Submitted to: American Society of Sugarbeet Technologists
Publication Type: Proceedings
Publication Acceptance Date: 5/5/2005
Publication Date: 5/31/2005
Citation: Jaronski, S.T., Grace, J.A., Schlothauer, R. 2005. Metarhizium anisopliae for biocontrol of sugarbeet root maggot: constraints and challenges. Proceedings, 33rd Biennial Meeting, American Society of Sugar Beet Technologists. pp. 185-187. Interpretive Summary: The entomopathogenic fungus, Metarhizium anisopliae, has been under study by USDA ARS and North Dakota State University as a potential mycoinsecti-cide for controlling Tetanops myopaeformis, the sugarbeet root maggot. These fungi can be used in the form of a granular formulation applied at planting, and/or as an aqueous, band-over-row spray of conidia to the bases of the plants just before peak fly oviposition. Ecological factors poten-tially affecting performance of Metarhizium anisopliae against the sugarbeet root maggot were examined in laboratory and field studies. If there is enough soil moisture to germinate seed, the fungus will grow out from granules and resporulate, amplifying spore titers. Seed coat fungi-cides, while toxic in vitro, do not affect the fungus on granules even 0.5mm from a coated seed. Conidia applied into surface soil in field plots persist and are infectious far longer than lab data predict. Infection by conidia in soil in proportional to soil moisture, as expected, but soil type/texture greatly affects this relationship. Soil temperatures at the in the arena and time critical for infection of target larvae are favorable for spore germination, infection and mycelial growth.
Technical Abstract: Ecological factors potentially affecting performance of Metarhizium ani-sopliae, Strains MA1200 and F52, against the sugarbeet root maggot were ex-amined in laboratory and field studies. In all tested soils, Metarhizium failed to grow out of granules at moistures below 0.99Aw (-1.5 MPa), but based on in situ measurements during field trials, soils were moist enough in seed zone for fungal outgrowth and sporulation. Optimal mycelial growth occurred at 20-32 degrees C.; below 20 and above 30 degrees C. growth slowed, ceasing at ~8-10 and 34 degrees C., resp. Whereas the fungicide thiram inhibited growth of Metarhizium in vitro, fungal granules, placed within 0.5 cm of treated seed or fungicide-impregnated filter paper disks on agar or moist soil, grew and sporulated abundantly. Mefenoxam and hy-mexazole had no effect. At least 4 Metarhizium granules/cc soil were neces-sary for >90% efficacy. Metarhizium titers, after application of spores to soil beneath plants, decreased after by less than 80% at one site, remained generally steady at a second and briefly increased to a steady state at the third site. These trends occurred in the face of greatly fluctuating soil moistures from saturation to less than 0.50Aw and soil temperatures of 17-25 degrees C. Infection by Metarhizium conidia was proportional to mois-ture, decreasing to 0-11% at 10%WHC. The fungus seems much less effective in dry soil such as often exists in the top cm where sprays are applied. In five other soils, differing in composition, efficacy ranged from 7-100% in the different soils at 15%WHC but 44-87% at 30%WHC, with the fungus re-sponding differently to moisture in the different soils (CFUs remained un-changed).