Weaver ants: the living pesticide

Weaver ants: the living pesticide

Wearver ant

Wearer ant nest

Scientific classification

Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hymenoptera
Family:	Formicidae
Sub family:	Formicinae
Tribe:	Oecophyllini
Genus:	Oecophylla Smith, 1860 (15-20 species).
Species:2 Diversity species	Oecophylla longinoda in blue,  Oecophylla smaragdina Fabricius, 1775 in red.

Introduction

Oecophylla is a genus of large arboreal ants of the African, Asian, and Australian tropics. The weaver ants belong to the ant genus Oecophylla (subfamily Formicinae) which contains two closely related living species: O. longinoda found in Sub-Saharan Africa and O. smaragdina found in southern India, southeast Asia, and Australia. They are provisionally placed in a tribe of their own, Oecophyllini. The weaver ant genus Oecophylla is known about 18-20 species from Africa to Asia in the tropics.

Weaver ants are best known for their remarkable nest construction. Using precise coordination, the weaver ants create very strong ant chains by linking legs to pull and bend leaves into desired tent like positions. The ants then use their own larvae to secrete a silk that is used to stitch leaves together to create a nest. They may have several nests dominating a few trees at once.

These conspicuous insects are weaver ants, creating nests by pulling living tree leaves together and securing them with silk produced by the ants' larvae.  Colonies are territorial, covering several trees and containing dozens of nests. 

They are very aggressive territorial ants and for over 1000 years they have often been used by farmers to control agricultural pests.

Oecophylla smaragdina workers have a vice like grip and tremendous strength. A worker has been recorded to support 100 times its own weight whilst standing upside down on glass!

Weaver ants are reddish ants that live in the tropical forests of Africa and India and many countries in southeast Asia. They are also found in Australia and the Solomon Islands. Their nests are found in forest trees.

A Queen of Oecophylla smaragdina that has shed its wings Weaver ant nest on a Mango tree nest in Kinnerasani Wildlife Sanctuary, Andhra Pradesh, India.

The life cycle

Weaver ants or Green ants (genus Oecophylla) are eusocial insects of the family Formicidae (order Hymenoptera).Weaver ants get their name from their habit of binding fresh leaves with silk to form their nests. Oecophylla weaver ants vary in color from reddish to yellowish brown dependent on the species. Oecophylla smaragdina found in Australia often have bright green gasters. In Vietnam they are called "kiến vàng" (yellow ants). The life cycle of the ant has four stages: egg, larva, pupa, and adult. 

Eggs of weaver ants

The queen ant starts the ants' nests/colonies. She flies and searches for mate/s. She can mate with one or a few males (one at a time) in the air, or on low vegetation, or on the ground. Once mated, she looks for a nest site, either on trees or open fields. Once situated, she gets rid of her wings, seals herself into a small chamber and lays a small batch of eggs. The eggs then hatch into larvae. The queen is located in one nest and her eggs are distributed to all the other nests where workers and soldier ants are found. She spends her life laying eggs. The workers are females and do the work in the nest. The larger ones are the soldiers who defend their colony. 

Fertilized eggs develop into females (workers and the queen) and unfertilized eggs into males. Female ants have 2 copies of each chromosome while males have one. 

Larvae and Pupa of weaver ants

The larvae feed on the unfertilized eggs as food which the queen lays especially for them. The first brood of workers are normally smaller since she can only provide a limited amount of food. Once the ants mature, they leave the nest and begin to look for preys. They bring food to the queen and their siblings so that later offspring are bigger. As the colony reaches maturity, it begins to produce the queens and males for the next generation. Males can remain in the nests for some months and most of them will die within a few days after leaving their nests. 

The larvae have special glands to produce lots of strong silks (adults do not produce silk). One colony is found over several nests that may be placed in various locations in a tree, or several trees, or in fields. 

The worker ants form a chain along the edge of the leaf and pull the edges together by shortening the chain by one ant at a time. Once the leaf edges are in place, each ant holds one larva in its mandibles and gently squeezes the larva to produce silk. The silk is used to glue the leaf edges together. 

Pupae are laid in nest with a short time before become adults.

Adult weaver ants

Adult weaver ants are reddish to brown in color and have 10-segmented antennae with 2-segmented clubs. Their eyes are relatively larger than those of other species of ants. They do not have stingers, but can give painful bites caused by the chemicals secreted from their abdomen. They make nests in trees or on leaves of legumes, or in bunds or levees of the fields. They have the most complex nests among ants' nests. They use fresh leaves to build nests . 

Queen 20-25mm, a strong ant, normally green and brown, monogyn (one queen per colony).

Workers 5-6mm. Mostly orange. Sometimes this species has green gasters. Minor workers tend to look after the brood and farm scale bugs for honey dew.

