Performance of Commercial Insecticide Formulations Against Different Developmental Stages of Insecticide-Resistant Tropical Bed Bugs (Hemiptera: Cimicidae)

Insecticide Resistance of Cimex lectularius L. Populations and the Performance of Selected Neonicotinoid-Pyrethroid Mixture Sprays and an Inorganic Dust

Insecticide resistance is one of the factors contributing to the resurgence of the common bed bug, Cimex lectularius L. This study aimed to profile the resistance levels of field-collected C. lectularius populations to two neonicotinoids and one pyrethroid insecticide and the performance of selected insecticide sprays and an inorganic dust. The susceptibility of 13 field-collected C. lectularius populations from the United States to acetamiprid, imidacloprid, and deltamethrin was assessed by topical application using a discriminating dose (10 × LD₉₀ of the respective chemical against a laboratory strain). The RR₅₀ based on KT₅₀ values for acetamiprid and imidacloprid ranged from 1.0-4.7 except for the Linden 2019 population which had RR₅₀ of ≥ 76.9. Seven populations had RR₅₀ values of > 160 for deltamethrin. The performance of three insecticide mixture sprays and an inorganic dust were evaluated against three C. lectularius field populations. The performance ratio of Transport GHP (acetamiprid + bifenthrin), Temprid SC (imidacloprid + β-cyfluthrin), and Tandem (thiamethoxam + λ-cyhalothrin) based on LC₉₀ were 900-2017, 55-129, and 100-196, respectively. Five minute exposure to CimeXa (92.1% amorphous silica) caused > 95% mortality to all populations at 72 h post-treatment.

Bacterial symbionts influence host susceptibility to fenitrothion and imidacloprid in the obligate hematophagous bed bug, Cimex hemipterus

The use of insecticides remains important in managing pest insects. Over the years, many insects manifested physiological and behavioral modifications resulting in reduced efficacy of insecticides targeted against them. Emerging evidence suggests that bacterial symbionts could modulate susceptibility of host insects against insecticides. Here, we explore the influence of host microbiota in affecting the susceptibility of insect host against different insecticides in the blood-sucking bed bug, Cimex hemipterus. Rifampicin antibiotic treatment resulted in increased susceptibility to fenitrothion and imidacloprid, but not against deltamethrin. Meanwhile, the host fitness parameters measured in the present study were not significantly affected by rifampicin treatment, suggesting the role of bacterial symbionts influencing susceptibility against the insecticides. 16S metagenomics sequencing revealed a drastic shift in the composition of several bacterial taxa following rifampicin treatment. The highly abundant Alphaproteobacteria (Wolbachia > 90%) and Gammaproteobacteria (Yersinia > 6%) in control bed bugs were significantly suppressed and replaced by Actinobacteria, Bacilli, and Betaproteobacteria in the rifampicin treated F1 bed bugs, suggesting possibilities of Wolbachia mediating insecticide susceptibility in C. hemipterus. However, no significant changes in the total esterase, GST, and P450 activities were observed following rifampicin treatment, indicating yet unknown bacterial mechanisms explaining the observed phenomena. Re-inoculation of microbial content from control individuals regained the tolerance of rifampicin treated bed bugs to imidacloprid and fenitrothion. This study provides a foundation for a symbiont-mediated mechanism in influencing insecticide susceptibility that was previously unknown to bed bugs.

Influences of Exposure Time and Mortality Assessment Interval on Bioassay Results of Insecticide-Resistant Tropical Bed Bugs (Hemiptera: Cimicidae)

Simple Summary

Bed bugs cause health and economic impacts due to their hematophagous behavior. The tropical bed bug, Cimex hemipterus (F.) is predominant in tropical and subtropical regions and modern strains demonstrate high levels of insecticide resistance making them a major challenge to control. In this present study, we investigated the effect of exposure time and mortality assessment interval on bioassay results. Bed bugs were treated separately with six insecticide products at label rates using the surface contact method, with multiple exposure times (5 min, 10 min, 30 min, 1 h, 4 h, and continuous exposure for 96 h or 14 days for Phantom). Post-treatment mortalities of bed bugs were assessed daily for up to 4 days (Tandem, Temprid SC, Pesguard FG161, Sumithrin, and Sumithion) and 14 days for Phantom. Results showed that longer exposure times led to rapid knockdown and better killing effect in bed bugs. Exposure times and mortality assessment intervals should be carefully considered in resistance and efficacy studies with insecticide products.

Abstract

We evaluated the influences of insecticide exposure time and mortality assessment intervals on the bioassay results of three insecticide-resistant tropical bed bug Cimex hemipterus (F.) populations (Madam Mo, Tanjong Tokong, and Green Lane). This was achieved using the surface contact method and tested with six commercial insecticide products: Tandem, Temprid SC, Pesguard FG161, Sumithrin, Sumithion, and Phantom applied at label rate on glass petri dishes. Six exposure times (5 min, 10 min, 30 min, 1 h, 4 h, and continuous exposure for 4 or 14 days for Phantom) were tested. A susceptible common bed bug Cimex lectularius L. strain (Monheim) was used as the reference strain as no susceptible C. hemipterus strain exists. In treatment with Temprid SC, the Tanjong Tokong strain showed significantly higher KT₅₀ values at 5, 10, and 30 min exposures than 1 h, 4 h, and continuous exposures. When all resistant populations were evaluated at continuous exposure to Sumithion, they demonstrated moderate resistance levels (RR₅₀ ranged from 6.0 to 7.9), while KT₅₀ and KT₉₅ of other shorter exposure times failed to be generated due to low knockdown rate. Higher mortalities were observed in Tanjong Tokong and Green Lane strains when tested at longer exposure times with Temprid SC, Pesguard FG161, Sumithrin, Sumithion, and Phantom. Better killing effect was observed in the treatment with Temprid SC (Tanjong Tokong and Green Lane strains), Pesguard FG161 (Tanjong Tokong and Green Lane strains), Sumithrin (all C. hemipterus strains), Sumithion (all C. hemipterus strains), and Phantom (all strains tested) at longer mortality assessment intervals. We demonstrated that insecticide exposure time and mortality assessment interval could potentially affect outcomes of product performance evaluations, resulting in underestimation or overestimation of insecticide resistance levels in field populations.

Infestation Pattern and Population Dynamics of the Tropical Bed Bug, Cimex hemipterus (F.) (Hemiptera: Cimicidae) Based on Novel Microsatellites and mtDNA Markers

The tropical bed bug, Cimex hemipterus (F.), has now emerged as an important public health pest in the tropics. Despite its alarming infestation rate, the information on its population genetics remains scarce. Here, we described the infestation structure and population dynamics of C. hemipterus in the tropics, especially Malaysia and Singapore, based on eight novel microsatellites and two mtDNA markers, including cytochrome c oxidase I (COI) and 16S rRNA genes. Across populations, microsatellite data revealed high genetic diversity with significant genetic differentiation and restricted gene flow. Analysis within populations revealed evidence of a recent bottleneck. Nonetheless, elevated genetic diversity in nearly all populations suggests that the propagule in C. hemipterus populations were much diverse, distantly related (mean r = 0.373), and not significantly inbred (mean F_IS = 0.24) than that observed in Cimex lectularius from previous studies. We observed seven mtDNA haplotypes across the 18 populations studied (Hd = 0.593) and several populations displayed more than one matrilineal descent. The two markers were generally congruent in suggesting a common, genetically diverse (especially at the nuclear region) source population with possibilities of multiple introductions for the bed bug populations in the present study.