The Sensitivity of Field Populations of Metopolophium dirhodum (Walker) (Hemiptera: Aphididae) to Seven Insecticides in Northern China

: Insect pests are primarily controlled by insecticides. However, the sensitivity decreases and insecticide resistance is problematic for the effective management of agriculturally important insects, including Metopolophium dirhodum , which is an aphid that commonly feeds on cereals. The insecticide sensitivity status and potential resistance of M. dirhodum ﬁeld populations remain relatively unknown. In this study, the susceptibility of 19 M. dirhodum populations from seven provinces in Northern China to neonicotinoids, pyrethroids, organophosphates, and a macrolide (abamectin) was determined in 2017–2019. The results indicated that two populations were highly resistant to thiamethoxam, with a relative resistance ratio (RLR) of 134.03 and 103.03, whereas one population was highly resistant to beta-cypermethrin (RLR of 121.42). On the basis of the RLR, the tested M. dirhodum populations ranging from susceptible to showing moderate levels of resistance to imidacloprid (RLR of 1.50 to 57.29), omethoate (RLR of 1.07 to 18.73), and abamectin (RLR of 1.10 to 25.89), but they were ranging from susceptible to showing tolerance or low levels of resistance to bifenthrin (RLR of 1.14 to 6.02) and chlorpyrifos (RLR of 1.11 to 7.59). Furthermore, a pair-wise correlation analysis revealed a signiﬁcant correlation between the median lethal concentrations (LC 50 ) for beta-cypermethrin and thiamethoxam, reﬂecting the cross-resistance between these two insecticides. The data obtained in our study provide timely information about aphid insecticide sensitivity, which may be used to delay the evolution of M. dirhodum insecticide resistance in Northern China.


Introduction
Aphids are important sap-feeding agricultural pests that adversely affect cereal, vegetable, and fruit crops worldwide. Approximately 2% of the Aphididae species (100 of 5000) have successfully exploited agricultural ecosystems, resulting in substantial economic losses [1]. For example, Metopolophium dirhodum (Walker) is a common aphid on winter cereals [2,3]. This aphid, which is native to the Holarctic region, has been the most common cereal aphid species in Europe for many years, but it is now distributed worldwide [4,5].    The following seven insecticides from five different classes were used in this study: 97% thiamethoxam, 96% imidacloprid, 95% beta-cypermethrin, 97% bifenthrin, 95% abamectin, and 97% chlorpyrifos (Beijing Green Agricultural Science and Technology Group Co., Ltd., Beijing, China), and 40% omethoate (emulsifiable) (Hebei Xinxing Chemical Co., Ltd., Baoding, China).

Bioassays
Bioassays were conducted using aphids that were within three generations of being collected from fields. A previously described leaf-dip method [10] was used for the insecticide bioassays. Briefly, the insecticide active ingredients were diluted six or seven times using 0.1% Tween-80 (prepared in water). Wheat leaves containing apterous adult aphids (excluding alatae) were dipped in the diluted insecticide solutions for 3 s. Three replicates of 30-50 aphids were used for each concentration. The mortality rate was calculated for each treatment. Aphids that did not move after being touched by a writing brush were considered dead.

Statistical Analysis
Concentration-mortality data were subjected to a probit analysis, with the data corrected for natural mortality [43]. The median lethal concentration (LC 50 ), 95% confidence interval, and slope were calculated using the IBM SPSS program (version 20). The relative resistance ratio (RLR) for each insecticide was calculated on the basis of the median lethal concentration (LC 50 ) for the most susceptible field population. The following RLR respectively indicated low, moderate, and high insecticide resistance: RLR ≤ 10, 10 < RLR ≤ 100, and RLR > 100. Pairwise correlation coefficients for the log LC 50 values of the field populations treated with imidacloprid, thiamethoxam, beta-cypermethrin, abamectin, and omethoate were calculated according to Pearson's correlation analysis using the SPSS software (IBM Corp., Armonk, NY, USA) to determine the cross-resistance among the insecticides.

