A Novel Simulated-Use Test for Determining the Efficacy of Insecticides Against Bed Bugs (Hemiptera: Cimicidae)

Repellency of N,N-diethyl-3-methylbenzamide (DEET) during host-seeking behavior of bed bugs (Hemiptera: Cimicidae) in binary choice olfactometer assays

The bed bug (Cimex lectularius L.) is one of the most prolific and burdensome indoor pests, and suppression of bed bug populations is a global priority. Understanding bed bug behavior is important to the development of new tactics for their control. Major gaps exist in our understanding of how host cues, insecticide resistance, and exposure modality impact the repellency of formulated products to bed bugs. Here, we validate the use of a binary choice olfactometer for assessing bed bug repellency behaviors using N,N-diethyl-3-methylbenzamide (DEET) in a dose-dependent manner, while considering the role of host-associated stimuli (with vs. without CO2), exposure modality (olfactory vs. olfactory and contact), and resistance status (susceptible vs. resistant) on repellency. We observed that host-seeking insecticide-susceptible bed bugs were repelled only when olfactorily exposed to high concentrations of DEET. However, exposure to DEET by contact repelled insecticide-susceptible bed bugs at 100-fold lower dose of DEET. Further, we demonstrate for the first time that insecticide-resistant bed bugs were significantly more responsive to DEET than susceptible bed bugs. We conclude that the 2-choice olfactometer is an effective tool for assessing the behavioral responses of bed bugs to spatial and contact repellents.

Methods for Testing Repellents Against Bed Bugs (Hemiptera: Cimicidae)

Bed bug repellents should not only prevent humans from being bitten but impede an infestation of personal belongings. Only a few test proposals for evaluating the efficacy of repellents against bed bugs have been published so far. In the present study, two test systems were assessed for efficacy testing with five potential bed bug repellents (cinnamon oil, icaridin, N,N-diethyl-3-methylbenzamide (DEET), permethrin, and margosa extract). The first test setup was a harborage choice test system that consisted of a crystallizing dish with a treated and an untreated harborage. Sixty minutes and 24 h after treatment, DEET, icaridin, and cinnamon oil showed the highest repellency with a median proportion of at least 99% repelled bed bugs. The second test system was a barrier test. Bed bugs were attracted by CO2 and heat to cross filter papers treated with the potential repellents. The repellency of substances was significantly lower in comparison to the harborage choice test, except for DEET. The latter showed the highest repellency (97%) against bed bugs 24 h after application compared to controls. Results show that bed bugs are less sensitive to repellents when searching for a bloodmeal than when searching for a shelter.

Detection of target-site and metabolic resistance to pyrethroids in the bed bug Cimex lectularius in Berlin, Germany

Knockdown-resistance (kdr) against pyrethroids in bed bugs (Cimex lectularis) is associated with the presence of several point mutations in the voltage-sensitive sodium channel α-subunit gene and/or an increased metabolic detoxification by cytochrome P450 monooxygenases (CYPs). In the present study, pyrosequencing assays were developed to quantify the presence of the kdr substitutions (V419L or L925I substitution) in bed bugs in Berlin, Germany. In 14 of 17 bed bug field strains, pyrosequencing revealed the presence of the substitution L925I with allele frequencies between 30% and 100%. One field strain additionally carried the substitution V419L with allele frequencies of 40% in males and 96% in females. In seven of the 17 field strains, mRNA levels of four CYP genes were examined using RT-qPCR. Relative to a susceptible laboratory reference strain, five field strains showed significantly higher mRNA levels of cyp397a1 with 7.1 to 56-fold increases. One of these strains additionally showed a 4.9-fold higher mRNA level of cyp398a1 compared to the reference strain, while cyp4cm1 and cyp6dn1 showed no significant differences. Our findings indicate that multiple resistance mechanisms are present in German C. lectularius populations simultaneously.

Sublethal exposure to deltamethrin impairs maternal egg care in the European earwig Forficula auricularia

The application of pesticides typically leads to lethal and sublethal exposure of non-target insects. Whereas our current understanding of these sublethal effects typically focuses on reproductive and physiological parameters, recent works emphasize that sublethal effects on behaviors such as maternal care could be of major importance in non-target species. However, it remained unknown whether these sublethal effects occur in insects. Here, we tested if exposure to sublethal doses of deltamethrin - a pyrethroid insecticide commonly used in crops - alters the expression of maternal egg care in females of the European earwig Forficula auricularia, a predator insect and pest control. Our results first reveal that deltamethrin exposure impaired the expression of three forms of maternal egg care: It decreased the likelihood of mothers to gather their otherwise scattered clutch of eggs, increased the time during which the female abandoned the clutch after a predator attack and reduced egg grooming duration. These sublethal effects did not reflect a lower activity of deltamethrin-exposed females, as these females increased their expression of self-grooming, and deltamethrin exposure did not affect females' exploration and mobility. Finally, we found that the negative effects of deltamethrin on egg care did not modify egg development, hatching rate and juvenile weight, possibly due to the transient effects of deltamethrin on maternal behaviors. Overall, our results reveal that sublethal exposure to a pesticide may diminish maternal egg care in a natural pest control and call for the integration of this measurement in assays on pesticides application.

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.

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

This study examined the presence of insecticide resistance in different developmental stages (adults, first instars, and eggs) of the tropical bed bug, Cimex hemipterus (F.) using several insecticide formulations. Adults and first instars of five strains (Queensland, Kuala Lumpur, Bukit Mertajam, Saujana, and Krystal Point) were evaluated using the surface contact method and compared with a susceptible strain (Monheim) of the common bed bug Cimex lectularius L. The insecticide formulations were used at their label rates in this study: Tandem (thiamethoxam [11.6%], lambda-cyhalothrin [3.5%]) at 183.96 mg/m2; Temprid SC (imidacloprid [21%], beta-cyfluthrin [10.5%]) at 106.13 mg/m2; Sumithion 20CS (fenitrothion [20%]) at 250 mg/m2; Pesguard FG161 (d-tetramethrin [4.4%], cyphenothrin [13.2%]) at 110 mg/m2; and Sumithrin 10SEC (d-phenothrin [10%]) at 100 mg/m2. Results showed a very high level of resistance to Pesguard FG161 (388.3 to >605.0 times) and Sumithrin (302.9 to >365.5 times) in all adults of the strains tested, whereas low to high levels of resistance were registered for Tandem (1.4-4.7 times), Temprid (7.3-16.7 times), and Sumithion (1.2-14.6 times) for adults of all bed bug strains. For first instars, resistance to the former two formulations were high to very high (31.4-118.1 times). In contrast, they showed lower resistance to Tandem, Temprid, and Sumithion (1.0-10.2 times). An immersion method used to test on bed bug eggs found high to very high resistance toward all tested formulations. Results demonstrate that the resistance level varies between bed bug developmental stages.