A SURVEY OF HUSBANDRY TECHNIQUES IN ATTA AND ACROMYRMEX spp.
By Andrew Stephenson.
Increasingly studied for their unique lifestyle and displayed for their curiosity value, Leaf-cutting ants are established as a productive area for research and as a money spinning attraction in Butterfly Farms and Zoos etc.. However in most of their range these ants are damaging agricultural pests exacting a huge economic toll on the countries where they occur.
Leaf-cutting ants comprise some 24 species of Acromyrmex and 15 of Atta. Whilst they are by no means alone in the ant and termite world in growing fungus for food, leaf cutters are by far the most specialised in this respect. Their complex social organisation and extraordinary productivity make them among the most remarkable of creatures and to watch a large colony engaged in its full activity is a truly awesome sight.
The challenge of bringing this society from its natural environment and maintaining it in an artificial situation is one of considerable undertaking. Recreating the precise humidity and temperature that the ants require is an exact science with little room for error. What is more, the provision of a constant and suitable food source, year round, is a substantial pressure, especially when in a healthy, mature colony, the appetite can equal that of an adult cow!
A BRIEF LIFE HISTORY
Atta and Acromyrmex are exclusive to the New World predominantly in the tropical portions of Mexico and Central and South America. They typically construct nests in the ground and forage in the vicinity on non specific food plants which they cut into disks approximately 1.5-2cm in diameter and carry back to their nest. The leaf material that they gather is not eaten but utilised in the construction of their large sponge-like 'gardens'. These, fertilised with faecal matter, are seeded with a symbiotic fungus which permeates the substrate and in time produces 'fruiting bodies' (like miniature mushrooms full of nutrients) thereby providing an inhouse production facility.
Partly because of their ability to produce limitless amounts of food from a never ending resource of vegetable material (as against the carnivorous ants which depend on a far less dependable supply of prey) and partly due to the queens immense reproductive capacity, leaf-cutting ant colonies can become enormous. Imagine a tennis court excavated to the depth of a two storey house to give some indication of the sheer volume occupied by a very large colony. It can only be guessed at how many individuals are present in such a colony but estimates exceeding 6 million cannot be unreasonable. Once the colony has matured the queen will produce winged individuals which are sexually mature. These males (40000+) and females (up to 7000) leave the nest, fly to a given altitude and copulate. Each queen will mate several times and store more than 200 000 000 sperm for use throughout her 12 or more year life-span.
Once her wings have been shed she excavates a chamber and removes a small wad of fungal mycelia from her infrabuccal chamber (located beneath the opening of the oesophagus behind the labium in all ants.). This she fertilises with faecal matter and begins to cultivate a small fungus garden. Around three months later the first workers emerge and begin to forage. From this point onwards the colony expands extremely rapidly. However because of the vulnerability of the nuptial flight and lack of protection afforded the lone queen and very young colonies, the success rate is dire. In several studies conducted on foundling colonies figures suggest that fewer than 1% of queens survive their first year!
LEAF-CUTTING ANTS IN CAPTIVITY.
They are aggressive, ingenious and resourceful. They require precise temperature and humidity levels to provide the correct conditions for the fungus to thrive. They will escape and forage far and wide given the slightest chance and will quite happily bite the hand which is thinking twice about feeding them.
Culturing Leaf cutting ants is an undertaking of significant proportions and as such is rarely done other than by establishments with appropriate facilities such as zoos, butterfly farms etc. But there are ways to successfully manage even a large colony, giving the ants the necessary space and conditions to flourish and at the same time enclosing them in a safe and controllable environment.
There are two criteria which are essential if the colony is to thrive. Firstly the temperature must be 25C with a maximum variation of +\- 2C. The thinking is that if the temperature exceeds 30C the queen will permanently cease laying eggs (possibly the sperm she is storing is destroyed). This will ultimately kill the colony and so beware the biggest cause of high temperature, that of the sun.
Secondly humidity. In nature the humidity is very high and such a large amount of rain falls that it is very unlikely that they ever experience dry conditions. Given that there will always be the danger of over compensating for this it is important to provide for drainage so excess water can run away without causing any problems.
In a mature colony the ants will carry sufficient vegetable material into the nest to maintain the humidity of the fungus gardens at the required level and so there will be little problems in an artificial environment with such a set up. Small colonies however are in danger of becoming dehydrated very quickly. Whilst the ants themselves will be untroubled by the dry conditions and will in general behave normally, the fungus will die. This will obviously have catastrophic results on the well being of the nest and so it is important to remember that prolonged drought is dangerous. ( It is vital to bear in mind that it is the fungus and not the ants which require the moisture and so even 'desert' leaf cutters such as Atta texana require their garden chambers to be wet. They get around this in the 'wild' by excavating deep chambers.)
