Biological control assays with entomopathogenic fungi against the greenhouse thrips Heliothrips haemorrhoidalis

Abstract: The Conservatory and Botanical Gardens of Geneva (CJBG), run under the organic
agriculture label of BioSuisse, has been battling without success an unidentified thrips
damaging its greenhouse collections since 2016. The infestation reached a critical threshold,
prompting the use of entomopathogenic fungi from the Plants and Pathogens laboratory of
HEPIA, which had proven effective in combating other insect pests. The 11 fungal isolates
tested included Beauveria bassiana, Metarhizium anisopliae, Akanthomyces lecanii (=
Lecanicillium lecanii), and Isaria fumosorosea, along with a commercial fungal biopesticide
previously used by the CJBG.
The greenhouse environment was analyzed to track temperature and humidity, and a list
of plants affected by thrips was compiled. The thrips were identified as Heliothrips
haemorrhoidalis, a common greenhouse pest. Temperature conditions were considered when
selecting the most effective fungal isolates, with trials identifying six isolates that performed
best. Mortality pre-tests were conducted on Phaseolus vulgaris beans to assess the viability of
thrips, using an immersion method proven effective in the laboratory.
The first screening trial showed that B. bassiana isolate 2.1 achieved a significant 75 %
mortality rate. The second in vitro trial, conducted at concentrations ranging from 105
to 108 CFU/ml with the most effective isolates from the first trial (B. bassiana 1.1 and 2.1, Genbank accessions UASWS 1865 and UASWS 1866, respectively, and M. anisopliae 33.1 and 34.2, Genbank accessions UASWS 1462 and UASWS 1468, respectively), showed a mortality rate of 85 % at the 108 CFU/ml concentration. Lethal concentration (LC50 and LC80) values were determined for each isolate, with isolate 1.1 showing the lowest LC80. A semi-in vitro trial with isolates B. bassiana 1.1 and M. anisopliae 33.1 on infested bean plants showed promising results: isolate 1.1 achieved 84 % mortality and isolate 33.1 reached 76 %. An in vivo trial at CJBG, on greenhouse plants, with isolate 1.1 showed 75 % mortality after 8 days. These results suggest that further trials with isolates 1.1 and 33.1 are necessary before considering approval as a phytosanitary product.