The lethal and sub lethal effects of the combined insecticide Emamectin benzoate + Acetamiprid on the life table parameters of Aphis gossypii (Hem.: Aphididae)

Background and Objectives
Aphis gossypii Glover is a polyphagous pest that infects a wide range of host plants in temperate, tropical and subtropical regions. Therefore, it has a great ability to maintain its population in different environmental conditions. Quantitative and qualitative damages of this pest are through feeding on plant sap, transmission of viral diseases, reduction of photosynthesis and secretion of honeydew. Various factors such as excessive use of chemical insecticides, the destruction of natural enemies and the emergence of resistant pest populations have increased the population of aphids and turned it into a damaging pest. However, the most common method of controlling aphids, despite its negative effects, is usage of chemical insecticides. The high potential of this pest in resistance to chemical compounds requires investigating and using of newer insecticides with different modes of action. Insect reproductive parameters as an ecological and indicator tools, can indicate the behavior and physiology of different types of pests after pesticides using. Studies have also been conducted on the effect of sub lethal concentrations of insecticides on behavior, biological characteristics and growth of insect population, which provides a more realistic estimate of the effect of insecticides on pests and natural enemies. Therefore, in this study, the lethal and sub lethal effects of the new combined insecticide Emamectin benzoate 4.8% + Acetamiprid 6.4% which has contact and oral function, was evaluated on the aphids biological and population growth parameters in greenhouse conditions.
Materials and Methods
The colony of cotton aphid used in the experiment was obtained from pinto beans fields in Urmia, West Azerbaijan (Iran), during September 2021. The aphid population was reared on pinto beans var. Khomein in plastic pots (12 × 6 cm) under controlled greenhouse conditions (21 ± 1°C, 50% ± 10% RH, and a photoperiod of 16: 8 [L: D] h). The LC₅₀ concentration of Emamectin benzoate + Acetamiprid for cotton aphid adults was 23.84 ppm 24 hours after treatment. The values of LC₁₀, LC₂₀ and LC₃₀ were estimated at 1.76, 4.31 and 8.22 ppm, respectively. The insecticide spraying method on leaves containing aphids was used in bioassay and life table studies against adults of aphid insects. The age stage, two-sex life table method was used to analyze the collected data. We used the bootstrap technique with 100,000 iterations to estimate the variance and standard errors of the biological and population parameters and used Sigma Plot software to draw graphs. The growth of the pest population in a period of 60 days was done with Timing-MsChart software.
Results
According to the results, the adult lifespan/longevity was significantly different from the control by LC₂₀ and LC₃₀ concentrations. The shortest lifespan of adult aphid in LC₃₀ treatment was recorded as 5.90 days. Fertility and the duration of Nymphal period were significantly reduced in sub lethal concentrations of the insecticide compared to the control. The highest value of pre adult period in LC₃₀ concentration was recorded 8.88 days and the lowest value in control treatment was 5.06 days. All biological table parameters of cotton aphid treated with sub-lethal concentrations were significantly affected by the insecticide compared to the control. The highest value of intrinsic rate of increase (r) was 0.39 per day in control. The net reproductive rate (R₀) decreased from 28.37 nymphs per female/generation in control to 6.90 in LC₃₀ concentration. The lowest value of finite rate of increase (λ) was observed in LC₃₀ concentration, which was 1.18 per day. The gross reproductive rate (GRR) in sub lethal concentrations was significantly lower than the control. The highest and lowest values of mean generation time (T) were estimated in LC₃₀ (11.28 days) and control (9.11 days), respectively.
Discussion
Based on the findings of the present study, considering the necessity of using chemical insecticides for aphid management, and the significant effect of this pesticide on the aphid Emamectin benzoate + Acetamiprid insecticide can be used to control and reduce population of cotton aphids and it can be a suitable alternative to old insecticides. In order to confirm the greenhouse results, it is necessary to conduct similar experiments in field conditions, and conduct laboratory and field studies on the effects of this compound on the natural enemies of A. gossypii especially bees and coccinellids.