Susceptibility Baseline of M. dirhodum to Seven Insecticides
At present, the baseline values for insecticide resistance in Metopolophium dirhodum have not been determined. In this study, the relative resistance ratio for each insecticide was calculated on the basis of the LC 50 for the most susceptible field population because of the lack of use on a contemporary susceptible reference strain during the bioassay ( Table 2).

Monitoring Sensitivity to Seven Insecticides in Northern China
The relative resistance levels varied among the field populations collected from various locations in Northern China (  Table 3). The bioassay results for the pyrethroids indicated that the NIS-2018 population was highly resistant to beta-cypermethrin, with an RLR of 121.42, but 15 populations were ranging from susceptible to showing moderate levels of resistance, with an RLR of 2.40-96.75. However, the tested populations were equally susceptible to bifenthrin. Regarding the susceptibility to the older generation organophosphate insecticides, the SHL-2018 and NIS-2019 populations were moderately resistant to omethoate, with an RR of 18.73 and 11.82, respectively. An analysis of the susceptibility to chlorpyrifos revealed that the tested populations were equally susceptible (Table 4). Moreover, 18 populations were ranging from susceptible to showing moderate levels of resistance to abamectin, with an RLR of 1.10-25.89 (Table 5).
SHY-2018 populations were highly resistant to thiamethoxam, with an RLR of 13 103.30, respectively. Additionally, 16 populations were ranging from susceptible ing moderate levels of resistance to thiamethoxam, with an RLR of 2.04-46.77, only four populations were moderately resistant to imidacloprid, with an RLR o 57.29 and 14 populations ranging from susceptible to showing tolerance or low resistance, with an RLR of 1.50-5.56. (Table 3). The bioassay results for the pyr indicated that the NIS-2018 population was highly resistant to beta-cypermethrin RLR of 121.42, but 15 populations were ranging from susceptible to showing m levels of resistance, with an RLR of 2.40-96.75. However, the tested populatio equally susceptible to bifenthrin. Regarding the susceptibility to the older gener ganophosphate insecticides, the SHL-2018 and NIS-2019 populations were moder sistant to omethoate, with an RR of 18.73 and 11.82, respectively. An analysis of ceptibility to chlorpyrifos revealed that the tested populations were equally sus (Table 4). Moreover, 18 populations were ranging from susceptible to showing m levels of resistance to abamectin, with an RLR of 1.10-25.89 (Table 5).

Insecticide Resistance at Five Locations in 2018-2019
The insecticide susceptibility of M. dirhodum collected from the same region in 2018 and 2019 was analyzed (Figure 3). Our results indicated that the sensitivity of M. dirhodum to five tested insecticides fluctuated between 2018 and 2019. More specifically, although the 2-year analysis revealed similar resistance levels in most regions, substantial differences between years were detected for the resistance to thiamethoxam in NIS and SHY ( Figure 3B), the resistance to beta-cypermethrin in NIS and SHL ( Figure 3C), the resistance to omethoate in SHL ( Figure 3D), and the resistance to abamectin in QIG and SHY ( Figure 3E).
Pair-wise correlation between the log LC 50 values of different insecticides. There were no significant correlations among the evaluated insecticides, with the exception of a significant positive correlation between thiamethoxam and beta-cypermethrin (Table 6).  Pair-wise correlation between the log LC50 values of different insecticides. There were no significant correlations among the evaluated insecticides, with the exception of a significant positive correlation between thiamethoxam and beta-cypermethrin (Table 6).

Discussion
Previous research determined the baseline toxicities of insecticides used to control wheat aphids as well as the corresponding susceptibility levels [15,20,21]. In the current study, we analyzed the susceptibility of the wheat aphid M. dirhodum to insecticides by comparing the lethal concentrations of various insecticides for 19 field populations. We also evaluated the resistance of M. dirhodum collected from 14 regions in Northern China to seven commonly used insecticides and determined the likelihood of cross-resistance among five insecticides.

