Weaver Ants are spinning gold

A research project at the Department of Bioscience at Aarhus University shows that African weaver ants are able to increase the production of cashew nuts by more than 80%. The ants eat other insects and in this way combat pests on orchard trees. For a country like Benin in West Africa this could lead to additional annual revenue of 120 million dollars.


Weaver ants catch and eat insect pests on orchard trees. This results in increased plant production and animal production, as the pests are converted to edible ant biomass rich in protein and polyunsaturated fatty acids. Above left a bush-cricket has been caught by a weaver ant, above center is a close-up of a weaver ant, and above right (top) a cashew nut is seen on a cashew apple and (below) is a basket with harvested ant larvae.
Figure: Joachim Offenberg

In connection with a DANIDA financed research project at Science and Technology we have recently been visited by the African PhD students we tutor and train in the use of weaver ants. With them the students brought the first data from study orchards in Benin and Tanzania, where we are studying the abilities of weaver ants as biological pest control. The results show that orchards with weaver ants provide an increased a yield of 53-82% compared to areas without pest control, and that the yield is equal to that in areas that spray expensive pesticides. As the ants are just as efficient as pesticides, but far less expensive, the gain is considerable, particularly African smallholder farmers who would never be able to afford pesticides. Ants are already in the immediate environment, and apart from having to invest in some string and scissors, using ants only costs time, and time is cheap in Africa. The weaver ant method is so low-tech that even the most resource-poor farmers can use it, as long as they are told about the existence of the method and we teach them how to manage the ants in the orchards.

Apart from a higher yield for less money and the ensuing environmental benefit, lucrative access to the European market for organic cashew nuts is also part of the scheme. In Europe, you can get up to four times the price if the nuts are certified organic. If the ant technique is fully applied in Benin and Tanzania, on the basis of the current export value and the above mentioned extra yield, it could lead to an increased export revenue of 120 and 65 million dollars a year, respectively, in the two countries – and that is prior to including the higher price for the organic nuts. All in all not bad business.

That pest control is efficient is perhaps not so strange. In another weaver ant project at Science & Technology, which is funded by The Danish Council for Independent Research, as a first PhD student Christian Pinkalski Stidsen has developed a model that in just a couple of minutes is able to estimate how many ants actually are in a tree. Previously we have not been able to do that. According to the model, a well-managed orchard tree has somewhere between 80- 120 thousand worker ants, equivalent to just under 1 kg. All these ants will constantly patrol all parts of the tree and make it very difficult for potential pests to take hold. The result of this is trees that are “cleansed” of pests. Even snakes and flying foxes stay away from trees with weaver ants.

Results from the same project show that urine from the ants contains important plant nutrients. E.g. PhD student Nanna Hjort Vidkjær found urea in ant fertilizer. Gardeners use this chemical compound as fertilizer, as it can be absorbed by plants directly through their leaves. We can therefore assume that the pests that the ants eat are converted into nutrients that benefit the plants. During the next phase of the project we will measure how large quantities of nutrients are delivered from the ants to the trees.

Another fascinating benefit from the method is that the pests are converted into edible protein, as they are eaten by the ants and as the ants themselves are edible for humans. The ant larvae, which are a delicacy, are rich in protein and polysaturated fatty acids and are easy to collect, as the ants gather them in their large leaf nests. In this way a system is attained/achieved in which the ants not only benefit crop production, but the very same process also creates animal products - a bi-product free of cost.