There are two main types of artificial nests employed by those who culture Leaf cutters. Depending on the purpose for which the colony is being maintained the ants can either be housed in a series of clear plastic receptacles linked by tubing and isolated by water or they can be given access to a large tank filled with peat and allowed to construct their own nest in the given area. The merits of these two designs are discussed here.
Type 1 is the traditional laboratory set-up where the ants are provided with clear plastic containers of a fixed size (approx. 30cm.x30cmx30cm) and it is in these that they construct their gardens. Initially it is a good idea to fill these with peat or sand to encourage the ants to begin excavation and in a very short time they will have emptied the contents of the tub and replaced them with fungus garden. As the colony expands the initial container may be linked to several others via a clear plastic tube of approx. 3/4" diameter. The whole colony is placed on a platform which stands in a tray of water, effectively isolating the ants and preventing escape. A feeding table can be within easy reach of the nest or can be some distance away, connected to the colony by a length of rope (this is suspended from above with fishing twine which the ants are unable to scale) and isolated in a similar style with water or oil. It has been found that the foraging ants will travel further than 100 metres to a food supply but an optimum distance of 30 metres is recommended. When the colony is small it is a good idea to feed in close proximity to the nest so the efforts of the relatively few foragers are not wasted in travelling to the food but rather concentrated on carrying plant material to the gardens. Leaf cutting ants are kept using this general technique all over the world and it has been proved many times over to be a successful method. There are however one or two problems associated with this set-up. Leaf-cutting ants build their gardens in darkness, the rearing of brood and fungus is an enterprise practised in the unlit world of the subterranean nest. To expose them to bright light and have them adapt and thrive ( which they seem to do ) is a testament not to clever artificial nest design but to the tolerance of the ants and their ability to make-do. Therefore such a set-up as explained above, though fine for butterfly farms and scientific research projects where behaviour(r))9¯ (r)):¯is not of prime importance, cannot be recommended where results of observation are the basis of deductions about the ants behaviour.
The second method is one in which the needs of the ants are of prime importance. The colony is housed in as large a tank as possible which is half full of peat. Do not fill beyond this level since the ants will undertake substantial excavations and as a result the peat will eventually reach the top of the tank. The ants are prevented from escaping by the construction of a moat around the top of the tank. This should be as deep as possible and can be filled with water or oil. Initially the ants will be fed in the confines of the nest, again preventing the waste of timeand energy associated with distant foraging. Later a table may be set up at a distance and the spectacle is complete.
It is important when using this method to darken the sides of the glass with sheets of thin plywood or heavy cloth. This ensures that the gardens will be built against the sides and as such makes the workings of the colony more visible. When in a situation of public display these screens can safely be removed for prolonged periods in dim lighting conditions without upsetting the ants too much. And to illustrate the preference of the ants against the first exposed laboratory technique, if the screens are removed permanently the ants will build a wall of substrate between the gardens and the glass to try to block out the light.
One of the advantages of this system is that the ants will construct their colony much along the lines of a natural nest, given the confines of their tank. The gardens find their own size which can be huge, and the ants are able to construct the entrances to the nest such that the gardens are ventillated by thermal airflow through the nest. (It is thought that in naturally occuring nests warm air generated in the centre of the colony rises up and out of the central entrances which has the effect of drawing air in through the peripheral entrances thus producing a constantly moving supply of fresh air.)
One problem of this system is that the ants of most species of atta dump their garbage downwards in 'dump chambers' built below the main body of gardens which in the 'wild' works well, refertilising the soil and disposing of a major waste product discretely and safely. In this tank set-up, the action of dumping at the bottom of the container produces a build up of waste which ultimately leads to the disrupting of the fungus gardens and damage to the colony. Because of this it is important to provide a separate vessel at the base of the tank into which rubbish may be dumped and periodically cleaned out.
Whichever type of system is used the regime of temperature and humidity will determine the success of the colony. These ants are nothing if not adaptive and will generally change to suit. The clear boxes of the laboratory system provide a more accessible visualisation of workings of the various castes whilst the natural setup offers a closer glimpse of the way a real colony operates.
FEEDING LEAF CUTTING ANTS IN CAPTIVITY.
Little has been written about the food preferences in Leaf-cutting ants in captivity. It is of limited use reading the volume of literature available on plant preferences in South and Central American tropical rain forests when trying to find something for the little blighters to eat in the middle of a typical British winter.
