Beta-cyfluthrin Resistance of Cigarette Beetle Lasioderma serricorne (F.) (Coleoptera: Anobiidae) and its Control Using Imidacloprid Space Sprays

INTRODUCTION

The cigarette beetles, Lasioderma serricorne (F.) (Coleoptera: Anobiidae), belong to a group of insects that infest stored products, and feed and live in many kinds of dried substances, such as cloth, paper, furniture, grain, coffee, and tobacco [1]. As a cosmopolitan and polyphagous pest, L. serricorne is generally considered to be one of the most destructive insects in tobacco processing and cigarette manufacturing, due to its capacity to develop in tobacco which is toxic for most other species [2-6]. In addition to weight losses of tobacco leaves caused by larvae feeding during tobacco processing and storage, if beetles get into final cigarette products, it can lead to customer complaints and even cause a cigarette brand to disappear from the market, resulting in huge economic losses [3, 7].

Like pest control in food processing and stored factories, management of L. serricorne in cigarette manufacturing areas has always depended heavily on sanitation, including promptly removing waste material and eliminating infected tobacco shreds [8]. However, merely cleaning in cigarette manufacturing area is not enough to control the pest completely, and the application of space aerosol insecticide is often used as a supplementary method [8, 9]. Application of residual and contact insecticide aerosols can effectively prevent pest infestation in food-processing facilities with several advantages, including low cost, short treatment time, and can be integrated with other management tactics such as sanitation [10, 11]. The application of insecticides as aerosols for managing stored-product pests is a well-established technique and was initially used for controlling L. serricorne in tobacco warehouses in the 1950s with dichlorvos (DDVP) aerosol [12]. It was reported that DDVP aerosol could effectively control stored-product pests, including black carpet beetles Attagenus megatoma (Fab.) (Coleoptera: Dermestidae), confused flour beetles Tribolium confusum (Jacquelin du Val) (Coleoptera: Pseudodontidae), L. serricorne, red flour beetles Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) and almond moths Cadra cautella (Walker) (Lepidoptera: Pyralidae) in laboratory and field [10, 12-15]. However, DDVP aerosol has been discontinued because of the development of pest resistance and its high toxicity to mammals and the environment. At present, widely used pesticides for such applications mainly include pyrethroids and insect growth regulators. There have been quite a few reports on successful control of various insects, for example, prallethrin aerosol for C. cautella, Indian meal moths Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae), saw-toothed grain beetles Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae), Trogoderma variabile Ballion (Coleoptera: Dermestidae) and T. castaneum under laboratory conditions [16], cyfluthrin aerosol for T. castaneum under laboratory conditions [17], esfenvalerate aerosol for Plodia interpunctella under laboratory conditions [18], and pyrethrin aerosol for T. castaneum and T. confusum in laboratory and a commercial food-storage factory [11, 19-22].

It’s well known that beta-cyfluthrin is a synthetic pyrethroid with low environmental persistence, low toxicity to non-target organisms, and no bioaccumulation. In addition, this pesticide is also a broad-spectrum neurotoxin with a demonstrable effect in killing L. serricorne [20, 23]. A chemical insecticide that can be used in cigarette manufacturing factories is limited, and the insecticide resistance of L. serricorne is of great concern. Beta-cyfluthrin aerosol, as an assistant to sanitation, has been used for years to control L. serricorne in Hefei Cigarette Manufacturing Factory (Hefei City, Anhui province, China). Therefore, it is imperative to determine the sensitivity level of L. serricorne to beta-cyfluthrin for maintaining control efficiency and resistance management.

Herein, we assessed beta-cyfluthrin susceptibility level in the population of L. serricorne collected from Hefei Cigarette Manufacturing Factory. After multiple rounds of bioassays with different insecticides (not shown in this paper except imidacloprid), we finally selected imidacloprid as an alternative insecticide. Subsequent field trials were carried out in the cigarette manufacturing area of Hefei Cigarette Manufacturing Factory to evaluate the control effect of space spray with imidacloprid aerosol in practice. Our study is conducive to timely recognizing the development of insecticide resistance of L. serricorne in cigarette manufacturing factories and provides key suggestions for pest resistance management.

