Can ants control plant diseases?

Summary

Ants are challenged with disease transmission and microbial hygiene due to a life in densely populated long-lived ant nests. To improve hygiene ants use different ways to reduce microbial growth. They groom their bodies with (i) antimicrobial excretions from their metapleural and poison glands, they (ii) associate with bacterial symbionts capable of producing antibiotics, or they (iv) eat microbial spores. These defenses extend beyond the individual ant, as ants may groom nest mates, food substrates or nesting material. Defenses may even extend beyond the nest and affect mutualistic partners. For example, ants in specialized myrmecophytic ant-plant interactions have been shown to reduce pathogen loads on their host plants. Here we tested if wood ants (Formica polyctena) used for biological control of insect pests in an apple plantation affected two fungal apple diseases. Two apple varieties were infected by brown rot (Monilinia fructigena) and four with apple scab (Venturia inaequalis). When comparing ant-trees with ant-free control trees, we found significantly more infected apples on the controltrees. According to variety, the number of apples with brown rot increased with 37 and 526 %, on control trees, whereas the number of apples with apple scab increased with 21, 47, 530 and -13 %, respectively. This is the first time the antimicrobial properties of ants are tested in an agricultural context. Further studies are needed to test its generality and applicability on a broader scale.