Weaver Ants : Interactive insects

THEY are not here anymore!” exclaims Emily Heffernan as she peers under the leaves of the cengal saplings in the hardwood tree nursery at the Forest Research Institute of Malaysia (FRIM) in Kepong, Kuala Lumpur.  

“Perhaps the rain had driven them away,” speculates the American entomology student with a tinge of sadness at the thought of her study subject disappearing over the weekend. 

However, there is some comfort in knowing that they may still be alive somewhere. For it is not unusual for a whole colony of the butterfly larvae to be evacuated to a safer place by their protective guardians – the weaver ants (Oecophylla smaragdina) which are locally called the kerengga, discloses the 25-year-old lass from Florida, the United States. 

The subject of Heffernan’s research, Anthene emolus, generally known as the Blue butterfly, belongs to the Lycaenid family which has been known to share a symbiotic relationship with ants. The ants attend to the caterpillar, keeping it safe from predators and parasitic organisms, until it transforms into a butterfly. 

 

Weaver ants make their nests from living leaves which they glue together at the edges with a sticky substance secreted by their larvae.

It has been observed that the protection service extends to physically relocating the larvae if the ants feel that the caterpillar is in danger.  

The Lycaenid is the biggest of the five butterfly families in the world and in Malaysia where there are 1,050 species of butterflies, 405 of them belong to the Lycaenid family. Worldwide, the Lycaenid has been noted to co-exist with ants of various species. 

The Lycaenid is the smallest of all butterflies, with the blue butterfly having a fore-wing length of 14mm to 17mm. Most of the Lycaenid are metallic blue or metallic orange-brown in colour; the colour of the butterflies is formed by the scales which cover the wings.  

During the larvae stage, a symbiotic relationship evolves between the two different insects.  

The caterpillars become totally dependent on the ants for their survival. If left alone they may be devoured by insects or killed by parasites. 

“There are a couple of things a mother butterfly will look out for before laying her eggs. She will choose the right type of plants, right down to the leaves, so that when the eggs hatch the caterpillars can feed on the soft leaves,” says Heffernan. “The next most important thing is the presence of ants. Their role can be likened to that of a babysitter. “Studies have shown that ants communicate via chemical secretions. The butterfly will know the whereabouts of the ants through the chemical trail on the host plant, and this will help the butterfly to determine where to deposit its eggs.” 

 

The weaver ant 'milking' a caterpillar in return for the protection it provides the vulnerable butterfly larvae.

Throughout the caterpillar stage which lasts for about three weeks, the ants will guard the vulnerable caterpillar. In return, they will be rewarded with a nectar-like secretion. 

“The ants are, in essence, milking the caterpillars and protecting them. It is a curious evolutionary development, how cooperation prevails over competition.  

“Studying this relationship may give us some clues and help us understand other symbiotic interactions that exist in nature,” Heffernan enthuses. 

There is added urgency to this research as other related species of this butterfly have become extinct or severely endangered. So studying the Anthene emolus which is still doing well here may give researchers clues to help save the other species, adds Heffernan. 

It is this unique relationship between the two insects that has Heffernan enduring the ferocious bites of the kerengga while snooping in between tree branches to unravel the mystery of the interaction. 

“It is not so much the beauty of the butterfly that attracted me to study them, but rather the symbiotic relationship which makes an interesting lifecycle that fascinates me. Especially when you consider the contrasting characteristics of the notorious kerengga which displays its tender side in guarding and tending to the larvae. 

“I believe this relationship plays an important part in the ecosystem, although the ecological roles need to be researched further,” says the Fulbright scholar who decided to specialise in entomology upon completing her first degree. Heffernan worked mostly with moths initially. “There were a lot of pest species of moths and it was easy for me to find jobs and research projects.”  

It was natural for her to turn her attention to butterflies as her interests lie in ecology and not pest control. The rich biological diversity of Malaysia’s tropical rainforest makes it a choice location for Heffernan, whilst the abundance of the two insects on FRIM grounds was a plus point. 

Incidentally, FRIM entomologist Dr Laurence G. Kirton is researching the ecological function of the weaver ant, and welcomes a research project that would add dimension to his biological study of the ants. 

Kirton and his assistant Grace Lim had carried out a preliminary study on the kerengga as a biological control agent of the shoot borer (Hypsipyla robusta), a pest that lowers the economic value of the mahogany tree as it cripples the formation of the trunk. 

Unlike research on larger animals, Heffernan finds the possibility of getting close to the research subject a big incentive. In fact, it was during her first degree field trip studying the biology of mammals like deer and wolves that she got distracted by the tiny creatures crawling around her. 

“It is nice to get up-close with the creature you study. To be able to touch and hold them in your palm is a great feeling. The field work is fun and satisfying too.”  

Heffernan is fortunate in that she does not suffer any acute reactions to the kerengga bite which is laden with formic acid. “It is a love-hate relationship,” she confesses. 

Heffernan’s 10-month research project which began in September includes both field and laboratory experiments. Out in the field, she observes the behaviours of the ants and notes the division of labour among them. 

Initial observations show that the smaller ants tend to knead the larvae, while the bigger ones collect the nectar and are more defensive. 

She also spends hours watching and recording the type of predators that attack the larvae and the correlation with the host plant.  

“I need to collect such data over time and over a few research areas to confirm the behavioural patterns.” 

To look for the butterfly larvae, Heffernan goes for bigger visual clues like the distinctive kerengga nest, rather than search for the tiny caterpillar that would be well-hidden among the leaves. 

In the laboratory, Heffernan could manipulate the conditions in a controlled environment and chart the growth of the caterpillar and the butterfly accordingly. 

“For example, I could separate one clutch of eggs into different containers with different tree leaves and observe which one would help the caterpillars to grow or which is their preferred food. This gives me an insight into the host plant,” she adds. 

Heffernan hopes that the project for her masters thesis would lead to a bigger PhD project that would enable her to peer into the lives of more butterflies in Malaysia. 

The field data gathered would enhance local knowledge of the winged insect that is so little-known in a country blessed with a thousand species of butterflies. More species await discovery. 

And if the butterfly is established to be a significant player in the food-chain of the insect world, with the ants as an important biological control agent, than more research and funding can be expected to be channelled to the field of butterfly research

Copyright © 1995-2002 Star Publications (Malaysia) Bhd (Co No 10894-D)
Managed by I.Star.