Major workers 8-10mm. Mostly orange, this ant has long strong legs, long flexible antennae and large mandibles. These ants forage, maintain and expand the nest.

A dealate queen of O. smaragdina having shed her wings after a mating flight.

O. smaragdina major workers inspecting and cleaning (allogrooming) another worker on its return to the nest.

Their lifecycle spans a period of 8 to 10 weeks.

The main characters

Diet

They farm scale bugs for their honeydew, and eat small insects.

Weaver Ants eat any small creatures that they can find, but they are particularly attracted to nectar. The weaver ants do not have a stinger, but inflict a painful bite which is aggravated by irritating chemicals secreted from their abdomen.

Like many other ant species, weaver ants prey on small insects and supplement their diet with carbohydrate-rich honeydew excreted by small insects (Hemiptera). 

Colony productivity  

Weaver ant colonies are founded by one or more mated females (queens). A queen lays her first clutch of eggs on a leaf and protects and feeds the larvae until they develop into mature workers. The workers then construct leaf nests and help rear new brood laid by the queen. As the number of workers increases, more nests are constructed and colony productivity and growth increase significantly.

Workers perform tasks that are essential to colony survival, including foraging, nest construction, and colony defense.

A colony may be dispersed over several nests which may be placed in various locations in a tree, or even span several trees. The queen is located in one nest and her eggs are distributed to the other nests.

Social organization

Weaver ants are Social insects. The exchange of information and modulation of worker behaviour that occur during worker-worker interactions are facilitated by the use of chemical and tactile communication signals. These signals are used primarily in the contexts of foraging and colony defense. Successful foragers lay down pheromone trails that help recruit other workers to new food sources. Pheromone trails are also used by patrollers to recruit workers against territorial intruders. Along with chemical signals, workers also use tactile communication signals such as attenation and body shaking to stimulate activity in signal recipients. Multimodal communication in Oecophylla weaver ants importantly contribute to colony self-organization.

Like many other ant species, Oecophylla workers exhibit social carrying behavior as part of the recruitment process, in which one worker will carry another worker in its mandibles and transport it to a location requiring attention.

Nest-building

Weaver ants' nests are among the most complex ants' nests. The ants choose living leaves to build nests. These provide well camouflaged protection from predators and the elements. To create their neat nest, chains of worker ants form along the edge and pull the edges together by shortening the chain by one ant at a time. Once the edges are in place, an ant holds one of their larvae in its mandibles and gently squeezes it so the larvae produces silk. The silk is used to glue the leaf edges together. The larvae have special glands to produce lots of strong silk. The adults do not produce silk.

The nest starts very simply. A group of worker ants finds a leaf that is soft and easy to bend. Several ants line up. Each holds an edge of a leaf in its mandibles and feet. Slowly, the ants pull the two leaf edges together. More and more workers join in. They link their feet and pull until the two leaf edges are nearly touching. Weaver ant nests begin small but can sometimes become so large they connect branches of neighboring trees.

A giant weaver ant nest may look like it is damaging the leaves and branches of a tree. But weaver ants actually protect the tree they are living in. The ants act like miniature bodyguards for the tree. They keep other animals like birds, reptiles, and other insects from living in the tree or eating it. Sharing the same resource or living space is called symbiosis.

Oecophylla weaver ants are known for their remarkable cooperative behaviour used in nest construction.The time required to construct a nest varies depending on leaf type and eventual size, but often a large nest can be built in significantly less than 24 hours. Although weaver ant's nests are strong and impermeable to water, new nests are continually being built by workers in large colonies to replace old dying nests and those damaged by storms.

Next, other worker ants carry larvae from the old nest and gently squeeze them between their mandibles. This causes the larvae to ooze a thin thread of silk. Then the workers get busy. Just like tiny tailors, they stitch the leaves together. In fact, another name for weaver ants is “tailor ants.” Treetop nests can become extremely large. Sometimes they even connect branches from two nearby trees.

Other features

Role in the habitat: Weaver ants are exploited by plants and animals. Some plants such as the Sea Hibiscus (Hibiscus tiliceaus) secrete nectar in their leaves to attract these ants, which in turn protect the plant from insect leaf eaters. The nasty bite of the ants also discourages larger herbivores.

Some other creatures also exploit the Weaver Ant's sweet tooth. Some caterpillars of the Lycaenidae and Noctuidae butterfly families secrete a honey dew that attracts these ants to protect them. Some of these caterpillars are more sinister and use their bribe to gain entry into the ant's nest and devour their larvae! Some jumping spiders look and more importantly, smell like ants, and in their disguise, enter the ant's nest to devour them and their larvae.

Miniature bodyguards for trees: A giant weaver ant nest may look like it is damaging the leaves and branches of a tree. But weaver ants actually protect the tree they are living in. The ants act like miniature bodyguards for the tree. They keep other animals like birds, reptiles, and other insects from living in the tree or eating it. Sharing the same resource or living space is called symbiosis.

