Laboratory studies assessing the microbial biocontrol potential of diverse strains of Beauveria bassiana isolated from coffee berry borer, with emphasis on strains from Hawai'i Island and comparisons to commercial strain GHA

Authors: WRAIGHT, STEPHEN - Retired ARS Employee; HOWES, REBECCA - University Of Arizona; CASTRILLO, LOUELA - Retired ARS Employee; GRIGGS, MICHAEL - Retired ARS Employee; GALAINI-WRAIGHT, SANDRA - University Of Hawaii; CARRUTHERS, RAYMOND - University Of Hawaii; Matsumoto Brower, Tracie; Keith, Lisa

Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/11/2022
Publication Date: 8/17/2022
Citation: Wraight, S.P., Howes, R.L., Castrillo, L.A., Griggs, M.H., Galaini-Wraight, S., Carruthers, R.I., Matsumoto Brower, T.K., Keith, L.M. 2022. Laboratory studies assessing the microbial biocontrol potential of diverse strains of Beauveria bassiana isolated from coffee berry borer, with emphasis on strains from Hawai'i Island and comparisons to commercial strain GHA. Journal of Invertebrate Pathology. 194. Article 107819. https://doi.org/10.1016/j.jip.2022.107819.
DOI: https://doi.org/10.1016/j.jip.2022.107819

Interpretive Summary: Use of Beauveria bassiana (Bb) strain GHA is a major component of an areawide pest management program for coffee berry borer (CBB) in Hawaii. During field studies to assess the efficacy of spray applications of this pathogen, numerous genetically-diverse, naturally-occurring strains of Bb were found infecting CBB. These strains exhibited a greater capacity to persist (recycle) and spread in CBB populations than commercial strain GHA, and coffee growers have expressed interest in their use as CBB biocontrol agents. Laboratory studies were therefore undertaken to assess the commercial-development potential of these fungi and other CBB strains of Bb from Africa and Latin America, including Puerto Rico. None of the strains proved more virulent than GHA; however, a strain representative of the most common Hawaiian genotype produced greater numbers of spores on killed beetles than GHA. Additionally, the spores of the Hawaiian strains were substantially smaller than the spores of GHA and were produced on mycelial strands that grew farther away from the dead CBB than those of GHA, suggesting greater potential for dispersal via air currents or contact with host-seeking CBB. Results indicate that the greater natural epizootic potential of these strains is derived not from greater virulence but from these morphological and behavioral differences. A few strains also produced greater numbers of spores than GHA on a commercial mass-production substrate (cooked barley). These finding indicate strong potential for development of these novel strains as CBB biocontrol agents, but additional characterizations, including human and environmental safety, insect host range, and conidia stability (shelf life) will be required before they can be recommended for further development.

Technical Abstract: A series of experiments investigated genetically diverse coffee berry borer (CBB) strains of B. bassiana. Objectives included assessment of their biocontrol potential, particularly in comparison to commercial strain GHA currently applied for CBB control, and identification of various attributes potentially contributing to their comparatively greater epizootic potential in CBB populations. Bioassays identified one strain from Hawaii Island and one from Puerto Rico with virulence greater than GHA based on equal weights of unformulated conidia powder (CP); however, the greater potency of the CPs was ultimately explained by their 2.4-fold greater conidia densities (ca. 3.1 vs. 1.3 x 1011 viable conidia/g CP). Density was explained, in large part, by conidia size, but not by size alone. Bb-inoculated CBB held on moist filter paper were more susceptible to infection than those held on cooked green coffee bean (CGCB). A CBB strain representative of the most common Hawaiian haplotypes produced 2.6x more conidia after death of CGCB-held beetles than GHA (19.1 vs. 7.3 x106 conidia/cadaver). Following host death, no difference was observed in time to emergence and initial conidia production by GHA and a selected group of Hawaiian strains; however, mass sporulation was initiated within 2 days by strain GHA compared to 4–5 days by the Hawaiian strains. In a preliminary evaluation of conidia mass-production potential, CP yields of several strains were comparable to GHA on a weight basis and significantly greater than GHA on a conidia basis (1.3–1.6 vs. 0.7 x 1013 viable conidia/kg barley substrate).