Invasive ant exploits fungus for food and potential shelter

Researchers have recorded for the first time the interaction between the invasive ant *Allomerus auropunctatus* and a living fruiting body of a fungus from the genus *Ganoderma*. Worker ants collected exudate produced by the fungus and used its structure as a foraging site and possible nesting area. The work also gathered citizen science data and reinforced the species' broad dietary flexibility.

The observation took place in Laranjal Paulista, São Paulo, in an area under strong rural and urban influence near the Sorocaba River. The site is located in the transition zone between the Atlantic Forest and the Cerrado. The researchers found groups of worker bees on the fruiting body during the night (doi 10.37486/2675-1305.ec08036).

Exudate secreted by the fungus

The ants were collecting an exudate secreted by the fungus. Several worker ants arrived at the feeding point in coordinated groups. The researchers also observed individuals entering and exiting through natural openings present in the fungal structure. This behavior indicated possible use of the site for nesting. The team did not confirm that the ants themselves opened the cavities.

Remains of arthropods appeared near possible nest entrances. The material included insect fragments, mostly from other ants. Researchers interpreted these residues as a possible result of opportunistic predation or food discard by the colony. Other ants also visited the fungus to collect the same substance.

Wood-decomposing fungus

The interaction is of added interest because it involves a wood-decomposing fungus. Species of the genus Ganoderma cause white rot in the roots and lower trunks of trees from different botanical families. The active mycelium can secrete polysaccharides in response to environmental changes, such as changes in temperature, pH, and mechanical damage.

In the observed case, the researchers consider a possible association between polysaccharide production and alterations in the mycelium. The accumulation of organic matter deposited by the colony may have influenced the pH. Mechanical damage may also have contributed. Adult Mycotretus flavomarginatus fed on the fruiting body and could cause lesions.

Causal relationship?

The study does not allow us to define a causal relationship between ant activity and exudate production. It also does not allow us to determine if this interaction occurs frequently in nature. The observation period was limited. A visit a week later did not allow for the collection of the colony and the fungus, as the area had been burned and cleared.

To broaden the analysis, the researchers examined 787 records of Allomerus auropunctatus available on iNaturalist, also accessible through the Global Biodiversity Information Facility. Blurred images, distant records, observations with bait, and instances of artificial feeding were discarded. The evaluation considered visible morphological characteristics and published taxonomic information.

Among the 787 records reviewed, 150 involved feeding behavior. After screening, 25 were excluded. The final set comprised 125 observations. Of these, 32 occurred within the species' native distribution area and 93 outside of it.

Food strategy

The collection of honeydew from hemipterans appeared as the most frequent feeding strategy in the survey. The records included interactions with membracids, aphids, and scale insects. The baseline also showed consumption or utilization of various arthropods, in addition to plant resources.

Approximately 22,6% of the feeding records involved insects used as prey or carcass material. Observations encompassed representatives of Hymenoptera, Diptera, Lepidoptera, Coleoptera, Isoptera, Orthoptera, and Hemiptera. Another 21% showed the use of plant resources. Floral and extrafloral nectar predominated in this group, with additional records of leaf tissue, pollen, sap, and fruits. Unidentified organic matter appeared in 9,4% of cases.

The record in the fungus remained unique among the observations evaluated. For the researchers, the dataset demonstrates the ability of Allomerus auropunctatus to exploit varied food sources in different environments. This plasticity helps to understand the ecological success of the species, considered one of the hundred worst invasive species in the world by the International Union for Conservation of Nature.

Taxonomic classification

The taxonomic classification of the species has also undergone a recent change. Phylogenetic evidence led to the synonymization of the genus Wasmannia under Allomerus. Thus, Wasmannia auropunctata came to be called Allomerus auropunctatus.

The researchers also highlight the potential interest of dietary flexibility for future studies in biological control. However, the interaction with the fungus does not appear to produce only benefits for the fungal organism. The ants may prey on insects associated with the fruiting body, but the accumulation of waste and activity within the structure can also cause negative effects. The study does not determine whether these alterations favor the production of polysaccharides used as food or attract new prey.

The work was carried out by scientists João PS Medeiros, Karine S. Carvalho and Edgar B. Crispino.