By Maneka Gandhi*
Sooner or later the world is going to have to stop using chemical pesticides. The statistics in India WHO says cancer shows an increase of 60% in just 15 years (1998-2013) and this could only be due to the chemical pollution caused by these unnecessary and vicious compounds that simply create money for their sellers and the people who give them licences and death for farmers and consumers.
We need to identify safer, more sustainable and low tech ways to control pests; and what better than the most intelligent, miraculous beings on earth – ants. These super-organisms could become super pest controllers.
Classical biological control has achieved tremendous successes over the past century, yet scientists refuse to take the natural controls, that nature has provided, seriously. Clearly we need to recognize that greater attention is needed to increase the impact of native natural enemies. Ants have an estimated world population of 10 to the power of 15 and are everywhere except Antarctica. There can be upto 200 species of ants in just one locality. They have a major influence in many habitats as pest eaters, pollination, soil improvement (they aerate and displace tons of earth every year) and nutrient recycling. While most live in the ground, some live in trees and bushes. They start disappearing in farmlands that rely on chemicals and this leads to even more pests coming in and destroying the crops. How odd then that few entomologists have studied them in a pest management context. In fact only 1% of all publications on biological control address the role of ants in integrated pest management.
Dr Joachim Offenberg is an ecologist from Aarhus University who has studied ants for decades. He has done a major review of 70 scientific studies, in the Journal of Applied Ecology, to provide evidence that ants can control pests even more efficiently and cheaply than chemicals. The studies are centred round 50 pest species on nine crops ranging from cocoa and citrus to palm oil and cedar across eight countries in Africa, south-east Asia and Australia.
According to Offenberg, “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.” Most of the studies in Offenberg’s review are on weaver ants (Oecophylla), a tropical species that weaves ball-shaped nests from leaves. Because weaver ants live in trees, 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 walk ways’ 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. In 304 AD a Chinese publication wrote “in the market the natives of Jiao Zhi sell ants stored in baskets of rush mats. The bags are attached to twigs and leaves which, with the ants inside the nests, are for sale. In the south, if the Gan (mandarin orange) trees do not have this kind of ant the fruits will all be damaged by many harmful insects and not a single fruit will be perfect.” 1700 years later this biological control is still practiced in Indo-China. In 1974, De Bach described ant husbandry practices in northern Burma (Myanmar) that were very similar to the ones in China: ant nests being collected and sold to farmers, and bamboo bridges being constructed between the citrus trees.
Crops, such as cashew and mango, can be exceptionally well protected from pests by weaver ants. One three-year study in Australia recorded cashew yields 49% higher in plots patrolled by ants, compared with those protected by chemicals. The quality of the nut was higher too. So, net income was 71% higher with ants than with chemicals.
Similar studies in 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. Ants control major pests, such as mango leafhoppers, thrips, seed weevils, fruit flies and tip borers. In various African countries, including Benin, Burkina Faso, Mali and Guinea, ants are also considered to protect mangos from being stolen. The Bambara name for Oecophylla is kowulu , meaning dog of the lowlands. African farmers commonly state that snakes will not enter those trees with ants, and that their presence makes climbing trees safer. Fruit loss caused by fruit flies was significantly less in cultivars and orchards where weaver ants are very abundant. Recent trends in exporting organic mangos and citrus from West Africa to European markets have created a demand for farmer training in weaver ant husbandry.
The citrus ant has emerged as the principal agent for the control of insect pests in orange-pomelo orchards. Vietnamese citrus farmers, who took care of weaver ants spent, on average, half the amount of money on agrochemicals compared with those farmers who did not have Oecophylla in their orchard, yet obtained similar yields (Van Mele & Cuc, 2000). Up to 20% of the orange growers produced their crop entirely without pesticide by making optimal use of the weaver ant. “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. Wood ants can 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,” says Offenberg.
In another major study “Role of Ants in Pest Management” by MJ Way and KC Khoo, in the Annual Review of Entomology, the scientists have studied red imported fire ants Solenopsis invicta which are often considered serious pests. They have found these ants to be extremely beneficial in destroying the major insect pests in cotton and soybean.
In 1952 the entomologist Swanzy wrote about the discovery in Zanzibar that the “gumming” disease of coconuts, caused by bugs sucking the nuts, could be biologically controlled by the large red ant Oecophylla longinoda. Because the coreid bug is a low-density pest (ten bugs per hectare can cause significant damage), this had never been observed before. So this ‘disease’ had never been successfully controlled. This was then taken up by planters in other places who reported that where Oecophylla is present, the coconut trees almost invariably bear well. Weaver ants in coconut were enhanced by interplanting with citrus, clove, cashew.
Research on Oecophylla, in cocoa plantations, shows that the ants drive off a range of pests, including weevils, coreid bugs, capsids and mirids. Although weaver ant husbandry is a centuries-old tradition in China and Vietnam, scientific research on ants in smallholder citrus took off in the 1980s after problems of insecticide resistance, and the ants were found to control citrus stinkbugs, aphids, leaf-feeding caterpillars , inflorescence eaters, coleopteran and various other pests .
Oecophylla smaragdina has been identified as a biological control agent in cashew, and controls the important pests; the tea mosquito bug, the mango tip borer, the fruit spotting bug and the leaf roller. The weaver ant also helps protect cocoa against rodents and oil palm from butterfly and moth defoliators.
The beneficial effect of weaver ants has been observed in Eucalyptus and other tropical trees. A study on the prospects for biological control of the mahogany shoot borer Hypsipyla robusta in Malaysia showed that the weaver ant is capable of reducing damage by this pest on Khaya ivorensis trees.
Formica ants kill many different defoliating insects in European forests, and their role has been recognised by Germany since the 19th century. In fact, Formica lugubris has been successfully transplanted to Eastern Canada to protect the forests.
Ants should be promoted because they can store food, and therefore continue to capture prey even when not immediately needed as food. They not only prey on eggs, but pick up insects that are much larger than themselves. Colony fragments can easily be transferred to other areas to establish new colonies. With the increased demand for organic produce, the ant must be lobbied for. Entomologists must be asked to provide input in farmer training programmes, and entomology must be part of the agriculture college syllabus. In West Africa, weaver ant husbandry has been introduced. Why can’t we do the same?
*About the author: Maneka Sanjay Gandhi is a Member of Parliament and leader of animal welfare movement in India. You may contact her at email@example.com, www.peopleforanimalsindia.org