Shao-Hung Lee

BACKGROUND: Insecticide baits are commonly used to control the German cockroach [ Blattella germanica (L.)]. Because bait performance is dependent on foraging and feeding behavior, forced external exposures with bait active ingredients are often used to estimate physiological susceptibility without behavioral confounders. However, relying on external contact methods may not be appropriate for baits because of their oral route of entry. To increase the utilization of oral exposures for German cockroaches, we describe a novel method of conducting precise oral dosing of insecticides in an accessible and practical manner. RESULTS: Peanut butter was used as the delivery medium in quantities of 1.5 mg per adult male. Oral lethal doses of fipronil, indoxacarb, and dinotefuran were determined for the laboratory susceptible UCR and field‐collected Dav strains, resulting in resistance ratios (RR ₅₀ ) of 14.4 (fipronil), 7.3 (indoxacarb), and 3.5 (dinotefuran) for the Dav strain. Bait bioassays using commercial fipronil, indoxacarb, and dinotefuran baits were performed to validate the method and compare with the lethal doses. CONCLUSION: The oral‐dosing method was compatible with both laboratory and field‐collected strains, providing an accessible way to enhance the accuracy of German cockroach bait toxicity studies in the future. © 2026 Society of Chemical Industry.

Background: For decades, insecticides have been central to controlling the yellow fever mosquito, Aedes aegypti (L.), but extensive use has driven resistance development. This study investigates resistance of Ae. aegypti to pyrethroids (permethrin, deltamethrin) and organophosphates (malathion, pirimiphos-methyl) and their underlying mechanisms across Malaysia, Thailand, Indonesia, and the USA. Methods: Adult female Ae. aegypti (3–5 days old, non-blood-fed) were subjected to World Health Organization (WHO) tube bioassays using 0.4% permethrin, 0.03% deltamethrin, 5% malathion, and 60 mg/m ² pirimiphos-methyl. Each assay included four replicates of 25 mosquitoes, with mortality assessed at 24 h post-exposure. Genomic DNA was extracted from 10 resistant individuals per population, and two coding regions of the voltage-gated sodium channel (VGSC) gene (domains II and III) were amplified and sequenced to detect known and novel kdr mutations. For biochemical analysis, 40 newly emerged, non-blood-fed females per strain were individually homogenized to quantify mixed-function oxidase (MFO), esterase (α- and β-EST), glutathione S-transferase (GST), and acetylcholinesterase (AChE) activity. Results: High resistance levels were recorded in Malaysian and US Ae. aegypti strains, with low mortality ranging between 9% and 22% for pyrethroids. New mutations T1520I (8–15%) and I1011M (10–15%) were identified in Malaysian populations, the first detection of T1520I in the country, while V1016I (10%) was newly detected in Indonesian strains. Malaysian mosquitoes had multiple kdr mutations (S989P, V1016G, F1534C, and T1520I) in triple- and quadruple-haplotype combinations. The US Riverside strain showed a nine- to 10-fold increase in β-EST and three- to fivefold increase in MFO and GST activity compared to the VCRU susceptible strain, indicating strong metabolic resistance. In contrast, the highly resistant Malaysian Hamna strain exhibited no significant upregulation ( P > 0.05) in detoxifying enzymes, suggesting resistance was driven primarily by kdr mutations. Thai strains lacked kdr mutations but exhibited altered AChE (20–35% remaining activity) and elevated GST (2–3 times higher than control). Conclusions: The detection of novel kdr mutations and diverse resistance mechanisms underscores the adaptability of Ae. aegypti to insecticide pressure and highlights the urgent need for continuous monitoring and integrated resistance management strategies. Graphical

The German cockroach, Blattella germanica (L.), is a major urban insect pest primarily controlled by insecticides, including pyrethroid sprays, fipronil baits, and indoxacarb baits. However, widespread pyrethroid resistance increases the probability that cockroaches will survive treatments in the field. Many insecticides are applied concurrently or repetitively on a periodic basis, meaning survivors have the chance to be re-treated, but any lasting physiological effects from initial survival may impact the performance of subsequent treatments. We investigated the effects of sublethal deltamethrin exposure on the susceptibility of susceptible (UCR) and resistant (Ryan) strains of German cockroaches. Ineffective pyrethroid treatment was simulated by treating cockroaches with the LD20 of deltamethrin. Survivors were treated again with the same dose of deltamethrin, a food source containing the LC50 of fipronil, or a food source containing the LC50 of indoxacarb. Both strains of cockroaches experienced greater mortality when treated with deltamethrin 3 d (49.8% to 67.6%), after the sublethal exposure. No impact on fipronil or indoxacarb susceptibility was observed after pre-treatment with deltamethrin. Because surviving a deltamethrin treatment did not induce tolerance to deltamethrin or other insecticides, this suggests no within-generation drawback of current conventional management practices of applying multiple treatments in the same site. Furthermore, sequential exposure to deltamethrin caused greater mortality than expected, indicating that this insecticide may remain useful even if initial efficacy is low. Comprehensive examinations of additional resistant strains treated with formulated products are warranted to understand whether these effects may impact field control.

The management of subterranean termite pests remains a major challenge in Southeast Asia, where these pests cause significant structural and economic damage. Termite baiting has emerged as an effective option to conventional soil termiticides, offering a safer pest management approach with reduced chemical input into the environment. In this paper, we review the history of termite research in Southeast Asia, highlighting the turning points of termite research, from agriculture and plantations to buildings and structures, and the transformative impact of termite baiting on the pest management industry in the region over the last 25 yr. We also discuss the outcome of a survey of pest management professionals on their baiting practices, bait performance, and reinfestation rates. All bait products eliminated termite colonies. There were significant differences in terms of the baiting period to colony elimination, with Xterm outperforming Sentricon, Exterra, and Exterminex. Above-ground (AG) baiting was preferred over in-ground (IG) baiting due to construction constraints and low IG station interception rates. While bait effectively controlled Coptotermes spp., challenges persist in managing fungus-growing termites such as Macrotermes gilvus Hagen. Reinfestation occurred in < 10% of baited premises.

