PERSISTENCE OF BEAUVERIA BASSIANA IN SOIL FOLLOWING APPLICATION OF CONIDIA THROUGH CROP CANOPIES

Abstract

AbstractThe influence of three formulations, water, oil, and a 5% oil emulsion, and two crops, alfalfa and crested wheatgrass, on the deposition and subsequent persistence of Beauveria bassiana (Balsamo) Vuillemin conidia in soil was investigated. The alfalfa canopy was considerably denser than that of wheatgrass. Leaf area indices for alfalfa ranged from 1.8 to greater than 2, those for wheatgrass ranged from 0.24 to 0.55. Initial populations of conidia averaged 1.2 × 103 to 2.6 × 104 colony-forming units (cfu) per gram of dry weight of soil under alfalfa, and 5.5 × 103 to 3.4 × 104 cfu per gram of soil under wheatgrass. There was no consistent influence of formulation or application method (high or ultra low volume) on penetration of conidia through the canopy of either crop. However, conidial populations under wheatgrass were larger than those under alfalfa in two of three trials. After 225–272 days (over winter), substantial populations (87 to 4.3 × 104 cfu/g) were recovered from soil. Although conidial densities decreased over time, reductions in population size over this period were generally less than one order of magnitude; neither crop nor formulation consistently influenced conidial persistence. In most instances, a rapid decrease in conidial populations was observed within approximately 20 days but thereafter, the rates of population decline abated. The initial decrease in conidial numbers did not appear to be related to precipitation. This study demonstrates that substantial numbers of B. bassiana conidia infiltrate crop canopies, are deposited on the soil surface, and subsequently persist in a clay–loam soil. The aerial application of B. bassiana conidia to vegetated roadsides may prove useful for the management of ovipositing grasshoppers and emerging nymphs.