Improving photoprotection of Beauveria bassiana conidia for biological control of the cactus pest Dactylopius opuntiae in the semiarid region northeast of Brazil

Abstract

A technical grade conidial preparation of the entomopathogenic fungus Beauveria bassiana isolate LCB63 was exposed to ultraviolet light ( UV) from artificial and natural sources in the presence of various photoprotectants to develop a viable formulation to control Dactylopius opuntiae, a limiting pest of forage cacti in the Brazilian semiarid region. In the first experiment, Conidial viability was assessed in preparations containing water and liposoluble photoprotectants at a concentration of 1% and exposed to solar radiation for 0, 60, and 120 min. In another experiment, preparations containing different concentrations of the selected photoprotectants were exposed to natural and artificial UV radiation for 120 and 5 min, respectively. Insecticide activity of technical grade conidial preparations containing selected photoprotectants were evaluated against first instar nymphs of D. opuntiae. In general, no evaluated compounds showed deleterious effects on conidial germination, and they provided a strong degree of protection over a short period of time. One-percent concentrations of starch and skim milk resulted in low photoprotection over a longer irradiation period, while 1% Neo Heliopan AV, Neo Heliopan E1000 and Oxybenzone provided a moderate degree of protection. Increasing the concentration of Oxybenzone increased germination rate to 70%, while treatment with 1% Neo Heliopan E1000 maintained germination at 50%. A technical grade conidial preparation containing 1% Oxybenzone was effective in promoting D. opuntiae nymph mortality. These results suggested that addition of liposoluble photoprotectants could provide better photoprotection for B. bassiana LCB63 conidia in bioinsecticide formulations.