This study investigated how management strategies influence resistance profiles in German cockroach (Blattella germanica (L.)) populations and their impact on the performance of commercial gel baits containing fipronil, imidacloprid, and indoxacarb. Field populations from premises managed under 3 different strategies: Baiting, random insecticide (RI) used, and insecticide rotation (IR) were tested. Almost all populations under RI and IR were resistant to deltamethrin, but low to moderate resistance was observed under the Baiting approach. Cytochrome P450 monooxygenases (P450) were involved in deltamethrin resistance in these resistant populations. All individuals under Baiting and RI were homozygous for the L993F mutation, but the populations under IR lacked homozygous-resistant individuals. Eighty-three percent of field populations with complete homozygosity for the Rdl mutation displayed low mortality upon exposure to 3× LD₉₅ fipronil. The effect of P450 and the Rdl mutation conferred high fipronil resistance in populations under the Baiting approach, recording moderate performance indices (PI) of 44–67 in fipronil bait. By contrast, those populations under RI and IR, in which involve glutathione S-transferases in fipronil resistance, had high PIs of 78–93. Almost 80% of populations exhibited over 90% mortality at 3× LD₉₅ indoxacarb treatment, accompanied by high PIs of 90–100 in indoxacarb bait. Partial mortality from 1× LD₉₅ imidacloprid occurred across all field populations due to the involvement of P450. PIs of imidacloprid bait ranged 5–57 and 20–94 in populations under RI and IR, respectively. Field populations demonstrate different resistance profiles depending on the treatment regimes, and the resistance mechanisms involved influenced gel bait’s effectiveness.

Isocycloseram is a new insecticide in the isoxazoline class that targets insect GABA-gated chloride channels. In this study, we evaluated a cockroach gel bait formulation containing 1% isocycloseram against a susceptible strain (UCR) and 5 field-collected strains (WM, RG386, Ryan, CDR, and SY) of the German cockroach, Blattella germanica (L.) (Blattodea: Ectobiidae), and compared it with several commercial insecticide baits in the laboratory. Using the Ebeling choice box method, we also tested a residual deposit of an SC formulation of isocycloseram against the UCR, RG386, and Ryan strains. The isocycloseram bait was among the fastest-performing treatments against adult males (mean survival time: 0.9–2.7 days) and mixed stages and sexes (mean survival time: 1.4–5.4 days) across all strains. Secondary transfer effects of the bait were demonstrated in the UCR strain by exposing new adult males to individuals killed by direct bait treatment. Physiological resistance was not detected in the WM, CDR, and RG386 strains with topical treatment of a diagnostic dose (3× LD₉₅) of isocycloseram developed using the UCR strain. However, topical assays revealed resistance ratios (RR₅₀) of 1.6 and 3.0× in the Ryan and SY strains, respectively. The performance of a 0.05% isocycloseram residual application against the Ryan strain was improved with the addition of piperonyl butoxide.

We investigated the expression of 4 cytochrome P450 genes (CYP4G19, CYP6J1, CYP6K1, and CYP4C21) in 4 field-collected strains (WM, RG386, CDR, and Ryan) of the German cockroach, Blattella germanica (L.), collected from California. The UCR susceptible strain was used as a comparison. Topical assays using a diagnostic dose (3× LD₉₅) of deltamethrin revealed decreased sensitivity in all field-collected strains with mortality ranging from 0% to 58%, and the addition of PBO before deltamethrin treatment increased mortality to 52.5%–87.5%. Using qPCR to investigate the expression levels of CYP4G19, CYP4C21, CYP6J1, and CYP6K1, we found that only CYP6K1 was significantly overexpressed (2.1–5.8× higher) in all field-collected strains when compared to the UCR strain. Next, we investigated the role of the CYP6K1 gene by performing gene knockdown using RNAi. After dsCYP6K1 treatment, the expression levels of CYP6K1 in WM and Ryan strains were significantly reduced (P < 0.01) by 91%–94% vs. those treated with dsEGFP (control) on the third and sixth day posttreatment. RG386, CDR, and Ryan strains were more susceptible compared to their respective controls to topically applied deltamethrin 6 days after treatment with dsCYP6K1. This study provides evidence of the involvement of the P450 CYP6K1 gene in pyrethroid resistance in some populations of German cockroaches.

Artificial or non-nutritive sweeteners are indigestible by most animals. Some sweeteners are orally toxic to insects and have received recent interest as potential safe insecticides due to their low mammalian toxicity. In this study, we investigated the oral toxicity of sucralose on insecticide-susceptible and resistant German cockroaches, Blattella germanica (L.). In a nonchoice test, we evaluated 5, 10, and 20% sucralose solutions. Depending on the cockroach strains, mean mortality ranged from 62.5 to 92.5%, 15 to 55%, and 2.5 to 27.5% for 20, 10, and 5% sucralose, respectively. Next, we measured the impact of a 20% sucralose treatment on water loss rates in the cockroach strains. All strains lost 23.0–30.29% of body water by 6 d. Dehydrated cockroaches were more prone to be killed by sucralose than nondehydrated ones. Lastly, we evaluated the effect of 20% sucralose treatment on gut bacterial composition and found the diversity of gut bacteria in treated cockroaches was significantly reduced after 3 days, implicating a rapid change in the alimentary environment.