A scientific review from Aarhus University has hailed the use of weaver ants as a "rare example of sustainable production that matches conventional yields" across a wide range of crops around the world, which in some cases has even led to higher yields with lower pest control costs.

The report 'Ants as tools in sustainable agriculture' was written by Dr. Joachim Offenberg and published in the British Ecological Society's Journal of Applied Ecology.

The review draws on the studies of more than 50 pest species on nine crops across eight countries in Africa, Asia and Australia.

Offenberg says because weaver ants live in their host trees' canopies near the flowers and fruit that need protection from pests, they are good pest controllers in tropical orchards.

All farmers need to do is collect ant nests from the wild, hang them in plastic bags among their tree crops and feed them a sugar solution while they build their new nests. Once a colony is established, farmers then connect the trees that are part of the colony with aerial 'ant walkways' made from string or lianas.

After that, the ants need little, except for some water in the dry season (which can be provided by hanging old plastic bottles among the trees), pruning trees that belong to different colonies so that the ants do not fight, and avoiding insecticide sprays.

A study in Tanzania found that cashew trees with weaver ant nests had a 27.8 percentage point fall in the amount of shoots damaged by the bug helopeltis sp, while the presence of ants in an Indian study of mango and citrus trees led to higher yields of 18% and 20% respectively.

In general, different studies showed weaver ants were particularly useful conservation biological control (CBC) methods for cashews and mangoes.

One three-year study in Australia recorded cashew yields 49% higher in plots patrolled by ants compared with those protected by chemicals. Nut quality was higher too, so net income was 71% higher with ants than with chemicals.

Similar studies in Australian mango crops found that ants could produce the same yield as chemical control, but because the ants were cheaper, and fruit quality better, net income from mangoes produced with ant protection was 73% higher.

"Although these are rare cases where the ants were superior to chemicals, many studies show that ants are just as efficient as chemical controls. And of course ant technology is much cheaper than chemical pest control," Offenberg says.

While ants are not a panacea for pest control, he believes that with more information and training for farmers, ants could be much more widely used in temperate as well as tropical agriculture and forestry.

"Weaver ants need a canopy for their nests, so they are limited to plantations and forestry in the tropics. But ground-living ants can be used in annual crops such as maize and sugar cane. European wood ants are renowned for controlling pests in forestry, and new projects are trying to use wood ants to control winter moths in apple orchards.

"Ants could even be used to fight plant pathogens because they produce antibiotics to combat diseases in their dense societies.

"Ants are great hunters and they work cooperatively. When an ant finds its prey, it uses pheromones to summon help from other ants in the nest. By working together, they can subdue even large pests."

Photo: Kim Aaen, NatureEyes

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