Environmental-Stress Tolerant Formulations of Metarhizium anisopliae var. acridum for Control of African Desert Locust (Schistocerca gregaria)

Abstract

Entomopathogenic fungi are highly susceptible to the damaging effects of solar radiation. Attempts to protect entomopathogenic fungi from solar radiation have been, for the most part, unsuccessful. A new strategy for formulating entomopathogenic fungi for protection from solar radiation and desiccation has been developed tested using the acridid entomopathogen Metarhizium anisopliae var acridum (IMI 330189). This strategy involves coating spores with water-soluble materials that provide protection from solar radiation and enhance spore survival during drying. Development of this formulation involved the following: 1) production of an infective spore-type in liquid culture that could survive drying; 2) coating spores during an air-drying process; 3) reducing formulation particle size for oil suspension; 4) testing the effects spore coating on spore-tolerance simulated sunlight; and 6) testing the effects of spore coating infectivity to Schistocerca americana.

Aerial conidia, submerged conidia, and blastospores produced in a high-osmolality liquid medium all had high desiccation tolerance relative to blastospores produced in Adamek's media. Blastospores produced in high osmolality medium were the most infective to S. americana in an aqueous 20% molasses solution followed by submerged conidia and aerial conidia, with LT50 values (95% C.I.) at 1 x 106 spores/insect of 7.8 d (6.7 to 9.0 d), 10.5 d (9.5 to 11.6 d), 14.6 d (11.9 to 18.0 d), respectively. Comparisons were made among cell-wall characteristics of these spore-types, including cell-wall thickness, lectin-binding, charge, and hydrophobicity.

An optimal spore-coating formulation was selected on the basis of spore survival and germination over time after air-drying, particle-size reduction, and storage at 28 Ë C. This spore-coating formulation, consisting of skim milk, Kraft lignin (Curan 10®) and glycerol, greatly improved the tolerance of aerial conidia and submerged conidia to simulated sunlight, increasing the LT50 (95% C.I.) of aerial conidia from 4.0 hr (3.1-5.1) to 17.0 hr (12.5-23.0). The spore coating formulation decreased the infectivity of spores in oil to adult S. americana; reducing the LT50 values of aerial conidia at a dose of 1 x 105 spores / insect from 5.8 d (4.9-6.9 d) to 8.2 d (7.3-9.3 d).