Effects of imidacloprid and thiamethoxam at LC30 and LC50 on the life table of soybean aphid Aphis glycines (Hemiptera: Aphididae)

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is a main pests of soybean that poses a serious threat to its production. Studies were conducted to understand effects of the different concentrations of the insecticides (imidacloprid and thiamethoxam) on the life table of A. glycines to provide vital information for its effective management. We found that the mean generation time, adult and total pre-oviposition periods in A. glycines specimens exposed to LC50 imidacloprid and thiamethoxam were significantly longer than those in the control group. However, when exposed to LC30 imidacloprid and thiamethoxam, the adult pre-ovipositional period was significantly shorter than that in the control group. The mean fecundity per female adult, net reproductive rate, intrinsic rate of increase, and finite rate of increase were significantly decreased in individuals exposed to LC30 and LC50 concentrations of imidacloprid and thiamethoxam, respectively (P < 0.05). Both insecticides produce stress effects on A. glycines, and specimens treated with LC50 concentrations of the two insecticides exhibited a significant decrease in their growth rates than those treated with LC30 concentrations. This study provides data that can be used as a reference to predict the effect of imidacloprid and thiamethoxam on the population dynamics in the field, and agricultural producers could attach importance to prevent stimulation the reproduction made by low-lethal concentrations during actually applying pesticides.


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Due to the different development rates between individuals, the age-stage specific survival rates 188 curves show obvious overlaps (Fig 1). The relative number of female adults in the LC 30 imidacloprid and 189 thiamethoxam treatment groups was higher than that in the respective LC 50 treatment groups.

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Age-specific survival rate is the probability that a newly hatched nymph will reach an age x, and the 191 curve of the age-specific survival rate is a simplified form of the curve of the age-stage survival rate, 192 disregarding developmental stages. After treatment with imidacloprid and thiamethoxam, the l x curve 193 decreased significantly (Fig 2).

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The highest peak of m x in the control group was higher than that in LC 30 and LC 50 treatment groups.

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The highest peak of m x in the control group appeared on day 8 (Fig 2), whereas that in the LC 30 imidacloprid group appeared on day 7, i.e., a day earlier than that in the control group. The highest peak 197 of m x in the LC 30 thiamethoxam group appeared on day 6, two days earlier than that in the control group. 198 The highest peak of m x in the LC 50 imidacloprid group appeared on day 10, two days later than that in the 199 control group, while the highest peak of m x in the LC 50 thiamethoxam group appeared on day 9; a day 200 later than that in the control group (Fig 2).

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The female reproductive values in the imidacloprid and thiamethoxam treatment groups decreased 205 compared with those in the control group; however, the female reproductive value in the LC 30 treatment 206 group was higher than that in the LC 50 group (Fig 3).

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The age-stage life expectancy curve (e xj ) is shown in Fig 4. In the curve, the highest peak values of 208 the first to fourth instar nymphs and female adults were lower in the treatment groups compared with the 209 control group.

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The life table parameters used herein reflect the total effect of imidacloprid and thiamethoxam on A.
211 glycines. We found that imidacloprid and thiamethoxam at LC 50 significantly increased the APOP and 212 TPOP and significantly decreased the mean fecundity per female adult compared with that in the control 213 group (P < 0.05). In contrast, the APOP in individuals exposed to imidacloprid and thiamethoxam at 214 LC 30 was shorter than that in the control group (P < 0.05, Table 2). In addition, according to the results in the age-stage two-sex life table, the R 0 , λ, and r also decreased significantly (P < 0.05, Table 3).

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Collectively, these results indicate that both imidacloprid and thiamethoxam have inhibitory effects on 217 the reproduction of A. glycines.

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The l x curve is a basis for the s xj curve. In this study, we found that the l x curves of individuals 219 exposed to imidacloprid and thiamethoxam showed a declining trend (Fig 2). During the first stage, A.
220 glycines failed to respond effectively when initially exposed to a high dosage of insecticide; 221 consequently, the l x decreased sharply, and only some surviving individuals entered the second stage.  table may   225 be also related to the regulation strategies of the species, such as self-metabolism and detoxification. In 226 the future research, we will be committed to metabolic detoxification, from the physiological indicators 227 and even molecular level to continue understand the deep impact of imidacloprid and thiamethoxam on 228 population dynamics. During the third stage, the survival rate continued to decline, less sharply than in 229 the first stage, but more sharply than in the second stage. During the third stage, the individual longevity 230 may also be one reason for the decrease of the l x ; this is consistent with the decrease in the age-stage life 231 expectancy curve with increasing age (Fig 4).

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In the present study, individuals in the LC 50 thiamethoxam and LC 50 imidacloprid treatment groups