Study On Occurrence And Control Of Two Main Alfalfa Weevils In Ningxia

Sitona cylindricollis and Sitona callosus are two important pests found on Ningxia alfalfa.The adult insects bite the leaves and seeds,while larvae feed on the roots and root tumors,leading to the decline in the quality of alfalfa,production reduction,and even causing plant death.Consequently,the prevention of these two pests should not be overlooked.In this study,the biology and ecology of these two pests were investigated.In addition,the screening of the effective pesticides and biocontrol fungus Beauveria bassiana were also studied.The goals of our research are expected to provide scientific basis for green control of alfalfa weevils.The main findings are as follows:1.The weevils were collected from the different alfalfa planting regions in Ningxia,and their morphological characteristics were observed by three-mean stereoscopic microscope systems.The total DNA of alfalfa weevils was extracted,and sequenced by PCR amplification to obtain COI gene sequences.The sequences were compared with those in the GenBank database,and the software MEGA 7.0 was used to calculate the base composition,site variation,and segmentation genetic distance,and the phylogenetic tree was constructed by the neighbor-joining method(NJ method).The results showed that there are two kinds of species in the Ningxia area,S.cylindricollis and S.callosus.The adults could be quickly distinguished by the compound eyes and the spacing between the eyes,which were small and flat in S.cylindricollis and large and convex in the S.callosus.The base composition of different loci of the two alfalfa weevils differed somewhat,with the S.cylindricollis having the highest T content at codon 3 among the three loci,while S.callosus having the lowest.2.Combining field observation and indoor feeding to clarify the life history of two alfalfa weevils,the results show that how that alfalfa weevils have one generation per year and overwinter as adults.The life histories of the two species were consistent.The overwintering adults start to emerge from the soil in the late April,the peak egg-laying period is in the middle and late May,the larvae hatching period is in the early June to middle June,the pupation period is in the late July to early August,the feathering period is in the middle to late August,and the adults gradually start to overwinter after October.3.The results of daily feeding of two alfalfa weevils showed that the feeding ofS.cylindricollis was greater than that of S.callosus,and the feeding of both alfalfa weevils increased with the increase of temperature.The maximum feeding area of S.cylindricollis weevil was 3.91 mm2 and the feeding amount was 2.49 mg;the maximum daily feeding area of S.callosus weevil was 2.48 mm2 and the feeding amount was 1.67 mg.4.The supercooling points of two alfalfa weevils were determined by SUN-V intelligent insect supercooling point tester.We found that the supercooling point and freezing point of S.cylindricollis were-19℃ and-16.77℃.The supercooling point and freezing point of S.callosus were-18℃and-15.97℃.5.Field surveys of two species of alfalfa weevils were conducted from May to September in 2020 and 2021.The results of the aggregation index analysis showed that both adult and larvae of alfalfa weevils were aggregated in the field distribution type,and the reasons for this aggregation pattern were related to their own habits and environmental factors;august was the peak month for the occurrence of adults of both species.6.In August,at the height of the alfalfa weevils outbreak,Huinong,Pingluo,Xixia,Hongsipu,Yuanzhou,Pengyang,and Longde 7 sampling points were investigated.The results show that the distribution of S.cylindricollis in Ningxia is decreasing in the north to the south,and the distribution of S.callosus is increasing by the north to the south.7.The toxicity of 9 insecticides to two kinds of alfalfa weevils was determined by infiltration method,the results showed that 2.5%biphenylene and 0.3%mazoin,showed very high toxicity force.Two pesticides and pesticide auxiliaries have been known to the synergies of S.callosus,2.5%biphenylene+huainongte and 2.5%biphenylene+qiangliyuan increased effect,the increased magnification is 1.45 and 1.79,respectively.0.3%mazadizoin +bijia and 0.3%mazadizoin+jijian increased effect,the increased magnification is 1.47 and 1.42,respectively.8.The pathogen B1 was isolated and purified from the alfalfa weevils,and was identified as B.bassiana by morphological and molecular identification.This strain had high pathogenicity to both kinds of alfalfa weevil.After treatment with spore suspension of B1 strain,alfalfa weevil compound eyes changed from black to red and brown,slow movement and reduced food intake.Mycelium first appeared in the mouthparts and appendage nodules of insect cadavers,then the whole insect body was wrapped by mycelium and broken when touched it,and death occurred on the 4th day.On 12 day,the corrected mortality of both alfalfa weevils treated with 1×108 spores/mL suspension was 100%.Different solid media had different effects on the growth of B.bassiana colony,among which potato sucrose agar medium and potato peptone sucrose agar medium were more suitable for B.bassiana colony growth than other media.In liquid culture,after inoculated 9 days,L-broth promoted the growth of this strain more than other cultures.According to univariate analysis,it was concluded that the temperature,pH of the culture medium and the sample loading volume all had certain influence on the growth of B.bassiana B1 strain in liquid culture.The strain is most suitable for the strain when the temperature is 26℃ or 30℃,pH of 7,and the loading sample is 1/4 or 1/5.