As revealed in a review published this week in the Journal of Applied Ecology, tree-borne ants are so effective at defending orchards that in many cases, they can be used as a cost-effective alternative to chemical pesticides. Professor Joachim Offenberg of Aarhus University conducted a review of more than 70 scientific studies demonstrating the effectiveness and cost-efficiency of using weaver ants to protect a variety of tree crops such as cashews, citrus, mango cocoa, oil palm, mahogany, and cedar.
Weaver ants are colony insects that live in trees, building nests from woven leaves and preying on other insects. Because ants work cooperatively, they are able to overbear insects that are many times larger. Attacks are coordinated using chemical scents called pheromones, and insects that would otherwise be pests to the tree are brought back to the nest to feed the colony.
The ants are managed by a number of low-cost measures such as artificial nests, tying ropes between trees to serve as ant bridges, providing sugar and water in the off season, and pruning trees to reduce fighting between neighboring colonies of ants. It is also important to limit use of chemical pesticides to prevent harm to the tree-defending insects.
The review shows that trees raised among weaver ants have a reduced number of pests, less plant damage, and increases in yield. Additionally among cashew and mango trees, weaver ants out-perform chemical pesticides to the extent that they can be used as an alternative with benefits to both the bottom line and the environment. In a three-year study conducted in Australia, orchards protected by ants produced yields 49 percent higher than those sprayed with traditional chemical pesticides. Use of these predatory insects also produced higher-quality nuts, resulting in a 71 percent increase in net income. Similarly, Australian mangos protected by ants had roughly equivalent levels of production as those protected by chemical pesticides, though ant management was cheaper and the quality of the fruit was higher, resulting in a 73 percent increase in net income.
Pest-controlling beneficial insects are an old idea. For decades farmers have used a variety of methods such as ordering ladybugs and parasitic wasps in the mail, and planting hedgerows and wildflowers to attract predatory insects. Megan Dunn, communities program director of the Northwest Center for Alternatives to Pesticides (NCAP), said: “[Chemical] pesticides are more of a Band-Aid. The more you can improve the soil and improve the ecology, the better a farm will do. Working within your ecosystem improves things all around.” It is also important to “… use scientifically backed insects that are proven to work in similar ecosystems. While ants are indeed found on every continent [except Antarctica], all the crops in this review were tropical species, so weaver ants can’t be used everywhere.”
Still, according to Offenberg, “Weaver ants are unlikely to be unique in their abilities to provide effective pest protection. Of the almost 13,000 described ant species, many other species are likely to hold similar properties.”