Effects Of Low Concentration Of Imidacloprid On Field Diffusion And Predation Capacity And Temperature Adaptation Of Hippodamia Variegata (Goeze)

Hippodamia variegata（Goeze）is an essential predatory natural enemy in cotton fields in Xinjiang and plays an important ecological role in controlling Aphis gossypii Glover populations.The residues of imidacloprid,a chemical insecticide commonly used to control cotton aphids,can hurt the H.variegata.In this experiment,based on the results of the toxicity determination of imidacloprid on cotton aphids,cotton aphids sprayed with LC₂₀(3.93 mg a.i./L,this concentration is the LC₂₀concentration obtained from the determination of cotton aphid virulence,referred to as"LC₂₀concentration")and LC₅₀(13.62 mg a.i./L,this concentration is the LC₅₀ concentration obtained from the determination of cotton aphid virulence,referred to as"LC₅₀ concentration")imidacloprid were fed to H.variegata to evaluate the effects of low concentrations of imidacloprid on the dispersal ability,field feeding ability and temperature adaptation of H.variegata,to clarify the response of H.variegata to low concentrations of the main findings of the study are as follows:（1）Low concentrations of imidacloprid reduce the dispersal ability of H.variegata adult in cotton fields and diminish the activity of related enzymes that metabolize energy substancesThe results of the four releases of H.variegata in two years showed that the dispersal distance of H.variegata in the LC₅₀ concentration treatment group decreased significantly on day one and day two after the release of H.variegata in the field,which was substantially different from that of the control group.On the third and fourth days after the release,the dispersal distance of ladybird beetles in the field of each treatment group showed that the control group>LC₂₀ treatment group>LC₅₀ treatment group,but the difference between the three groups was not significant;on the fourth day after the release,the average dispersal distance of the control group was 60.63 m,while the average dispersal distance of the LC₅₀ treatment group was only 11.67 m.The LC₂₀ and LC₅₀ concentrations of imidacloprid significantly reduced the activities of Trehalase（THL）,Glyceraldehyde-3-phosphate Dehydrogenase（GADPH）and Lacate Dehydrogenase（LDH）enzymes in H.variegata adults;the activity of 3-phosphate glycerol dehydrogenase（GDH）enzymes increased with increasing imidacloprid treatment concentrations,but the differences between control and imidacloprid-treated groups were not significant.Compared to the control,LC₅₀ concentration of imidacloprid significantly reduced Trehalase enzyme content of H.variegata but had no significant impact on the content of glyceraldehyde 3-phosphate dehydrogenase,lactate dehydrogenase and 3-phosphate glycerol dehydrogenase.Imidacloprid at LC₅₀concentration significantly increased the Trehalase content of both female and male adult H.variegata;both LC₂₀ and LC₅₀ concentrations of imidacloprid significantly increased the glycogen content of H.variegata males but had no significant effect on females.The experiment results indicated that low concentrations of imidacloprid reduced the activity of the enzymes that metabolise energy substances in H.variegata,weakening its ability to utilize energy substances and ultimately leading to a reduction in the dispersal ability of H.variegata in cotton fields.（2）Low concentrations of imidacloprid weaken the field feeding ability of H.variegataIn the field predation of ladybird beetles,cotton aphid populations in the adult and larval groups at three density gradients of cotton aphids were as follows:treatment group without ladybird beetles>LC₅₀ concentration>LC₂₀ concentration>control group.In the predation of individual female adult test,the number of cotton aphids in the LC₅₀ treatment group was significantly higher than in the control group on day 1.After that,the number of cotton aphids in the two imidacloprid treatments was not significantly different from that in the control group.In the individual male test,the number of cotton aphids in the LC₅₀ treatment group was significantly higher than that in the control group only on day1,at a density of 400 aphids/plant.In the individual larvae test,the number of cotton aphids did not differ significantly between treatments throughout the survey period at a density of 100 aphids/plant;however,as the density of cotton aphids increased to 300 aphids/plant,the number of cotton aphids in the LC₅₀ treatment group was significantly higher than that in the control group throughout the survey period.In the adult population predation test,the number of cotton aphids in the LC₅₀ treatment group was significantly higher than that of the control group on day 9 and 11 when the density of cotton aphids was 200 aphids/plant;when the density of cotton aphids increased to 400 aphids/plant,the numbers of cotton aphids in the LC₂₀ and LC₅₀treatment groups were significantly different from that of the control group respectively on day 7 and 5.In the larval population test,the number of cotton aphids was significantly higher in the LC₅₀ group than in the control group throughout the survey period when the density of cotton aphids was 100 and 300 aphids/plant,but the number of cotton aphids in the LC₂₀group was not significantly different from the control group.The results showed that the low concentration of imidacloprid reduced the predation ability of adults and larvae of H.variegata and impaired its pest control function in the field.The effect was more evident as the concentration of imidacloprid and the density of the cotton aphid population increased.（3）Low concentrations of imidacloprid reduce temperature acclimatization of H.variegataAt different temperature treatments,LC₅₀ concentrations of imidacloprid significantly reduced the longevity,female egg production,reproductive calendar and egg-hatching rate of H.variegate.The pre-oviposition period of females was significantly prolonged at 31℃,34℃and 37℃.In particular,under the high temperature of 37°C,the average life span of females and males in the LC₅₀ treatment group was the shortest among all treatment groups,at 6.83 d and 7.78 d,respectively;egg production was the lowest at 24.50 eggs/female,egg hatching rate was the lowest at 11.24%,the reproductive calendar was the shortest at 2.43 d,and pre-laying period was the longest at 4.31 d.The LC₅₀ concentration of imidacloprid caused a greater negative impact on the vital parameters of H.variegata at different temperatures and reduced its adaptability to temperature,this adverse effect is especially evident under high temperature conditions.At the same temperature,compared with the control groups,the fecundity of the G0 generation of A.Gossypii treated with LC₅₀ concentration of imidacloprid significantly decreased,but the longevity did not show significant changes;‘Net reproductive rate rate’R₀,‘Intrinsic rate of increase’r_m and‘Finite rate of increase’λof the G1 generation of A.Gossypii significantly decreased,and the population doubling time and the mean generation period were prolonged.at 25℃,34℃and 37℃,LC₂₀ and LC₅₀ concentrations of imidacloprid reduced the abundance and diversity of H.variegata female symbiotic populations at 25℃,34℃and 37℃;the same trend was observed for males and females at 34℃;PCo A analysis revealed that LC₂₀ and LC₅₀ concentrations of imidacloprid significantly affected the diversity of female symbiotic bacteria at OTU level.The results indicated that the LC₂₀ and LC₅₀ concentrations of imidacloprid affected the population abundance and diversity of the symbiotic bacteria of H.variegata and A.Gossypii.This study reveals that the decrease in cotton field flight diffusion capacity of H.variegata caused by LC₂₀ and LC₅₀ concentrations of imidacloprid is due to the inhibition of its energy metabolizing enzyme activity.It confirms that low concentrations of imidacloprid weaken the ecological pest control function of H.variegata and its adaptation to temperature.The study results have scientific significance for the conservation and use of H.variegata and the scientific use of chemical control of cotton aphids and help to improve the integrated control strategy for cotton aphids.