Selection and yield optimisation of a Beauveria bassiana isolate for the biological control of leaf cutter ants

Abstract

Leaf cutter ants are one of the most important pests to agriculture and forestry in the Neotropics. One environmentally friendly solution to control them is biological control with the use of conidia of some fungi, such as Beauveria bassiana. We tested several isolates of this fungus (obtained from leaf cutter ants) by inoculating ants from seven colonies of Acromyrmex lundii with 5 × 106 conidia ml−1 suspensions. All the isolates proved to be good controllers, but isolate 5 (B5) was found to be the best performing option, because it caused the death of 92.3% of the ants in 4–5 days. Isolate 7 (B7) was the second-best option and used for comparative purposes. Further assays were performed with the isolate B5 to optimise yield production. Three types of rice (brown, parboiled and white rice), soybean hull and yerba mate (Ilex paraguariensis leaves) were tested as substrates, and following that, different conditions of solid-state fermentation (SSF) were changed so as to select the most productive combination. Among treatments, we selected brown rice as substrate and a temperature of 28°C and 75–85% relative humidity in the SSF chamber because this allowed a production of 6.4 × 108 and 1.2 × 109 conidia of B5 and B7 per gram of final substrate with a 98% and 97.5% of germination, in 10 days, respectively. The differences in yield obtained under the same conditions for both isolates show that yield is isolate-dependent. A description of the customised design of the SSF chamber used is included.