| Abamectin is an important pesticide compound used in agricultural production.However,its efficiency can be hindered in various field applications due to environmental limitations.For example,strong soil adsorption can prevent its full effectiveness against root-knot nematodes when applied to soil.When used as a foliar spray,abamectin’s shelf-life is significantly reduced due to its high susceptibility to photolysis.Additionally,abamectin poses a risk to non-target organisms,further limiting its use.Therefore,it is evident that the development of compounds must consider both biological activity and the demands of field applications.This study used avermectin B1a as a model compound to develop a pesticide functionalization strategy by exploring and resolving the functional group sites that affect the efficacy of avermectin B1a.This approach provides new ideas for designing highly efficient and low-toxicity pesticide compounds.The main results are as follows:1.Establishing of avermectin B1a derivatives for efficient control of root-knot nematodesSoil samples were taken from various areas,along with three avermectin-based compounds（avermectin B2a,avermectin B1a,and emamectin benzoate）,to investigate their impact on soil adsorption.The study aimed to study the mechanism of soil adsorption of avermectin-based compounds with respect to their chemical structure and soil sorbent properties.The results revealed that the soil adsorption of avermectin B2a was the strongest,followed by avermectin B1a and emamectin benzoate,suggesting that the-OH group of avermectin-based compounds serves as the primary adsorption site for soil.In the soil,organic matter is the main sorbent,and humic acid constitutes the primary component of organic matter.Therefore,we extracted humic acid fractions for adsorption tests on these compounds to further elucidate the soil adsorption mechanism.The results indicated that the humic acid fraction exhibited the highest adsorption of avermectin B2a,followed by avermectin B1a and emamectin benzoate.This suggests that the adsorption of these compounds by the soil arises from interactions between the-OH of the compounds and the humic acid fraction.Elemental and 13C NMR analyses revealed that this interaction is an intermolecular force between the compounds and the polar groups in the humic acid fraction.Quantum chemical calculations validated that this intermolecular force corresponds to hydrogen bonding between the-OH of the compounds and the amino,hydroxyl,carboxyl,and carbonyl groups in the humic acid.These findings suggest that the mechanism of soil adsorption of avermectin-based compounds involves hydrogen bonding and that the-OH of the compounds are the primary sites of hydrogen bonding with soil organic matter.The derivative-1 and derivative-2 series were synthesized by modifying different-OH groups of avermectin B1a.Soil adsorption of avermectin B1a was found to be strongest,followed by the derivative-1 series,and weakest for the derivative-2 series.This suggests that modifying the-OH groups of avermectin B1a resulted in a change in soil adsorption of the compound.The strongest sorption of the humic acid fraction to avermectin B1a,followed by the derivative-1 series,and the weakest was for the derivative-2 series.Analysis of the relationship between the number of hydroxyl groups in the structure of the compounds and the strength of adsorption once again demonstrated that the mechanism of adsorption of the derivatives by the soil is still hydrogen bonding between the-OH groups of the compounds and the polar groups of the humic acid fraction.The 24h-LC50 and 48h-LC50 of avermectin B1a against second instar larvae of root-knot nematodes were 0.47 and 0.27 mg/L,respectively.The biological activity of the derivatives against root-knot nematodes was reduced to varying degrees,with the 24h-LC50 and 48h-LC50 of the derivative-1 series ranging from 7.86-16.79 mg/L and 2.44-4.66 mg/L for derivative-2 series,and 5.15-13.23 mg/L and 4.16-7.88 mg/L for derivative-2 series,respectively.The derivative-2 series was selected to verify the efficacy of the derivatives against root-knot nematode disease.In the soil thin layer