Huqi Liu

Insecticide Resistance and Cross-Resistance in Alabama and Florida Strains of Culex quinquefaciatus

Abstract Insecticide resistance and cross-resistance was determined for three strains of Culex quinquefasciatus Say in the southeastern United States. HAmCq and MAmCq strains were collected in 2002 from Huntsville and Mobile, AL. The VBFmCq strain was collected in 1998 from Vero Beach, FL. VBFmCq, HAmCq, and MAmCq larvae showed resistance to permethrin with resistance ratios of 13, 100, and 940, respectively, compared with the susceptible S-Lab strain. Levels of resistance in HAmCq and MAmCq larvae were 200- and 830-fold to resmethrin and 4- and 70-fold to malathion, respectively. VBFmCq, HAmCq, and MAmCq strains all demonstrated a great ability to develop tolerance and/or cross-resistance to different insecticides, including deltamethrin (50-, 100-, and 300-fold), chlorpyrifos (150-, 33-, and 720-fold), fipronil (10-, 5-, and 15-fold), and imidacloprid (7.5-, 5- and 10-fold, respectively). Comparison of resistance ratios for pyrethroids, organophosphates, and imidacloprid at LC50 and LC90 and gradual slopes of dose-response curves indicated that VBFmCq, HAmCq, and MAmCq were heterozygous in response to these insecticides. All three strains showed high levels of susceptibility to Bacillus thuringiensis variety israelensis (Bti) and spinosad, although these mosquitoes had been extensively exposed to Bti. Thus, we conclude that Bti and spinosad may be valuable for the management of Cx. quinquefasciatus, especially in situations where local strains are highly resistant to other insecticides.

Insecticide Resistance in Alabama and Florida Mosquito Strains of Aedes albopictus

Abstract The susceptibility of four strains of Aedes albopictus (Skuse) to permethrin, deltamethrin, resmethrin, chlorpyrifos, malathion, propoxur, fipronil, imidacloprid, spinosad, and Bacillus thuringiensis variety israelensis (Bti) was determined. The HAmAal and MAmAal strains were collected in 2002 and 2003, respectively, from Huntsville and Mobile, AL, and the VBFmAal and SFmAal strains were collected in 1998 from Vero Beach and southern Florida, respectively. The HAmAal strain showed a 22-fold elevated level of resistance to deltamethrin compared with the susceptible Ikaken laboratory strain, whereas the VBFmAal strain showed a six-fold lower sensitivity to deltamethrin compared with Ikaken. However, comparison of resistance ratios for deltamethrin at LC50 and LC90 (21-fold) and the gradual slopes of dose–response curves indicated that the field population of this mosquito strain was heterogenous in response to deltamethrin. All four mosquito strains showed elevated levels of resistance to chlorpyrifos, with resistance ratios from 10 to 33. Nevertheless, except for the relatively low resistance to deltamethrin and chlorpyrifos, all mosquito strains showed a similar susceptibility or lower tolerance to the remaining insecticides tested compared with the susceptible Ikaken strain, even though some, such as permethrin, resmethrin, malathion, and Bti, have been used in the field for a long time, especially in Alabama. These results indicate that the development of resistance to insecticides in Ae. albopictus is slow and conventional insecticides, such as permethrin, resmethrin, malathion, and Bti, and relatively new insecticides, such as fipronil, imidacloprid, and spinosad, may all be valuable for the management of this important mosquito.

