Studies on infectivity of the green muscardine fungus, Metarihzium anisopliae, to the sweet potato weevil, Cylas formicarius

• Main Authors: Chih-Chieh Shu, 徐芷婕
• Other Authors: Li-Cheng Tang
• Format: Others
• Language: zh-TW
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/80984338186448949412

碩士 === 國立中興大學 === 昆蟲學系所 === 98 === Virulence of the entomopathogenic fungus, Metarhizium anisopliae (Metschnikoff) Sorok (Ascomycota: Hypocreales) was evaluated against the sweet potato weevil, Cylas formicarius (Fabricius), (Coleoptera: Curculionidae）through a series of tests in the laboratory. After inoculating with surface spray of conidial suspensions, the LC50 of female adults was 2.6×105 conidia/ml while that for males was 7×104 conidia/ml. At 15, 20, 25, 30, or 35℃, weevils treated with the same concentration of 1×107conidia/ml resulted in mortalities, which increased with rising temperature regimes. At 30℃, the mortality was highest reaching 100% for both male and female adults. Three types of sterilized soils, i.e. clay loam (black soil), clay (red soil) and sandy loam (sand), were assayed to estimate the effect of soil characteristics on M. anisopliae pathogenicity to C. formicarius. The soils tested were dehydrated and adjusted with sterilized water to give the water contents as 5、10、15、20 and 25% (w/w) for different treatments, with spraying 1 ml of 1×107 conidia/ml conidial suspension and with exposing adult weevils to the contaminated soil for 48 h. The mortalities of weevils were highest at a water content of 15%, while extreme water contents at 5 and 25%, weevil mortalities were below 50%. The efficacy of application was estimated under a water content of 15%, and by surface spray or mix of a 1×107 conidia/ml conidial suspension on or with soils tested. The weevil mortalities after being exposed for 48 h were higher when conidia sprayed on than mixed with clay loam or clay, while those between spray and mix inoculations in sandy loam were not significantly different. In assays with different soil types, the weevil mortalities were highest in clay loam, followed by clay, and sandy loam, being 72.5 ± 4.3%, 59.2 ± 1.4%, and 54.2 ± 2.9 %, respectively. Therefore, our results showed that M. anisopliae causes various levels of virulence to C. formicarius in different soil textures. After assaying the residual time of the fungus, the mortalities were varied with soil types, being moderately decreased by the residual time in both clay loam and sandy loam, while those on clay were increased. Using foliage, stem and tuber parts of sweet potato as inoculating substrates for adult weevils to be exposed to 1 ml of conidial suspension at 1×107 conidia/ml for 12 or 24 h, the mortality in the foliar spray was highest in both exposure times. In general, weevil mortality was increased as prolonged the exposure time. The horizontal transmission of mycosis in C. formicarius was tested by pairing bioassays. Male adults walking through Petri dish containing dry conidia were paired with healthy females for 1, 6, 12, 18, 24, 48 or 72 h. The conidium-carrying males themselves succumbed to 100% mortality in 6 days, and were confirmed to be possible carriers to transmit M. anisopliae. The mortality of female was getting higher, reaching above 80% when extended pairing time with male carriers for longer than 18 h. The pheromone-based autodisseminators were placed in polystyrene incubating boxes in sizes of 41×25×20 cm and 55×36×25 cm with clay soil and sweet potato veins, and then 50 females and 50 males were pairing for observing the transmission of M. anisopliae conidia from males contaminated in the autodisseminator to untreated females in test arena. Female mortality in the box of 41×25×20 cm was moderately higher than that in 55×36×25 cm one, indicating that the effectiveness of horizontal transmission might potentially be affected by the moving range of the weevils in the field. In addition, the pheromone-based mass trapping in combination with M. anisopliae applications was carried out to investigate an integration of non-pesticide control measures into IPM system as an effective strategy.