate resistance of the GRH A. hybridus population was mainly governed by the triple mutation TAP-IVS found of the EPSPS target site, but the impaired translocation of herbicide also contributed in this resistance.

BACKGROUND: Insecticide resistance threatens malaria control in sub-Saharan Africa. Knockdown resistance to pyrethroids and organochlorines in Anopheles gambiae sensu lato (s.l.) is commonly caused by mutations in the gene encoding a voltage-gated sodium channel which is the target site for the insecticide. The study aimed to examine risk factors for knockdown resistance in An. gambiae s.l. and its relationship with malaria infection in children in rural Gambia. Point mutations at the Vgsc-1014 locus, were measured in An. gambiae s.l. during a 2-year trial. Cross-sectional surveys were conducted at the end of the transmission season to measure malaria infection in children aged 6 months-14 years.
RESULTS: Whilst few Anopheles arabiensis and Anopheles coluzzii had Vgsc-1014 mutations, the proportion of An. gambiae sensu stricto (s.s.) mosquitoes homozygous for the Vgsc-1014F mutation increased from 64.8 to 90.9% during the study. The Vgsc-1014S or 1014F mutation was 80% higher in 2011 compared to 2010, and 27% higher in the villages with indoor residual spraying compared to those without. An increase in the proportion of An. gambiae s.l. mosquitoes with homozygous Vgsc-1014F mutations and an increase in the proportion of An. gambiae s.s. in a cluster were each associated with increased childhood malaria infection. Homozygous Vgsc-1014F mutations were, however, most common in An. gambiae s.s. and almost reached saturation during the study meaning that the two variables were colinear.
CONCLUSIONS: As a result of colinearity between homozygous Vgsc-1014F mutations and An. gambiae s.s., it was not possible to determine whether insecticide resistance or species composition increased the risk of childhood malaria infection.

The brown dog tick, Rhipicephalus sanguineus sensu lato (Latreille), is a cosmopolitan ectoparasite and vector of pathogens that kill humans and animals. Pyrethroids represent a class of synthetic acaricides that have been used intensely to try to control the brown dog tick and mitigate the risk of tick-borne disease transmission. However, acaricide resistance is an emerging problem in the management of the brown dog tick. Understanding the mechanism of resistance to acaricides, including pyrethroids, is important to adapt brown dog tick control strategies. The main objective of this study was to determine if target-site mutations associated with pyrethroid resistance in other pests could be associated with phenotypic resistance detected in a brown dog tick population from Florida. We amplified segment 6 of the domain III of the voltage-sensitive sodium channel protein, using cDNAs synthesized from pyrethroid-susceptible and pyrethroid-resistant tick strains. A single nucleotide point mutation (SNP) identified in a highly conserved region of domain III S6 in the resistant ticks resulted in an amino acid change from phenylalanine to leucine. This mutation is characteristic of resistance phenotypes in other tick species, and is the first report of this mutation in R. sanguineus. Molecular assays based on this knowledge could be developed to diagnose the risk for pyrethroid resistance, and to inform decisions on integrated brown dog tick management practices.