Some improvisation is therefore necessary. It would however be sensible to show what plant species that Leaf-cutting ants commonly take in their natural environment to help paint the full picture. To this end the following table illustrates some forest trees and plants and also agricultural crops most damaged by the activity of the attines in their natural environment.
FAMILY SPECIES COMMON NAME
Leguminosae Dioclea guianensis
Burseraceae Bursera gummifera
Tiliaceae Muntingia calabura
Annonaceae Annona trinitensis
Sterculiaceae Guazuma ulmifolia
Anacardiaceae Spondias mombin
Boraginaceae Cordia alliodora
Combretaceae Terminalia amazonia
Flacourtiaceae Casearia guianensis
Lauraceae Nectandra martinicensis
Melastomaceae Miconia acinodendron
Rosaceae Licania membranacea
Acanthacae Megaskepasma erythrochlamys
Agavaceae Dracaena spp. White Rheo
Amarantaceae Amaranthus spp. Spinach
Anacardium occidentale Cashew
Mangifera indica Mango
Spondias cythera Golden Apple
Spondias mombin Hog Plum
Aracaceae Cocos nucifera Coconut Palm
Bactris major Roseau Palm
Asteraceae Bidens pilosa Railway Daisy
Eupatorium odoratum Christmas Bush
Lactuca sativa Lettuce
Begoniaceae Begonia spp. Begonia
Bignoniaceae Crescentia cujete Calabash
Tabebuia rasea Pink Poui
Bombacaceae Ochroma pyramidale Bios Flot
Boraginaceae Cordia collococca Manjack
Cordia curassavica Black Sage
Brassicaceae Brassica chinensis Pak Choi
Brassica oleracea capitata Cabbage
Brassica oleracea verbotritis Cauliflower
Caesalpiniaceae Cassia fructicosa Cocrico Bush
Mora excelsa Mora
Cannaceae Canna edulis Canna Lily
Cucurbitaceae Citrullus lanatus Water Melon
Cucumis sativus Cucumber
Cucurbita moshata Pumpkin
Fabaceae Cajanus cajan Pigeon Pea
Erythrina glauca Swamp immortelle
Erythruna micropteryx Mountain "
Inga venosa Wild Pois Doux
Moghania strobilifex Money Bush
Vigna unguiculata Cow Pea
To provide the huge quantity of food that a large colony of Atta needs means finding alternatives which are both acceptable to the ants and easily obtainable. Based largely on personal experience, the following table illustrates some food types which in the past have proved to be of use. They are ranked in rough order of preference based largely on the reaction of the ants when first offered the food.
TABLE 1.0; LIST OF FOODS TAKEN BY ATTA AND ACROMYRMEX spp.
GROUP A GROUP B GROUP C
Raspberry Leaves* Lentils Coconut
Black Cherries* Barley Oats
Kibbled Maize* Tea Bags (used) Raw Rice
Orange Rind* Fresh Peas Cooked Rice
Raisins Split Peas Raw Macaroni
Sultanas* Cooked Corn Dog Meal
White Grapes Various Cereals; Rabbit Pellets
Black Grapes Rice Crispies Carlans
Rose Leaves Sugar Puffs Ivy Leaves
Rose Petals* Holly Leaves Oak
Mixed Flowers Lemon peel
Cooked Ham* Split Yellow Peas
Cooked Chicken* Fruit cake
Shelled Sunflower Sponge Cake
Cabbage Glace Cherries
The above table contains 3 headings;
GROUP A - Those foods that are taken at all times even when the colony is not 'hungry'.
GROUP B - Those foods that are taken most times but are not favoured over those in GROUP A.
GROUP C - Those foods which are normally taken only wnen foods of groups A or B are not present.
These are especial favourites and are rarely turned down.
It can be seen that virtually all foods are available all year round so it will be possible to provide the ants with a variety no matter what the weather is doing outside. Please note that ants get tired chewing the same food all the time and if offered the same food for more than two days in a row (even a GROUP A food) they will cease foraging activity until a new food is found. Therefore only feed at one time a quantity of food that the ants can remove completely in around 4 hours. The next day feed something different and so on until perhaps 10 days later you can offer the original food again and they will again forage with great enthusiasm. (This is a built in safety device so that the ants rotate their attentions from tree to tree thus avoiding the destruction of portions of the forest.)
At the end of the day and following a great deal of hard work, not a small amount of pain and a good measure of panic, you will have the satisfaction of having a genuine 'super organism' in your charge. Thirty million years have prepared these ants for you, how long will you prepare for them?