Interactions Of Metarhizium Anisopliae With White Grubs And Population Persistence In Field

Metarhizium anisopliae, an entomopathogenic fungus, was applied in soil to control white grubs. Weinvestigated interactions of the fungus with white grubs and population persistence in field. Thepurpose was to understand impact factors of control effect and possibility of technical manipulation.1. Pathogenicity of Metarhizium anisopliae against white grubs There were significantly differencebetween pathogenicity of M. anisopliae isolates based on bioassays with eggs and various larvalinstars. Grub mortality was positive correlation with fungal dosage. Three isolates (5-8, Br818and2-2) were high virulent to Holotrichia parallela, H. oblita and Anomala corpulenta.1st-instar larvaeof A. corpulenta were much more sensitive than2nd-and3rd-instar ones. But there were no diversityin larvae of H. oblita. Mortality and viewable infection of A. corpulenta eggs were to68.3%and51.5%. So high virulent isolate was need and grub development should be inspected for pest control.2. Behavior taxis of white grubs to M. anisopliae and plant White grubs moved random in soil.When with peanut plant,83⁸6%of grubs assembled around the plant. When with golf turf,55⁶9%of grubs did under and around them. It indicated that grubs strongly trend to host plants but had nopositive or negative tendency to the fungus. So the fungal agent should be delivered round the rootsto increase opportunity of grub infection.3. Application technique of M. anisopliae in field a) For peanut grubs: In potted treatments whenseeding, harvest survey showed that bad fruits decreased22.4%,amount and weight of fruitsincreased31.2%and33.7%in the fungal treatment compared to control. For phoxim treatment, badfruits decreased58.9%,but amount and weight of fruits decreased30.0%and26.7%. It indicated thatthe chemical effectively depressed grubs, but may be harmful to peanut. Mixture of half dosage ofthe fungus and phoxim could not suppress grubs effectively but still harmful to peanut. In fieldexperiment, bad fruits decreased10.9%, both amount and weight of fruits increased about40%.When the fungus and manure were used together, fruit amount had no significant difference to thecontrol, but fruit weight evidently higher. It means manure could weaken the fungal action but makefor peanut growth. b) For lawn grubs: In cage treatments, M. anisopliae could decrease27.3-87.3ï¼…neonate of lawn scarab A. cupripes under three dosages. The fungus presented strong infection in1st-instar larvae,90%mortality in6days at6.67×109spores/kg soil. LT50for2nd-and3rd-instars were11d and18d respectively. Therefore, fungal release at younger phase of grubs would be a preferablestrategy to enhance efficiency. For fungal release mode,sprinkling or scattering was better thanburying and stiletto-scattering. In large area, the effect of control grubs were78.8⁹0.7%.4. Population dynamic of M. anisopliae in field a) A selective medium, CA, was determined forisolating and quantity evaluating for M. anisopliae from soil. Isolating efficiency was72.6%.Directly dilute extraction provide a higher and more stable isolating efficiency than sugarcentrifugation. b) The soil inspection was carried out17times lasting for185weeks after inoculatingfungus. The results showed that M. anisopliae population in soil descend obviously at the trend ofstarting down slowly-went on down quickly-decelerated gradually-fluctuated or revived up. The attenuation process underwent38weeks then gradually stabilized at10²0%lever of originalpopulation for2³years. Half quantity was at26.7weeks. c) From fungal inoculation when seedingto harvest time, the quantities of M. anisopliae at various places of different depth and distance topeanut plant all declined quickly in30d-60d, then relaxed. The quantities would descend to20%¹0%of original inoculation at90d or about, then maintained or return appreciably. Thedecrease extent at3cm depth was smaller than at15cm depth. The fungal population would returnearlier and more quickly at12.5cm distance to peanut plant than those places more far from plant. d)Although three dosage of the fungus were inoculated in soil at seeding time, their quantity remain anear lever at last. Adding manure or phoxim, the fungal population would bring forward comingdown and reviving. Inoculation of grubs at anthesis did not impact obviously on the fungalquantity at lasting80days. e)At the dividing line around plant of11.5cm radius, the fungus couldhave the trend of growing from outside soil inoculated the fungus to inside soil without the fungus.5. Preparation of M. anisopliae granule and improvement of population revival Alginate gelpellets were formed with reaction of2%alginate sodium and0.2⁰.3M Ca⁺⁺.4¹0%of attapulgitewas filled in gel to uphold inner space.0.5%corn powder or dextrin added into the gel pelletscould advance60%of spore multiplication. The outer membrane formed by alginate and polyvinylalcohol could resist strike from environmental microbes and improve population reviving.