Discussion
Previous research determined the baseline toxicities of insecticides used to control wheat aphids as well as the corresponding susceptibility levels [15,20,21]. In the current study, we analyzed the susceptibility of the wheat aphid M. dirhodum to insecticides by comparing the lethal concentrations of various insecticides for 19 field populations. We also evaluated the resistance of M. dirhodum collected from 14 regions in Northern China to seven commonly used insecticides and determined the likelihood of cross-resistance among five insecticides.
Our results indicated that some M. dirhodum populations (4 of 19) exhibited moderate levels of resistance to imidacloprid. Additionally, two populations (NIS-2018 and SHY-2018) were highly resistant to thiamethoxam. An earlier investigation assessing the effect of coating seeds with imidacloprid on laboratory populations of four wheat aphids confirmed that the seed treatment could effectively control R. padi, S. avenae, and S. graminum, but not M. dirhodum [13]. This is particularly important considering M. dirhodum may overtake R. padi and S. avenae as the primary aphid species on wheat plants derived from neonicotinoidtreated seeds [10]. Furthermore, different wheat aphids often harm crops at the same time. Consequently, the effect of neonicotinoid seed treatments on the prevention and control of M. dirhodum infestations in the field will need to be investigated. The data presented herein may be useful for optimizing the use of neonicotinoids and decreasing the environmental effects associated with the application of multiple pesticides.
Regarding pyrethroids, the limited M. dirhodum populations collected in 2019 exhibited low or no resistance to bifenthrin. In contrast, higher levels of resistance to betacypermethrin were detected, including one highly resistant population and moderately resistant populations. Clearly, determining the likelihood an insecticide will fail to control M. dirhodum is warranted. In Argentina, the most widely used insecticides for controlling M. dirhodum are chlorpyrifos, dimethoate, and pirimicarb [44]. Organophosphates were used to control M. dirhodum for a long time, but they are rarely used now [45]. Moreover, because organophosphates effectively minimized damages caused by M. dirhodum, there has been relatively little attention paid to this aphid species. The bioassay results of the current study, in which M. dirhodum was susceptible to organophosphates, were as expected. Additionally, M. dirhodum was also relatively susceptible to abamectin.
To improve the prevention and control of M. dirhodum infestations, we examined the resistance patterns among pesticides to identify the best compounds for managing M. dirhodum. Significant cross-resistance was detected between β-cypermethrin and thiamethoxam. A thiamethoxam-resistant cotton aphid strain reportedly developed increasing resistance to several pyrethroids [46]. A recent study proved that the resistance to λ-cyhalothrin is related to the resistance to thiamethoxam in R. padi [47]. Some cytochrome P450 monooxygenase genes encoding CYP6 and CYP3 family members are associated with the resistance to both neonicotinoids and pyrethroids [48,49]. Furthermore, there is evidence that NADPHcytochrome P450 contributes to the resistance to β-cypermethrin and imidacloprid in N. lugens [50]. Despite the intensive use of chemical treatments, M. dirhodum has still managed to invade new areas and damage crops in northern China [13]. Previously or seldomly used insecticides are still effective for managing pests, including pyrethroids, organophosphates, sulfoximines (e.g., sulfoxaflor), and macrolides (e.g., abamectin).

Conclusions
Our results suggest that M. dirhodum field populations are resistant to the neonicotinoid and pyrethroid insecticides most frequently used to control this aphid species in SHY (Yangling, Shaanxi) and NIS (Shizuishan, Ningxia) of China. The efficacy of neonicotinoid seed treatments for controlling M. dirhodum should be evaluated in future studies. Our bioassay results indicate that abamectin and bifenthrin are effective against M. dirhodum and can be used as alternatives to insecticides to which M. dirhodum has evolved high levels of resistance. Unfortunately, rotating the use of thiamethoxam and beta-cypermethrin to control M. dirhodum in parts of China should probably be limited. Overall, we recommend that monitoring and control programs should be strengthened in regions where M. dirhodum is distributed.