In Media Res

Location of 3/6 plots

Today is a big day. I have 90 coffee berries across 3 2x2m plots in Costa Rican coffee fields and today I will collect them. Those berries will provide the control data for the 90 berries I collected from 3 other plots yesterday. What are these plots and berries?

Those questions culminate 10 months of thinking and reading. The goal in September was to come up with a question keen enough that I could get sent to Costa Rica to answer it. The goal now is to discern the potential for biological control of the coffee berry borer Hypothenemus hampei by the ants Solenopsis geminata, Wasmannia auropunctata, and Pheidole radoszkowskii. The fact that I now know those 4 spellings by heart shall be listed among my life's primary accomplishments.

The coffee borer, called la broca or the drill by farmers, is the most serious pest of coffee worldwide. It's a beetle (a weevil, if you must know) smaller than a comma in standard typeface. Very small. It drills into coffee berries and has kids, who excavate a cavern inside the coffee bean and destroy it. Ants are the main predators of the pest. Ant predation of la broca has been seen in the lab and in some contrived field experiments, but I'm looking simply to see how realistic it is as a control strategy. The fact that ants eat beetles in a petri dish does not necessarily imply that they reduce beetle populations in the field.

La broca adult

La broca 'drill' on a berry

Without examining the gut content of ants in the field, it's hard to actually know if they're eating the berry borer. I can find out indirectly, however. If I set up an experiment which modifies ant populations (but nothing else) and the rate of la broca disappearance from berries increases or decreases, this implies that the ants were doing something. If I can increase ant populations and increase la broca removal, this would imply that the ants actually are eating the beetle in the field. Even better, increasing ants to kill more beetles suggests a future control strategy for la broca: throw more ants at 'em. So that's my goal, to increase ants and increase la broca removal. How do you increase ants?

    Competition: S. geminata
        worker with an angry
W. auropunctata on her leg. 

Some ecology: ants are everywhere always. Ants are enormously succesful insects. They are so successful that they often have no competitors except other ants. Some ants are better at eating la broca than others (in the lab), so what if I remove an ant species which probably isn't good at eating la broca? Could I increase the population of an ant which is good at eating la broca? Could I then show that more la broca get eaten in plots where I increased that good ant (by removing its competitor, the bad one)?

So that's the experiment. I removed S. geminata and hoped to see an increase of W. auropunctata and/or P. radoskowskii. The latter two are much smaller than S. geminata, small enough to crawl into the holes made by the berry borer and eat the beetle. They have also been shown in the lab to eat more la broca. So, hopefully, when they increase la broca will decrease. I'll know this afternoon.

A W. auropuntata worker,
seen through a microscope.

   How to know one increased ants?
   By baiting with tuna, of course!