chromatography and soil column leaching tests,avermectin B1a was distributed horizontally at 0-3 cm and vertically only at 0-5 cm.The derivative-2 series was distributed between 6-15 cm horizontally and 10-25 cm vertically,indicating the successful construction of an avermectin B1a derivative system with enhanced soil migration capacity.The efficacy of abamectin B1a against root-knot nematode was40.67%at 30 d and 35.32%at 60 d in the field.After treatment with abamectin B1a,only the 0-5cm area of the plant roots were not infested,and root knots were evident in the root area below 5 cm,and the degree of root knots increased with time.The derivative-2 series showed a maximum efficacy of about 80%against root knot nematode at 30 d and about 70%at 60 d.The roots of the plants were almost uninfested for 30 d after treatment with the derivatives,and after 60 d of treatment a small amount of root nodules appeared in the lower root zone of the plants.2.Construction of an efficient and low-toxic avermectin B1a derivative system for foliar spray scenariosThe degradation of avermectin B1a was about 82%after 15 min of UV irradiation,while the degradation of its derivatives was between 30-60%.After 60 min of irradiation,avermectin B1a’s degradation was over 90%,and the degradation of its derivatives was between 50-80%.The LC50 and LC90 of avermectin B1a against Tetranychus cinnabarinus were 0.041 and 0.1327 mg/L,respectively.The biological activity of the derivatives against T.cinnabarinus was slightly reduced,with LC50and LC90 values ranging between 0.094-0.665 and 0.348-2.272 mg/L.The main factor contributing to the reduced biological activity against T.cinnabarinus was the change in the derivatives’binding mode to the target protein.The foliar residual bioactivity of the derivatives were better than those of averamectin B1a against T.cinnabarinus for the same time.In the pot trials,the control efficacy of avermectin B1a on T.cinnabarinus decreased from 100%to 70%after 9 d of application,whereas the control efficacy of the 5,4-（2-Tolyl）-carbamate-avermectin B1a,5,4-（3-Tolyl）-carbamate-avermectin B1a,5,4-（4-Tolyl）-carbamate-avermectin B1a,and 5,4-（4-Fluorophenyl）-carbamate-avermectin B1a remained above 90%.The LC50 of the derivatives against Bombyx mori ranged from 1.39-228.08 mg/L,revealing them to be 3-5 orders less toxic than avermectin B1a.The diminished binding energy of the derivatives to target proteins was the main cause for the notable reduction in their biological activity against B.mori.Comparatively,the species selectivity coefficient for avermectin B1a stood at a mere 0.018 between B.mori and T.cinnabarinus,while that of the derivatives ranged from 0.9-2.3.In terms of effect on the growth state and feeding behavior on B.mori,the derivatives had a much lower impact than avermectin B1a.The 48h-LD50 of avermectin B1a was 0.0006 mg/bee for the Apis mellifera,whereas the 96h-LC50for zebrafish was 0.02 mg/L For Trichogramma ostriniae,the safety factor was 3.59.As for the derivatives,the 48h-LD50 for A.mellifera was ranged from 1.02 to5.21 mg/bee,and the 96h-50 for zebrafish was ranged from 0.25 to 2.04 mg/L,with a safety factor for T.ostriniae between 53.26 and 243.16.The selectivity coefficients for avermectin B1a（I1 and I2）between A.mellifera and T.cinnabarinus were 1.5 and 4.5μL/bee,respectively.In comparison,the derivatives exhibited a significant increase in selectivity coefficients（I1 and I2）ranging from 734.5-1685.4 and 2185.5-4895.4μL/bee,respectively.This suggests that the derivatives have reduced the risk of exposure to A.mellifera by being selective between A.mellifera and T.cinnabarinus.The selectivity coefficient I1 and I2 of avermectin B1a between the T.cinnabarinus and zebrafish were 0.07 and 0.19,while the selectivity coefficient I1 and I2 of the derivatives increased to 0.08-0.39 and 0.39-0.91.The selectivity coefficient I1 and I2of avermectin B1a between the T.cinnabarinus and T.ostriniae were 67.8 and 265.8.The selectivity coefficient I1 and I2 of avermectin B1a between T.cinnabarinus and T.ostriniae were 67.8 and 265.8,while the derivatives increased to 135.1-290.0 and549.1-956.4,improving the safety against T.ostriniae. |