MATERIALS AND METHODS

Insecticide, pheromone trap, atomizer, and spray tower

Insecticides used in this study were commercial formulations, as follows: beta-cyfluthrin (12.5% [a.i.]), suspension concentrate (SC), provided by Jiangsu Yangnong Chemical Co. LTD. (Jiangsu, China), imidacloprid (100 g [a.i] L^-1, SC), provided by Zhejiang Haizheng Chemical co. LTD. (Zhejiang, China).

Pheromone traps were used to monitor the number of insects captured in the cigarette manufacturing area of Hefei Cigarette Manufacturing Factory, provided by Japan Fuji Flavor Co. LTD. (Japan). A lure permitting the controlled release of sex pheromones is mounted on the sticky board that catches beetles for counting.

Electric atomizer (Nanjing Minghai Health Technology Development Co. LTD., Nanjing, China) was used in field trials for delivering spray particles of 50 ± 10 μm in size, with spray efficiency of 500 mL/min, Hand-held ultra-low volume atomizer gun (Aima Instrument Factory, Guangdong, China) with calibration was used to produce mist in spray toxicity bioassay.

The cylindrical glass spray tower was made by Beijing Wuyi Glass Instrument Factory (Beijing, China) with spray settings at a volume of 10 L and, a height of 40 cm. The inner diameter of the tower bottom is 20 cm, and 3 cm for the tower mouth. The Spray tower has no upper and lower bases but has a glass tower cap.

Insect strains and rearing conditions

Tobacco leaf fragments and tobacco shorts were carefully collected from processing machinery in the cigarette manufacturing area of Hefei Cigarette Manufacturing Factory and brought back to the laboratory. With the aid of a stereoscopic microscope, larvae of L. serricorne were searched and collected from fragments and tobacco shorts and reared into adults (LS_H). LS_H strain was cultured for more than three generations in the laboratory before bioassays. Reference susceptible strain (LS_LAB) was obtained from our laboratory with no pesticide exposure for several years. Both strains were maintained in dark incubators with a temperature of 28 ℃ and relative humidity of 65-75%. Wheat raw material, previously frozen for more than 7 days under -18 degrees, was crushed into small particles. L. serricorne was reared on wheat particles mixed with active yeast (10:1, w/w). Unsexed adults of 1-3 days were used for the following bioassays.

Spray toxicity bioassay

In this study, spray toxicity bioassays were conducted to study the toxicity of beta-cyfluthrin and imidacloprid against L. serricorne in the laboratory as suggested by relative literature [24, 25]. Preliminary trials were carried out to determine appropriate ranges of testing concentration, with 156.25 mg mL^-1, 83.64 mg mL^-1, 41.82 mg mL^-1, 13.94 mg mL^-1, 6.35 mg mL^-1, 3.17 mg mL^-1 for LS_H strain, and 57.11 mg mL^-1, 19.04 mg mL^-1, 6.35 mg mL^-1, 3.17 mg mL^-1, 0.63 mg mL^-1, 0.13 mg mL^-1, 0.03 mg mL^-1 for LS_LAB strain, and distilled water as the control in beta-cyfluthrin spray toxicity bioassay. In imidacloprid spray toxicity bioassay, testing concentrations of LS_H strain and LS_LAB strain were 1.75 mg mL^-1, 0.35 mg mL^-1, 3.50 ´ 10^-2 mg mL^-1, 3.50 ´ 10^-3 mg mL^-1, 3.50 ´ 10^-4 mg mL^-1 and 3.50 ´ 10^-5 mg mL^-1, and distilled water as the control. In the laboratory, at 27 °C, the spray tower was placed on a flat marble tabletop to ensure that the bottom of the spray tower fitted perfectly with the tabletop to prevent insects from escaping. After slowly introducing 20 adults to the tower bottom from the tower mouth, 0.5 mL liquid was sprayed into the spray chamber at the tower mouth with a hand-held ultra-low volume atomizer gun, at a distance of 35 cm. The tower mouth was covered with a glass cap immediately (not sealed). Dead individuals were documented 2 and 24 hours after treatment under dark conditions. An adult was considered dead if its appendages did not move when prodded with a hairbrush [26]. Three replicates were conducted for each dose.

