Effect Of Entomopathogenic Nematodes Against Sweetpotato Weevil Cylas Formicaruis (Fabricius)

Sweet potato weevils （Cylas formicaruis） are important pests of sweet potato in southernChina, not only causing huge production losses, but also seriously affect the quality of sweetpotato. Chemical insecticides have been the main method to control sweet potato weevils,although it did has an inhibitory effect on controlling weevils of rampant hazards, butlong-term use will not only cause sweet potato weevils resistance but also environmentpollution, which poses a great threat to human and animal health. Therefore, searching for asafe and effective control method is imminent. Entomopathogenic nematodes（EPN）is a greatpotential biological control agent, it can effectively prevent the cryptic pests as well as soilpests. They had a wide host range, easy to produce and environmentally safe, which madethem been widely used as biocontrol method against a variety of pests, but few studies onEPN controlling sweet potato weevils had been reported.In order to efficiently and rationally use EPN preventing sweet potato weevils, virulenceof different EPN species and populations were compared, and the optimal infection conditionsfor the EPN population with highest virulence was determined. In addition, Beauveriabassiana strains with better control efficacy to adults of C. formicaruis, and insecticideswhich can improve EPN invasion ability were also screened, the main results are as follows:1. Virulence of8Steinernema species （Steinernema feltiae, S. carpocapsae, S.ceratophorum, S. litorale, S. monticulum, S. hebeiense, Steinernema sp1, Steinernema sp2）and3Heterorhabditids species （Heterorhaditis megidis, H. bacteriophora, H. indica）againstlast instar larvae of C. formicarius were evaluated in laboratory conditions and S. feltiaeshowed the highest virulence. On this basis, the virulence of11S. feltiae populationsoriginated from different geographical locations were further determined, S. feltiae JY-17wasthe best one against the last instar larvae of C. formicarius, The mortality of C. formicariuswas96.67%, and EPN invasion rate was46.67%.2. Virulence of S. feltiae JY-17against C. formicaruis under different conditions werecompared, the results showed that the sensitivity of insect stages at25℃were in a sequence of last instar larvae> young larvae> pupa> pre-pupa> adult; the virulence of S. feltiae JY-17to C. formicarius at different temperatures were in an order of25℃>20℃>30℃>15℃>10℃>35℃. The LD₉₀of S. feltiae JY-17to young larvae and last instar larvae of C.formicarius were10.26IJs/larva and18.10IJs/larva，LD₅₀of nematodes to pupa was90.49IJs/pupa in the density of6.36insect/cm², the adult of C. formicarius was less sensitiveto S. feltiae JY-17’s infection so adult mortality had not reached50%in all EPN dosages. LT₅₀of S. feltiae JY-17to the last instar larvae of C. formicarius was44.31h when the ratio of EPNto insect was5:1.3. Pathogenicity of two Metarhizium anisopliae strains and six B. bassiana strainsagainst adult of C. formicaruis were determined by dipping method in laboratory conditons. B.bassiana HFW-05and TM-01were screed to be optimal strains at the concentration of107spores/ml. On this basis, pathogenicity to adult of C. formicaruis at differentconcentrations of the two optimal strains were compared, which HFW-05shows the highestvirulence with a LC₉₀values of6.903×10⁷spores/ml.4. Toxicity of eight insecticide（semamectin benzoate, methomyl, chlorpyrifos, phoxim,imidacloprid, nitenpyram, hexaflumuron and beta-cypermethrin）on S. feltiae JY-17andnematode survival were determined in laboratory conditions. The results showed that S.feltiae JY-17treated with low concentrations of imidacloprid or nitenpyram couldincrease the nematode infectivity. When treated with1/50RC （the recommendedconcentration） imidacloprid, the lethal rate of S. feltiae JY-17to C. formicaruis was76.67%, and EPN invasion rate was21.33%. When treated with1/2RC or1/10RCnitenpyram, the lethal rate of S. feltiae JY-17to C. formicaruis were73.33%and76.67%respectively, and EPN invasion rate were27.33%and31.33respectively.5. Inhibition of C. formicaruis by different concentrations of S. feltiae JY-17weredetermined in a pot experiment under greenhouse conditions. The results showed S.feltiae JY-17has a certain inhibition to C. fromicaruis population at all concentrationstested, the inhibition rate was between25%-30%. There was no significant differencebetween S. feltiae JY-17treatments and phoxim treatment. On this basis, S. feltiae JY-17 treated with insecticids and mixed use with B. bassiana were tested in order to see if it canincrease efficacy for controlling C. formicaruis. The results showed that the inhibitory effectof all treatments were higher than the blank control, the inhibition rate were in an order of S.feltiae JY-17mixed with B. bassiana HFW-05（53.74%）> S. feltiae JY-17treated withnitenpyram（48.94%）> S. feltiae JY-17（47.98%）> imidacloprid（34.36%）> S. feltiaeJY-17treated with imidacloprid（26.68%）> B. bassiana HFW-05（20.73%）. EPN mixed usewith B. bassiana are promising for controlling C. formicaruis.