Leaf Cutter Ants Draft #2

This discussion topic submitted by Alex Covert on 5/13/98.

After extensive reading and the consideration of many topics, I choose leaf-cutter ants because of a sentence written by Alexander F. Skutch in his book entitled, A Naturalist on a Tropical Farm. "As predation, parasitism, and every form of ruthless exploitation of one organism by another are proof of evolution's tragic failure to create a harmonious community of living things, so every instance of fruitful cooperation is evolution's triumph, a sample of what it might have achieved with a different sequence of mutations over the ages, or better, if it had been guided by a wise, compassionate Intelligence- a sample, too, of what the living world may be on some far-distant planet happier than our own." Mr. Skutch lives in Costa Rica and is talking about symbiosis, particularly that between leaf-cutter ants, fungi, and specific tropical plants. It is my intention to discuss to the class a little bit about leaf-cutter ants and their role(s) in nature. Specifically, I will talk about their preference for certain species of plants and why this is so.

I. Symbiosis - living together of species in which both partners benefit
-Leaf Cutter Ants have this type of relationship with a fungus

II. Leaf Cutter Ants or Parasol Ants
A.) General Description
-Genera Acromyrmex and Atta (15 species)
-Range is in the Americas from Louisiana and Texas south to Argentina
-A dominant herbivore of the neotropics
-usually found in savannas or move into disturbed areas, i.e. human impact, and prove to be the major determinants of the success or failure of a crop or plantation.
-Susceptible crops include: citrus trees, cocoa, coffee, maize, cotton eucalyptus, pines, and bananas.
B.) Ants as pests
1.) Example of destructive capability
-leaf cutters remove approximately 17% of the total annual leaf production in intact forests.
-a colony can eat as much as a grown cow in a day
-ALSO, do good: turn over soil and aerates it and circulates nutrients
2.) How much damage is done?
-estimates based on pesticide costs and crop replacement is subjective.
-includes such questions as, "Are insecticides over used?" and "Are they
effective?"
3.) Means of control
-have used non-specific toxic baits(insecticides)
-this has caused an increase in the extinction of some unique ant species
(And probably others!)
C.) Foraging
-some debate of how the ants choose the vegetation types
1.) Optimal Foraging Theory - theory that animals should behave so as to
maximize energy intake for the time spent foraging.
2.) Conservational Grazing System
-has been noticed that they don't forage for vegetation immediately around
the nest.
-the ants scout mainly from the ends of their trails rather than from
the nests
-this spreads the grazing pressure over the whole foraging
area and minimizes over-exploitation.
-also, minimizes the over-exploitation of preferred plant
species, so that the quality is conserved as well as the
quantity.
-allows them to exist for about 20 years, the life of a colony
-usually forage on material near their trails.
a.) Factors that affect foraging.
1.) The trail system developed.
2.) The ants' previous experience with certain plants
3.) Higher energy expenditure (plant distance)
4.) Competition with rival colonies
5.) Presence of aggressive ant species
6.) Seasonal changes
7.) Foraging phases in colonies
D.) Leaf Cutters are selective in the plant species they harvest.
-exhibit preferences for individual plants within a species and for specific leaves
of an individual.
- Factors that affect plant selectivity (Important information to help control)
1.) The presence of secondary compounds toxic to ants and/or their
fungus.
2.) The presence of secondary compounds that may reduce ant or fungus
ability to digest the plant.
e.g. Tannins - are strong inhibitors of fungi and their enzymes.
-this would affect growth of the symbiotic fungi
3.) The plant's nutritive value for the fungus
-workers obtain a large portion of their metabolic needs by
ingesting plant sap.
-this increases the quality of the sap and/ or enhance the growth of
the fungus.
4.) Amount of nitrogen(protein), carbohydrates, and lipids.
5.) Moisture content (Contradictory)
-one study states that the ants prefer leaves with more water.
-another states that they choose wilted leaves, perhaps due to
repellent substances having evaporated.
6.) Leaf toughness.
E.) Life Cycle of Colony
1.) Nuptial flights
-heavy virgin queens fly upwards and mate with as many as 5 males
-retains sperm for 14 years or more
2.) Queen creates nest and raises first crop or worker ants.
-carries a wad of thread-like hyphae in a pocket at the bottom of her mouth
-after enough workers are present to run the nest, the queen becomes an
egg factory for the rest of her life.
3.) The ants begin to forage.
-they sample many different plants and, depending on the season, choose
certain ones.
-the plants are cut by the largest ants or foragers.
-these pieces are picked up by smaller workers who further cut them up
and take them to the nest.
-still smaller ants crush and mold into moist pellets and puts them into piles
-before ant workers innoculate the harvested leaf fragments with fungus,
they deposit fecal fluid containing previously ingested fungal enzymes,
mostly pectinases and proteases, onto the leaf fragments to initiate leaf
digestion and enhance subsequent fungal growth.
-still smaller ants pluck strands from places of less dense growth and place
them on the newly constructed vegetable-paste substrate.
-finally, the smallest ants clean the hyphae and pluck out alien species
(Thus, the fungus is given conditions and care that allow it to prosper, and
the ants are receiving a food source.)

Nicholsorians, C.M. "Environmentally Induced Differences in Plant Traits - Consequences for Susceptibility to a Leaf-Cutter Ant." Ecology. 72:(5) 1609-1623. Oct. 1991.

Folgarait, P.J. L.A. Dyer, R.J. Marquis, and H.E. Braker. "Leaf-cutting ant preferences for five native tropical plantation tree species growing under different light conditions." Entomologia Experimentalis Et Applicata. 80:(3) 521-530. Sept. 1996.

Vasconcelos, J.L. and J.M. Cherrett. "The effect of wilting on the selection of leaves by the leaf-cutting ant Atta laevigata." Entomologia Experimentalis Et Applicata. 78:92) 215-220. Feb. 1996.

Skutch, Alexander F. A Naturalist on a Tropical Farm. University of California Press. 1980.


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