# Application Method Determines Effects of Beauveria bassiana on Eucalyptus grandis Growth and Leaf-Cutting Ant Foraging

## TL;DR

This study shows that applying the fungus Beauveria bassiana to eucalyptus plants can boost their growth and reduce ant damage, offering a greener alternative to chemical pesticides.

## Contribution

The study demonstrates that foliar application of Beauveria bassiana is most effective in promoting eucalyptus growth and reducing ant foraging.

## Key findings

- Foliar spraying of Beauveria bassiana significantly increased leaf number and shoot weight in eucalyptus seedlings.
- The same treatment reduced leaf-cutting ant foraging activity by over 50%.
- Soil drenching at sowing also enhanced plant height and shoot fresh weight.

## Abstract

Leaf-cutting ants pose a severe threat to South American eucalyptus plantations, leading to significant economic losses. Traditional management relies on synthetic chemical baits, which are notorious for their environmental toxicity and long-term persistence in the soil. Consequently, alternative control strategies are urgently needed. While entomopathogenic fungi can be applied directly to ants or their nests, they also offer a novel approach to plant protection through plant tissue colonization. These fungi can act as endophytes, living within the plant, to provide protection and stimulate growth. However, the effectiveness of this protection and growth stimulation often depends on the inoculation method used. In this study, we tested three inoculation methods. The most promising results occurred when eucalyptus leaves were sprayed with fungal suspensions 20 days after planting. This treatment both stimulated plant development and significantly reduced leaf-cutting ant foraging activity. This research provides a foundation for reducing our reliance on chemical pesticides, offering a greener solution that benefits both eucalyptus growers and the broader ecosystem.

Beauveria bassiana can colonize plants, acting against insect pests and promoting plant growth. This study evaluated how different fungal inoculation methods affected Eucalyptus grandis growth and the foraging behavior of ants. An isolate (LPP 139) was identified as B. bassiana based on ITS sequences. Seedlings were submitted to three inoculation methods using fungal suspensions at 1 × 108 conidia mL−1: (1) soil drenching at sowing (SD), (2) soil drenching 20 days after sowing (20SD), and (3) foliar spraying 20 days after sowing (20F) when compared to controls. SD treatment enhanced plant height (mean 25 cm with a 31.6% increase compared to the controls; p = 0.0353) and shoot fresh weight (mean 1.5 g, a 50% increase; p = 0.0154), while 20SD increased leaf number (141.4% increase; p = 0.0419). The 20F treatment increased leaf number (287.9% compared to the controls; p = 0.0006), shoot weight (mean fresh weight 1.5 g, a 50% increase; p = 0.0213 and mean dry weight 0.7 g, a 75% increase; p = 0.0236), and reduced leaf-cutting ant foraging (mean 26 cm2, a reduction of 53.6%; p = 0.0134). These findings highlight the dual action of B. bassiana, promoting plant growth and reducing the activity of ants.