Applying ant hygiene: can ants protect crops against diseases?

Summary

1. Ants provide ecosystem services to agriculture by controlling arthropod pests. Plant diseases, however, can be more detrimental to plant production than herbivory. Here, we investigate a new type of potential ant service – namely that ant hygiene may extend beyond the ants themselves to also cover their host plants, potentially resulting in protection against plant diseases.
2. We review the literature for support to the idea that ant hygiene, including ant-derived antibiotics, can protect plants against pathogens, and we provide empirical pilot data from a Danish apple plantation further supporting this idea.
3. In a literature search, we found nine studies showing ants reducing pathogen incidence on host plants. These studies cover nine ant species reducing the incidence of 10 fungal and bacterial plant pathogens on eight different plant species, including a commercial crop, mango. In many cases, the presence of ants led to several-fold reductions in pathogen incidence compared to ant-free control plants.
4. Data from the apple plantation showed that wood ants (Formica polyctena), depending on apple variety, reduced the incidence of two fungal pathogens on apples. On the varieties Alkmene and Collina the proportion of apples with brown rot (Monilia fructigena) were reduced on ant-trees with 24% (P=0.23) and 84% (P=0.036), respectively, compared to ant-free control trees. Similarly, on the varieties Alkmene, Collina, Holsteiner Cox and Resista the proportion of apples discarded due to apple scab (Venturia inaequalis) were reduced on ant-trees with 5% (P=0.19), 19% (P=0.16), 91% (P=0.034) and -16% (P=0.56), respectively.
5. Synthesis and applications: These results stress that ants may provide a hitherto underestimated service to plants, namely protection against pathogens. This service may be utilised in agriculture, either as a by-product when ants are used for herbivore protection, or as a pathogen protection measure in its own right. We draw attention to this service to managers and call for future research investigating its mechanisms, generality, applicability and economic feasibility. Knowledge on responsible mechanisms may help to move this service into an applied context and may lead to the discovery of new antimicrobial compounds important to agriculture.