Chlorpyrifos Resistance in Mosquito Culex quinquefasciatus

Abstract Two mosquito strains of Culex quinquefasciatus Say, MAmCq and HAmCq, were collected from Mobile and Huntsville, AL, respectively, after the control of mosquitoes with insecticides proved difficult. A synergism study showed that resistance to chlorpyrifos in MAmCq and HAmCq was not suppressed by piperonyl butoxide (PBO) and S,S,S,-tributylphosphorotrithioate (DEF), suggesting that P450 monooxygenase- and hydrolase-mediated detoxication does not contribute to chlorpyrifos resistance in either strain. Diethyl maleate (DEM) did not cause any significant change in the level of chlorpyrifos toxicity to HAmCq. However, DEM enhanced toxicity of chlorpyrifos to MAmCq 2.5-fold, indicating that glutathione S-transferase (GST)-mediated detoxication may play a minor role in the resistance of MAmCq. An inhibition study of acetylcholinesterase (AChE) by chlorpyrifos showed that bimolecular rate constants (Ki) of chlorpyrifos for the inhibition of AChE in adults and larvae of the susceptible S-Lab strain were 2.2- and 1.9-fold higher, respectively, than in the HAmCq strain and 3.4- and 3.8-fold higher than in the MAmCq strain. The single mutation, G119S, resulting from a single nucleotide polymorphism (SNP), G to A, in ace-1 acetylcholinesterase gene was present in HAmCq and MAmCq mosquitoes. The frequency of the heterozygote for the G119S mutant allele in the HAmCq and MAmCq mosquito populations was 0.25 and 0.45, respectively, and no individuals in either of these mosquito strains were homozygous for the A allele. It thus seems likely that the presence of heterozygous individuals for the G119S allele in HAmCq and MAmCq populations may be a response to the insensitivity of AChE observed in these two mosquito strains.

Sodium sulfite is a potential hypoxia inducer that mimics hypoxic stress in Caenorhabditis elegans

Physical and chemical hypoxia have been widely used in the study of hypoxic injury; however, both of these hypoxia models have their own limitations. Physical hypoxia is usually difficult to control and maintain. Chemical hypoxia, which is usually induced by chemical hypoxia-mimicking agents, such as CoCl2, may result in heavy metal toxicity or impose security threats. To develop a more suitable hypoxia model, we focused on sodium sulfite (Na2SO3) and evaluated its ability to remove dissolved oxygen in aqueous solutions. Our results showed that sodium sulfite successfully induced hypoxic conditions. The degree of hypoxia and the guarantee period of the sodium sulfite solution could be easily controlled by the concentration of soluble sodium sulfite. In addition, we used sodium sulfite to create a hypoxia model in Caenorhabditis elegans. Similar to physical hypoxia, the sodium sulfite solutions induced hypoxia-related death in the worms and led to morphologic cell defects and C. elegans hypoxia inducible factor 1 stabilization. Taken together, our data show that sodium sulfite is a potential hypoxia inducer that mimics hypoxic stress in C. elegans.

GLB-13 is associated with oxidative stress resistance in Caenorhabditis elegans.

Globins constitute a superfamily of heme‐binding proteins that is widely present in many species. There are 33 putative globins in the genome of Caenorhabditis elegans, where glb‐13 is a homolog of neuroglobin (Ngb) based on sequence analysis and specific expression in neurons. Here we examined whether glb‐13 as well as Ngb is also associated with resistance to reactive oxygen species (ROS) induced by paraquat. Our results showed that the mRNA level of glb‐13 was significantly upregulated after paraquat exposure. Expression of a green fluorescent protein (GFP) reporter gene directed by the glb‐13 promoter was increased by paraquat exposure. The mutant C. elegans strain glb‐13(tm2825) was sensitive to paraquat‐induced oxidative stress. Overexpression of human Ngb (hNgb) in C. elegans neuronal cells can rescue the paraquat sensitive phenotype of the mutant strain. glb‐13 mutation or hNgb overexpression did not affect the expression of antioxidant enzymes such as superoxide dismutase (SOD). To examine the ROS‐scavenging capabilities of hNgb and glb‐13, we further examined the level of ROS in glb‐13 mutant and hNgb transgenic (hNgb‐Tg) worms. There was no statistical difference in ROS levels in the untreated controls; however in paraquat‐treated worms, the ROS level was statistically repressed in the hNgb‐Tg relative to enhanced green fluorescent protein (EGFP)‐Tg worms or wildtype animals. Additionally, the ROS level of glb‐13 mutant was statistically higher than the wildtype animals. Furthermore, hNgb overexpression diminished the ROS level of glb‐13 mutant. In conclusion, hNgb can rescue the ROS sensitive phenotype of the glb‐13 mutant strain. The protein GLB‐13 seems to have an hNgb‐like function, suggesting the importance of the globin protein family in maintaining the homeostasis of ROS signals. Our data provided evidence for the first time that glb‐13 is associated with the resistance against oxidative stress‐induced toxicity.