Submitted to: American Society of Sugarbeet Technologists
Publication Type: Proceedings
Publication Acceptance Date: 4/16/2009
Publication Date: N/A
Citation: Interpretive Summary: In three years of sugar beet field trials where an entomopathogenic fungus, Metarhizium anisopliae, has been applied at planting time for subsequent control of the sugarbeet root maggot, persistence trends depended on the formulation used. Spores sprayed into the furrow in water gave immediately high titers but degraded rapidly. With fungus applied as spores on nutritive granules, initial titers were very low; time was needed for the fungus to grow out and sporulate thereby increasing the titer of spores in the soil. Thus, granular formulations of the fungus offer efficacy advantages in addition to greater compatibility with existing farm practice over sprays of fungus spores into the open furrow. In the presence of an oat or rye cover crop, where the soil is heavily colonized by the grains’ root systems, there was at least a numerically greater increase in spore levels, in comparison to soil without the cover crop, possibly due to a higher soil moisture or to a changed biotic environment.
Technical Abstract: The sugarbeet root maggot, Tetanops myopaeformis (Röder), is a major insect pest of sugarbeet, Beta vulgaris L., in North Dakota, Minnesota, and Idaho. Three field trials using the insect pathogen Metarhizium anisopliae (Metch.) Sorok. ATCC 62176 in conjunction with cover crops were conducted in 2002-2004. Granular and aqueous spray formulations of M. anisopliae were applied in furrow to replicated plots at 8 x 10E12 viable conidia/ha. Oat (Avena sativa L.) and rye (Secale cereale L.) cover crops were planted prior to sugarbeet at three rates to create different microenvironments for the fungus. Soil samples were collected at 0, 30, or 60 d after treatment (DAT). Significantly higher numbers of conidia were detected in soil samples collected immediately after application in fungus spray plots compared to granule plots. This suggested delayed activation and proliferation of M. anisopliae conidia on granules, which has also been observed in the laboratory. There was also a 90% decline in conidial viability for the aqueous formulation within 30 DAT. In 2002, a 1.5 to 7.7-fold increase in conidial density per gram of soil occurred between 0 and 60 DAT in plots treated with M. anisopliae granules. This increase was numerically higher in cover crop plots compared to non-cover plots. Soil moisture tension in cover crop plots was higher (i.e., average of -27 kPa) compared to no cover plots (-17 kPa). It appears that granular formulations of M. anisopliae can persist in low soil moisture microenvironments that occur under a cover crop canopy.