Sugar-sweet symbiosis in the insect kingdom

AMost are eager hunters, after all, their brood needs a high-protein diet. Similar to wasps, ants also love sweets. Sugar serves as a source of energy that keeps them going. Since they have to search for the sweet fuel on foot, they rarely have the opportunity to grab cakes or soft drinks. In addition to sources of nectar offered by plants, ants mainly use the sweet excretions of plant suckers, which include aphids, scale insects and cicadas. When these insects pierce the sieve tubes of plants to feast on the products of photosynthesis, they ingest sugar in abundance. That is why most of them are eliminated immediately. What then sticks to leaves and twigs as so-called honeydew is also collected by honey bees, who use it to make “forest honey”.

Ants, on the other hand, often harvest directly at the source: they tap with their antennae on the abdomen of an aphid, which then releases a droplet of honeydew. The fact that aphids are actually milked in this way can be seen in the garden, for example: the black garden ant grows on various plants (Lasius niger) Aphids like dairy cattle. In return for sweet gifts, it protects the tender-skinned insects from ladybirds, lacewings and other attackers.

Ants are also among the animals that can be dangerous to aphids. It is well known that the cow that you want to milk is not allowed to be slaughtered. In the symbiosis with plant teats, the ants also take this to heart, the little crawling animals actually like to eat. Luis F. Camacho and Leticia Avilés from the University of British Columbia in Vancouver have observed that they go easy on their partner even when he cannot deliver a sweet meal. In the east of Ecuador, they studied ants that milk certain species of cicadas in the rainforest of the Jatun Sacha nature reserve as busily as local ants milk their aphids. In order to find out how much resistance the ants have against their symbiotic partners, they were confronted with different prey.


Ant milking aphids
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Image: dpa

Assertive thanks to honeydew

As the Canadian zoologists report in the Biology Letters, Free-living ants were most reliable in sparing those cicadas that usually provide them with honeydew. Even with dead animals that could no longer deliver, the ants shied away from being attacked for hours. Termites, on the other hand, were attacked immediately. Unless the researchers attached a droplet of honey to them. Then the ants mostly stayed peaceful and enjoyed the honey. When there was nothing left of it, they became more aggressive. But even then, some of these termites were left untouched, while all those who had never delivered anything sweet were invariably carried away as prey.

Presumably, ants recognize their symbiotic partners by means of special fragrances, even without honeydew. But they can be soothed even more by sweet gifts. Even common prey animals are spared if they have something to offer. No wonder, after all, honeydew plays a central role in the diet of many ant species. How much ants benefit from this food source was shown by biologists working with Jonathan Shik from North Carolina State University in Raleigh using the example of the Argentine ant (Linepithema humile) proven.

Originally only native to South America, the Argentine ant is now widespread almost worldwide in the tropics and subtropics and is feared as an invasive species. In their new home, they often multiply so rapidly that they drastically decimate the long-established insect fauna and thereby turn entire ecosystems upside down. Without a supplier of honeydew, the Argentine ant would be less assertive. Because, as Shik and his colleagues have found, a diet high in protein and low in carbohydrates is not good for you. With a high-sugar diet, however, the workers of the Argentine ant survive much longer and raise more offspring. A similar dependence on symbiotic partners was found in the red fire ant (Solenopsis invicta), which also comes from South America. A particularly aggressive species, this invasive ant is notorious for its painful stings. Their colonies also thrive best when they can harvest plenty of honeydew.

Plant teats, which excrete excess sugar, have existed for more than 260 million years. Ants did not develop until much later, a good 100 million years ago. Over time, these wasp related insects evidently found a taste for the sweet snacks that prey animals such as aphids and cicadas offered them. When evolution led to the first symbiotic relationship with producers of honeydew remains an open question. The fact that ants developed into the dominant predators in the crown area of ​​tropical forests around 50 million years ago is due to their ability to use honeydew as a source of food: with a diet that consists largely of carbohydrates, they can build up much larger populations than if they had to rely on their hunting luck alone.

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