Mutualism of the Month: Lac-insects and their bodyguards

Every month we showcase a relationship between two or more species or groups of species that can be considered a mutualism: a relationship where both members benefit. This month’s mutualism is the relationship between a group of ants and lac-insects, an interesting group of insects known for producing a waterproofing resin called “lac”.

Ants frequently provide protective services to another species in exchange for food or housing.  For an example, check out an earlier mutualism of the month on ants protecting acacia trees from elephants. Ants often protect honeydew-producing insects from predators and parasitoids. They are analogous to the rancher, protecting his cows from wolves. Relationships where an ant protects a honeydew-producing insect are normally detrimental to human agricultural efforts as honeydew-producing insects damage crops by sucking out the plant’s sap. Aphids, a group of sap-sucking insects, often protected by ants, decimate crops and ornamental plants when allowed to build up in numbers.

An ant tending lac-insects. The lac-insects are busy sucking sap within their red tubes of lac. This lac will be scraped off by a farmer later and processed to produce shellac or some other economically important compound. Photo credit.

An ant tending lac-insects. The lac-insects are busy sucking sap within their red tubes of lac. This lac will be scraped off by a farmer later and processed to produce shellac or some other economically important compound. Photo credit.

Unlike aphids, lac-insects are desirable and are cultured for their production of economically important lac. Lac is a red resin which protects the lac-insects from water-loss, waterlogging from rain, and predation. The insects produce a hollow tube with the lac, where they live inside, happily sucking out the sap of their host tree. Lac is widely utilized by food, textiles, and pharmaceutical industries and is worth approximately US$92 million annually. Raw lac can be ground up, purified, and mixed with alcohol to make shellac, which is used as a food glaze, a wood finish, and in some nail polishes. Shellac was the dominant high-gloss coating on furniture until lacquer was introduced in the 1920s and 30s.

The cultivation of lac-insects necessitates organic methods (no pesticides). The chemicals that could be used to kill their predators and parasitoids (animals that are parasitic for part of their life) would also kill the lac-insects - here’s where ants come in. Certain species of ants will protect lac-insects from predators and parasitoids in return for sugar-rich honeydew.

An aphid being "milked" by an ant. A droplet of honeydew can be seen being produced at the back-end of the aphid, which the ant is consuming. Photo credit.

An aphid being "milked" by an ant. A droplet of honeydew can be seen being produced at the back-end of the aphid, which the ant is consuming. Photo credit.

Until very recently (earlier this month!), the protective behavior of ants against parasitoids had never been quantified. Researchers went out to a lac plantation in China and did just that. For some trees, they removed all ants from the tree and placed a barrier of insect glue at the base of the tree. This excluded ants from the tree, removing ant protection for the lac-insects. Another set of trees was designated as “low ant attendance”. These trees had multiple species of ants, but the ants only visited the lac-insects as individuals. A third set of trees was found that contained active colonies of the ant Crematogaster macaoensis, which aggressively tends lac-insects night and day. This species of ant also defends its trees from other ant species and often constructs protective shelters over the lac-insects. These trees were considered “high ant attendance” trees. Parasitoids were caught as they emerged from the lac-insects with a mesh trap and were counted. All the parasitoids were small wasps. These wasps lay eggs inside the lac-insects, then bust out when they are ready to mate and find more lac-insects. It’s tough being a lac insect. For a cheesy natgeo video of aphids being parasitized by wasps, check out the video below.

Simply put, the more ants there are, the less parasitoids there were! The parasitoids were being deterred by the ants; ants would attack parasitic wasps before they could inject their eggs. The more aggressive ant species, particularly C. macaoensis, were better protectors than the more passive ants. These results were to be expected, but interestingly, some of the rare species of parasitic wasps were most abundant in the presence of the most aggressive ants. The authors of the paper postulated that perhaps some of the parasitic wasp species may cue in on the ants, rather than the lac-insects. That’s a scary prospect when your protectors are bringing the very thing you need to be protected from!

To read the original scientific article from the journal PLOS ONE click here.

Don’t forget to check out more mutualisms of the month and all the other great articles on FTDM! Stay hungry!

Check out more mutualisms of the month!