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Something Borrowed, Not Quite New

April 30, 2011

Over the course of these past two weeks I have become more familiar and curious about a small and inconspicuous resident here in the garden.

The Weaver or Green Ant (Oecophylla smaragdina) is an incredibly common though easily missed resident of Dokmai Garden.  They have made their presence known to those of us who wander aimlessly through the vegetation.  They deliver a painful bite followed by an application of acetic acid to the bitten area.  I have since learned to be slightly more aware of my surroundings while I wander.  Over the past two weeks I have repeatedly stopped to watch and sometimes prod them while they march.

The Weaver Ant is a part of the very successful sub-family (Formicinae) whose lineage can be traced back into the fossil record to the Eocene and Miocene periods.  They owe their evolutionary success to their ability to use silk (squeezed from their young)  to construct nests of living leaves within the forest canopy.  Their colonies can be quite extensive, consisting of up to 100 nests and half a million workers.  They are a beneficial addition to many gardens here as they aggressively feast on many of the insect pests found here.  My curiosity peaked (resulting in a “Hmmmm…” followed by a chin scratch) when a visitor to the garden remarked that historically throughout China and Southeast Asia these ants have been introduced as a form of biological pest control.

Here at Dokmai, applications of chemical insecticides are avoided if at all possible and as with any organic or semi-organic garden there are bound to be some patients in need.  One plant that I have had my eye on was a Carunda (Carissa carandas) bush begging for some affection.  It was suffering under a nasty infestation of Mealy Bugs (Fam: Psudococcidae) feasting on its sap and exuding excess sugar onto the plant’s foliage.  This caused discolouration and potentially opens the door to future fungal invasions.  This problem was compounded by an unidentified species of black ant (Fam: Formicidae) who were happily tending to the mealy bugs…unaware that an easily intrigued Canadian had plans afoot.

Taking a cue from history (and advantage of the Weaver Ants unique nesting style) I decided to introduce a nest of Weaver Ants to the immediate area banking on their irritable nature and healthy appetite in order to establish themselves, take out the enemy ants (Braveheart style) and then pillage their mealy herds thereby saving the shrub and bringing glory to Team Canada.

That was best case scenario.

Initially I found many smaller Weaver nests within the leaves of Tabernaemontana divaricata (a kind of Milkwood) that was serving as the grafted root-stock for Tabernaemontana pachysiphon visible above ground. As with many grafted plants the root-stock tends to periodically send up shoots competing with the grafted shoots.  If these new shoots are not pruned in time they can overtake the alien parts of the plant converting it back to its original form.  Luckily (for me) this did in fact happen.  Since there are no broad-leaved trees or shrubs next to the afflicted Carunda (and the Weavers are particular to a nice broad leaved shrub) I decided to transplant the one Milkwood that had reverted to its former self, providing a substrate and base of attack for the coming invasion.

After the Milkwood was successfully transplanted I harvested a couple of smaller ‘satellite’ ant nests and placed them in the Milkwood.  There was no real guarantee that there would be a queen present inside these nests but the results were favourable.  The ants defended their mobile homes well, easily outmatching the black ants and set up a defensive perimeter within the Milkwood.

The second wave shall start once the Milkwood has had some time to establish itself.  Then one of the large nests (housing a queen) shall be placed  within its boughs.  Sadly though I shall not be here to witness first hand as they establish themselves and (in a roundabout way) bring me glory!

Advocating the introduction of a non-native insect predator under the guise of pest control to a foreign country is ill advised at best, so the applications back home are limited.  Though here in Southeast Asia the Weaver Ants have proved their worth as a chemical free (aside from the acid thing) method of controlling insect pests in both private and commercial gardens.  Maybe that is enough to give countries pause in order to review what is already present within native ecosystems before enlisting help from abroad.  The Weavers are not without their flaws though.  Its edibility and the removal and elimination of defoliating insects is something to be put in the “+” column, Weaver Ants do have a habit of discouraging pollinators.  This in severe cases can cause reduced fruit production.  The ants can also prevent mammalian and avian herbivores from feasting on the fruit which limits the dispersal of seeds.  Those…would be negatives.

I am very curious to see the outcome of my short=lived military career and as long as they don’t take control and start raising their own mealy bugs things shall be fine.  If not, well I won’t be here!

Shane McCarroll, Tropical Gardening School Student

The new home tree, Tabernaemontana divaricata, planted in front of the infested patient Carissa carandas.



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