Persistence of the Entomopathogenic Fungus, Beauveria bassiana, on Phylloplanes of Crested Wheatgrass and Alfalfa

The influence of three formulations (water, oil, and a 5% oil emulsion) and two canopy positions (top and middle) on the persistence of conidia of Beauveria bassiana on phylloplanes of alfalfa (Medicago sativa) and crested wheatgrass (Agropyron cristatum) was investigated. Penetration into the canopy and coverage of leaves by conidia in oil applied with an ultralow volume (ULV) applicator was equal to that of conidia formulated in water-Tween or the oil emulsion applied at high volumes. Numbers of B. bassiana recovered from the top of the canopy immediately following application ranged from 8.3 × 103 to 1.2 × 105 colony-forming units (cfu)/cm2 for alfalfa and 5.1 × 103 to 3.5 × 104 cfu/cm3 for wheatgrass. In alfalfa, but not wheatgrass, the canopy influenced penetration of conidia as evidenced by the significant effect of sampling height. Conidia were relatively short lived on leaves at the top of the canopy in both crops and, by 4 days, conidial populations were reduced by more than 75%. At the middle of the canopy, conidia persisted longer on phylloplanes of both crops. This observation was more pronounced in alfalfa than in wheatgrass; at 16 days populations were reduced by more than 99% on wheatgrass leaves and by 28 to 85% on alfalfa leaves. Formulation had no obvious effect on the persistence of conidia. Mortality of grasshopper nymphs (Melanoplus sanguinipes) fed leaves sprayed with conidia from the top of the canopy corresponded to the conidial population data. Immediately after application of conidia, nymph mortality attributed to B. bassiana ranged from 31 to 58%, whereas mortality of nymphs fed leaves 2 days postapplication of conidia ranged from 0 to 5.5%. Methods to increase the persistence of conidia are required if plant surfaces are targeted in a strategy to control grasshoppers with B.bassiana.