Field trials

Field trials were carried out in the cigarette manufacturing area (shred-making section, storage section, blending section, cigarette-making section, and ripper section) of Hefei Cigarette Manufacturing Factory in Hefei, Anhui, China. Before the test, five sections worked normally and were cleaned normally following written sanitation procedures. Pheromone traps were used to monitor the number of insects captured [8, 11, 27-31]. The number and hanging height of the pheromone trap in each section were based on CORESTA [8]. Traps were placed on steel nails of the wall or column and marked for position. The numbers of L. serricorne caught by traps were recorded once a week, and traps were replaced every 30 days.

The details of the cigarette manufacturing area in the Hefei Cigarette Manufacturing Factory and the locations of traps in each section can be seen in Figure 1. Shred shredding section has a construction area of 1600 m² (20 m ´ 80 m), a height of 10 m, and thus, a volume of 16000 m³. A total of 9 pheromone traps were placed evenly in this section, with 8 traps placed on 80-meter-long walls, at 1.5 m above the ground, and one trap placed on the center column at 5 m above the ground. The storage section has a construction area of 600 m² (20 m ´ 30 m), a height of 5 m, thus, a volume of 3000 m³. A total of 3 pheromone traps were placed evenly on three non-entrance walls, at 1.5 m above the ground. The blending section has a construction area of 600 m² (20 m ´ 30 m), a height of 5 m, thus, a volume of 3000 m³. A total of 3 pheromone traps were placed evenly on three non-entrance walls, at 1.5 m above the ground. The cigarette-making section has a construction area of 1200 m² (20 m ´ 60 m), a height of 5 m, thus, a volume of 6000 m³. A total of 6 pheromone traps were placed evenly on 60-meter-long walls, at 1.5 m above the ground. The ripper section has a construction area of 200 m² (20 m ´ 10 m), and a height of 5m, thus, a volume of 1000 m³. Only one pheromone trap was placed on the center column, at 1.5 m above the ground. Temperature was at 26 ± 2 ℃ and relative humidity was 60 ± 5% in all, but the ripper section.

Table 1. Toxicity of insecticides to LS_LAB strain and LS_H strain of L. serricorne at 2 h and 24 h in bioassays (in the laboratory, at 27 ℃).

^aLS_LAB, reference susceptible strain; LS_H, L. serricorne collected from Hefei Cigarette Manufacturing Factory.

^bFiducial Limit.

^cDegree of freedom.

^dP ³ 0.05 indicates a significant fit between the observed data and the expected linear regression model in Probit analysis.

^eNumber of insects used totally

^fResistance Ratio: LC₅₀ of LS_H strain/LC₅₀ of LS_LAB strain.

