Controlling leafcutter ants

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Controlling leaf cutter ants in the tropics

Leaf cutter ants are commonly found in tropical forests and are an important part of the ecosystem. Their large deep nests aerate the subsoil and the ants move large amounts of organic matter below the surface, most of which eventually decomposes and enriches the soil. Other than humans, they are one of the only other species known to do agriculture. The ants cultivate a specific fungus in farms deep within their underground nests, which is the primary food source for young ants.

Leaf cutter ants can be highly destructive to trees and a large colony can completely strip the leaves from a tree in one night. These leaves are not directly eaten by the ants, but are used to feed the farmed fungus. In tropical regions, leaf cutter ants are a big problem for agriculture, as they can quickly defoliate crops. Many indigenous trees have evolved anti-fungal chemicals in their leaves, which protect them from the ants.

The ants are very difficult to get rid of, as the nests can be enormous, extending out to a radius up to 80 m, and containing up to 8 million ants. The colony can travel several hundred metres to find food. Hence, the nests can be difficult to locate as the ants may be coming from a neighbouring property. In general, low-grade pesticides or fungicides are used by farmers to kill the ants. The poison is mixed with a bait food, which is taken down into the nests by the ants, and either directly kills the ants, or kills their fungus food source.

At Kadagaya, we are trying to follow organic agricultural methods and strictly limit the amount of industrial chemicals used (preferably to zero). Hence, we have been investigating numerous organic options for controlling leaf cutter ants. Here we summarize some of the methods we have tried and their effectiveness.

Barrier methods

Barrier methods focus on protecting valuable trees by placing a physical barrier around the lower trunk. It is common in South America to see the trunks of trees painted white. Some people claim this deters the leaf cutting ants, although we had no success with this method. Placing plastic plates and tape around the trunks of small trees was effective for some months, until the ants realized they could simply walk over it. Placing PVC plumbing tubes and plastic buckets around the trunks of larger trees is quite effective, but does not always fully protect the trees. Grass and branches can fall on the tubes and give the ants easier access to the trunk. The most effective barrier method we found was to paint an agricultural adhesive onto PVC tubes around the base of the tree. This adhesive is a very sticky glue that catches any insects walking up the trunk. The disadvantage of this method is that it is non-selective and also catches beneficial insects. In summary, barrier methods are reasonably effective, but need regular maintenance, and are probably too labour intensive for large plantations.

Fungicide methods

Fungicide methods focus on killing or reducing the growth of the farmed fungus to starve the ants and encourage them to move their colony. One method is to introduce a competing fungal strain that will infect the nest and compete with the farmed fungus. The growth of undesired fungus is a common problem for the ants, and they need to constantly clean the fungus farm and remove other strains. The escovopsis fungus is known to cause colony collapse of leaf cutter ants and is being researched as an organic method for controlling them via a biological fungicide. There are various methods for producing home-grown fungus that is said to have the same effect on the leaf cutter colony.  For example, cultivating fungus on white rice and placing around the entrances to the nest has been shown to be effective in some cases. However, the fungal species that grows on the rice will depend on the spores available in the local environment and will vary depending on the location. We had no success with this method.

We also tried spraying valuable trees with natural fungicides, such as tea-tree oil and neem oil. This is labour intensive and needs to be done often, especially in the rainy season, and was not very effective. Our neem tree is one of their favourites! A very effective method is flooding the nest with large amounts of water mixed with wettable sulfur, which is a good fungicide. Sulfur powder is commonly used in the garden to fight snails, slugs, and fungal diseases. In addition, the sulfur powder can be left at the entrance to the nest; we observed that the ants took it into the nest and ant activity greatly declined in the following days.

Disturbing the nest

Control of the colony can be achieved by sufficiently damaging the nest. The ants will be occupied in rebuilding the nest and have fewer resources to damage crops. However this is a temporary solution. If the queen can be killed, the colony may move the nest. We tried damaging nests by pouring boiling water into the entrances, and digging up large sections of the nest. The colony was disabled for some days, but quickly recovered. In addition, we try to attract predators to eat the ants. We train our young chickens to eat ants, but leaf cutter ants are mainly active at night when the chickens are roosting. Our food forest has been designed to provide habitat for wild animals, such as armadillos and ant eaters, which can help control ant populations.

Sacrificial crops

If the nests themselves are not causing direct damage, it is possible to peacefully co-exist with the ants. The ants can be provided with a food source that they like more than the crops, such as plants from the melastoma, brassica, and legume families. We have observed that the ants do have favourite foods, but also try out nearly everything they can access. In addition to leaves, they also destroy plastic bags, clothes, sacks, and other materials that they simply cut and drop, seemingly with no intention to take back to the nest. Hence, this is probably not a sustainable long-term solution, and could add to the problem if the colony is being fed well and grows.

Repelling the ants

Most ants are repelled by strong smells, such as citrus, lemongrass and other essential oils.  We have tried spraying emulsions of essential oils on our valuable trees and around the base of tables in the greenhouse, but this had little success. All such sprays should be mixed with an agricultural gum, which helps it stick to the leaves and last longer (especially during times of high rainfall). We have planted lemongrass around valuable trees, which has been somewhat effective. In addition, some species of leafcutter ants can be deterred by collecting refuse from the nest and sprinkling it around the trees.

In summary, controlling leaf cutter ants is an ongoing challenge in tropical regions. At the start of the wet season, there is a plague of new leaf cutter queens searching for nest sites. Hence, it is important to find these new nests and get them under control quickly. We found that it is most effective to combine several control methods, including protecting valuable trees with barriers and aromatic herbs, planting sacrificial crops, attracting predators, and controlling large nests with targeted application of low-toxicity fungicide when absolutely necessary.