Enabling mycelial application of Hirsutella thompsonii for managing the coconut mite

Abstract

Laboratory and field studies were conducted to examine the prospect of mycelial application of Hirsutella thompsonii as an alternative to the use of mycelial–conidial formulations of the fungus in the suppression of the coconut mite, Aceria guerreronis. In a series of laboratory experiments, glycerol, yeast extract powder and dehydrated malt extract broth were found to be the best among nine substances investigated as possible adjuvants for use on coconut palms in the field along with H. thompsonii mycelia. H. thompsonii biomass in the presence of adjuvants not only produced more colonies but also yielded more conidia per pellet. In terms of the density of conidia generated on a mycelial mat the treatments varied highly significantly in two methods, with glycerol showing an average of 106% increase over control. Though irradiance with simulated sunlight resulted in reduced conidiogenesis, in general, adjuvant-treated pellets, both exposed and unexposed to simulated sunlight, produced substantial conidiation compared with control, irrespective of the two incubation conditions. Better conidiation was observed under alternating light–dark regime than under total darkness in all the treatments. Glycerol boosted the pathogenicity of H. thompsonii by 16.5% over control. In the field, a newly developed mycelial formulation of H. thompsonii applied after tank-mixing separately with the three selected adjuvants brought down the post-treatment population of the coconut mite by 85.6–97.1%. Application of the fungus in combination with glycerol resulted in a tolerable mean nut damage grade of 2.0 during the pre-harvest stage, compared with an acute score of 4.0 in control palms.