兩種斯氏線蟲與黑殭菌(Metarhizium anisopliae)對黃條葉蚤之防治效果

Abstract

In this study, two entomopathogenic nematodes, Steinernema abbasi and S. carpocapsae, and the green muscardine fungus, Metarhizium anisopliae, were used to infect the striped flea beetles, Phyllotreta striolata, on crucifers. In the laboratory bioassays, infection of P. striolata adults and larvae with M. anisopliae was evaluated at different temperatures. Inoculation of the beetle adults with five concentrations of conidial suspension caused increase in cumulative mortality with increasing the conidial concentration, its LC50 being 3.2 × 106 conidia/ml. The LT50 values tested with 1 × 107, 1 × 108 or 1 × 109 conidia/ml were 9.39, 3.97 and 3.55 days, respectively. When the concentration was above 1 × 108 conidia/ml the fungus could cause more than 80% mortality. At a conidial suspension of 1 × 108 conidia/ml, the corrected cumulative mortality of adults increased with elevating the temperature ranging from 10 to 25°C. The optimal temperature for M. anisopliae infection was 25-30°C. When inoculated with 1 × 108 and 1 × 109 conidia/ml, larval mortalities of P. striolata were both more than 80% at 8th day post-inoculation, their LT50 values being 3.46 and 3.01 days, respectively. Additionally, larvae reared in a modified rearing method could attain over 80% survival. Field trials were studied in opening fields and a screen house. The numbers of P. striolata adults captured using sticky papers were adopted to monitor changes in populations, and the numbers of holes per leaf were used to evaluate the injury level. The results of field trials were not distinct from all treatments with S. abbasi, S. carpocapsae, and M. anisopliae, most treatment groups reduced the population densities and caused some degree of injury at 1 week post-treatment, and some of M. anisopliae treated group at 1 × 107 conidia/ml did not reduce the beetle population. Comparing efficacy with cartap and high concentrations of S. abbasi, both controlling agents were similar. Efficacy of two nematodes against P. striolata was studied using outdoor screen cages to simulate the field trials in summer. Application of nematodes once only reduced the population densities at 1 week post-treatment, while successive applications of nematodes, S. abbasi was more effective against P. striolata than S. carpocapsae, probably resulting from high temperatures at the experimental period. Using oblong pots to simulate field trials at the same place in winter, two nematodes and M. anisopliae were treated at different concentrations. Due to high pathogen concentrations and low temperatures, the numbers of adults captured were under 10 insects in all of the treatment groups without significant difference among various treatments. The weight of radish roots was measured at 52nd day after seeding, the radish roots in S. carpocapsae treated groups were generally heavier than S. abbasi treated groups, possibly due to its optimum for S. carpocapsae at low temperatures. The weight of radish roots in M. anisopliae treated groups using 1 × 108 conidia/ml was not significantly different with the control group, possibly resulting from being diluted the conidial concentration by soil beneath the ground surface.