A sweet deal: Weaver ants have a “sweet tooth” that some creatures use to their advantage. Certain butterfly caterpillars produce drops of a sweet liquid called honeydew. The honeydew attracts weaver ants to the caterpillars. The ants then protect the caterpillars.

Weaver ant workers take great care of the colony’s larvae. They feed them and are very careful when they move them. The larvae produce the special silk that holds the colony’s nest together.

The uses of weaver ants

Ant eaters

Weaver ant pupae are harvested and sold as food in markets in Thailand and the Philippines.The taste of the pupae has been described as creamy flavor. People also eat adult weaver ants. Their taste is described as lemony or creamy and sour. The Dayaks in Borneo mix adult ants with rice for extra texture and flavor. Weaver ants are fierce biters, so people who harvest them have to be extra careful!

Use as tradictional medicines

People who live near weaver ants sometimes use them as a type of medicine. The ants have a strong chemical in their bodies called formic acid. The ants use the formic acid to protect their nests. People have discovered that they can collect a few of the worker ants and crush them to make a special mixture. The mixture is then used to fight infections. This kind of medicine is called traditional medicine. Studying traditional medicines like this may help scientists find new methods to cure diseases.
The larvae and pupae are collected and processed into bird food, fish bait and in the production of traditional medicines in Thailand , Vietnam and Indonesia.

Use as a living insecticide

The ancient Chinese as early as in 300 AD, exploited the voracious appetite of these ants by using them to control insect pests in their citrus orchards. They use them to control insect pests in their citrus orchards. To do this, they first put a weaver ant nest in an orchard. Then, they place bamboo strips among the trees to serve as "ant bridges." These ant bridges encourage the ants to colonize all the trees. More fruit growers are now bringing back this traditional practice of using weaver ants for pest control. It is a cheaper way of dealing with insects that have developed resistance to chemical insecticides.

Large colonies of Oecophylla weaver ants consume significant amounts of food, and workers continuously kill a variety of arthropods (primarily insects) close to their nests. Insects are not only consumed by workers, but this protein source is necessary for brood development.

Because weaver ant workers hunt and kill insects that are potentially harmful plant pests, trees harboring weaver ants benefit from having decreased levels of herbivory. They have traditionally been used in biological control in Chinese and Southeast Asian citrus orchards from at least 400 AD. Many studies have shown the efficacy of using weaver ants as natural biocontrol agents against agricultural pests.

The use of weaver ants as biocontrol agents has especially been effective for fruit agriculture, particularly in Australia and southeast Asia. Fruit trees harboring weaver ants produce higher quality fruits, show less leaf damage by herbivores, and require fewer applications of synthetic pesticides.

Farmers in southeast Asia often build rope bridges between trees and orchards to actively recruit ants to unoccupied trees. Established colonies are often supplemented with food to promote faster growth and to deter emigration.

Today in plant production, Weaver ants are usud in biocontrol to kill many kinds of insects on plant fruit trees. Their type: generalist predator and their hosts: citrus stinkbug, leaf-feeding caterpillars, aphids, citrus leafminer, leafhoppers, plant hoppers, bugs, moths, adult black bugs, and small animals.
In Mekong delta of Vietnam, weaver ants are used as living pesticides to kill many insect species on fruit plant trees from hundreds of years to now a day.

Conservation and management

Weaver ants thrive well in undisturbed places and plenty of green leaves. Plant fruit trees or shrubs in or around your new citrus orchard however, banana, sapodilla, and papaya are less suitable.

Introduce only native weaver ants to the orchard when no black ants' species are present to ensure the establishment of a weaver ant colony. 

Provide them with food during the dry season such as dried fish and shrimp, cut into pieces that are small enough for the individual ant to carry. 

Put bamboo or wooden strips between trees to guide the ants to transfer from one tree to another for them to build new colonies in other trees. 

To expand weaver ants' colonies to other field crops, tie a rope to a tree where they live, to guide them to the areas you want them to colonize. Monitor regularly the ant colonies. Like other insects, ants are easily being killed by pesticide. 
                                                                                     Edited and posted by Hồ Đình Hải

References

1-http://www.myrmecos.net/formicinae/oecophylla.html

2-http://en.wikipedia.org/wiki/Oecophylla_smaragdina From Wikipedia.

3-http://en.wikipedia.org/wiki/Weaver_ant From Wikipedia.

4-http://www.antark.net/ant-species/weaver-ant-oecophylla-smaragdina.html

5-http://www.springerlink.com/content/l5n03712j4825517/

6-http://www.globio.org/glossopedia/article.aspx?art_id=11&art_nm=Weaver+Ants

7-http://www.new-ag.info/03-2/develop/dev02.html

8-http://www.naturia.per.sg/buloh/inverts/weaver_ants.htm