Pyrethroids are the widely used insecticides to control L. serricorne in cigarette manufacturing in China, but given that pyrethroids have been used for a long time, it is imperative to investigate the control effect in the laboratory and field. Our study indicated that the LS_H strain had developed high resistance (up to 15.10-fold) to beta-cyfluthrin after years of exposure, and this insecticide failed to control pests in the laboratory and field. In this case, pyrethroids were incapable of being used for suppressing L. serricorne for a while. Our study also showed that the LS_H strain had high sensitivity to the neonicotinoid insecticide, imidacloprid. As we know, cross-resistance of insects to insecticides must be taken into account when selecting alternation of insecticides in insecticide resistance management programs. Metabolic mechanisms of insect resistance to pyrethroid and neonicotinoids have been extensively studied. Widespread resistance to neonicotinoid insecticides was associated with the elevation of cytochrome P450 activity and overexpression of cytochrome P450 genes [32]. Research also suggested that enhanced cytochrome P450 activity contributed to the development of pyrethroid resistance and that might be the main factor of pyrethroid resistance. In addition, the decrease in sodium channel sensitivity was also an important pyrethroid resistance mechanism that could not be ignored [33-37]. In this study, although these two insecticides have similar metabolic mechanisms of resistance, our results indicated that the LS_H strain exhibited no cross-resistance to imidacloprid with a cross-resistance ratio of only 1.00-1.33 folds (Table 1), which was an important factor that imidacloprid was able to effectively suppress L. serricorne in the field. Several studies have shown that there was no cross-resistance between insecticides that had similar metabolic mechanisms of resistance. Chen et al. [38] suggested that the overexpression of three cytochrome P450 genes and enhanced cytochrome P450 activity contributed to metabolic resistance to neonicotinoid dinotefuran in Aphis gossypii Glover (Lepidoptera: Noctuidae), and leaf-dipping bioassay results revealed that Aphis gossypii with 74.67 folds resistance to dinotefuran showed only 1.1 folds resistance to imidacloprid (no cross-resistance), but 15.3 folds resistance to neonicotinoid thiamethoxam. Ling and Zhang [39] indicated that Nilaparvata lugens (Hemiptera: Delphacidae) showed 7.42 folds resistance to nereistoxin bisultap, but only 1.02 folds resistance to imidacloprid (no cross-resistance) [39], and results of the biochemical analysis showed that cytochrome P450 played roles in metabolizing bisultap in Nilaparvata lugens. The above situations may be attributed to the fact that although the increased activity of cytochrome P450 was the main reason for the resistance of insects to insecticides, the ultimate result of insect resistance may be jointly determined by metabolic mechanism, target site sensitivity, and body surface penetration [35, 40].

Several pieces of literature have reported that imidacloprid, as a systemic neonicotinoid insecticide, could effectively control sap-sucking pests including thrip, aphids, and mosquitoes [41-43]. However, only a few pieces of literature reported the use of imidacloprid for controlling stored-product pests. Wakil and Schmitt [44] conducted field trials on farms and implied that imidacloprid could be effectively used for the protection of wheat stored against four major grain pests. Nayak and Daglish [45] studied the control effect of imidacloprid against four species of psocids and indicated that imidacloprid had the potential as a grain protectant to control stored-product pests. Athanassiou et al. [46] reported that simultaneous use of beta-cyfluthrin with imidacloprid was not more effective than beta-cyfluthrin alone to control stored-product pests, and the efficacy of both formulations varies with target species. Arthur and Fontenot [47] studied the effectiveness of spray and dust formulations of dinotefuran (another nicotinoid insecticide) to kill pests, and indicated that dinotefuran could be incorporated into management plans for the control of stored product insects. In these studies, imidacloprid or other nicotinoid insecticides were added to the grain as a protectant, or used for warehouse surface spraying to control stored-product pests. Different from above, in our work, we sprayed imidacloprid aerosols into a confined space to knock down and kill L. serricorne. In field trials, imidacloprid aerosol droplets produced by atomizer were thin and suspended in space for a certain period, which could effectively kill L. serricorne by using its knockdown effect and touch-killing effect.

CONCLUSION

Integrated Pest Management for food and tobacco industries mainly focuses on source control of raw materials, physical prevention, and sanitation. Meanwhile, space spray with insecticide aerosol can also be used as a supplementary means for pest control. Our work indicated that when L. serricorne develops resistance to pyrethroids, and the use of organophosphate and carbamate insecticides is limited due to toxicity or odor, application of space spray with imidacloprid aerosol in a confined space is a sensible and reliable choice for pest control. Our findings implied that, in addition to cigarette beetles, imidacloprid had significant potential to manage other Coleoptera insects resistant to pyrethroids in processing and stored facilities.

ACKNOWLEDGMENTS: We are grateful for the support of Hefei Cigarette Manufacturing Factory (Hefei, Anhui) for monitoring the amounts of L. serricorne in five plants and this work was supported by a grant from the Guangxi Province Tobacco Corporation (2021450000340033).

CONFLICT OF INTEREST: None

FINANCIAL SUPPORT: Guangxi Province Tobacco Corporation (2021450000340033).

ETHICS STATEMENT: None