Are you thinking “Gross!” as you look at this photo of German cockroaches? While they might trigger disgust, they are part of one of the most successful groups of animals on the planet. Cockroaches have been on Earth for over 300 million years, and about 3,500 species exist worldwide. The species that coexist with humans thrive in warm, moist areas, and scavenge for food that humans drop in the kitchen or leave on dirty dishes in the sink. As they scavenge, they spread germs that can cause food poisoning, diarrhea, dysentery, and other gastrointestinal illnesses.
The United States spends $11 billion annually to get rid of roaches and other pests. In the mid-20th century, humans started using synthetic organic chemicals to kill cockroaches. To draw the roaches out of crevices that sprays couldn’t reach, the poisons were mixed with corn syrup to attract the roaches. Within a few years, resistant populations of roaches started to appear.
A typical situation was observed in Gainesville, Florida. Starting in 1983, the poison hydramethylnon baited with corn syrup had been used successfully in an apartment building, but by 1988 the cockroach mortality rate had dropped to about 40%. If, however, the roaches were sprayed directly with hydramethylnon, they all died, showing that the roaches were not physiologically resistant to the poison. Instead, it appeared that most of the roaches never ate the poison at all – they just didn’t go for the bait.
In this exercise, you will take on the role of a scientist to figure out why some cockroaches reject poison baits in their environment.
Your first step will be to determine which ingredient in the poison bait triggers aversion behavior.
Next, you will figure out if the aversion behavior is learned or inherited, and therefore if natural selection can act on the trait.
Finally, you will determine when the aversion behavior does and does not benefit the cockroach population.
Part A - Are cockroaches refusing poison baits because of one particular ingredient?
To find out which ingredient the cockroaches are refusing, you can carry out a controlled experiment. Hydramethylnon is dissolved in oleic acid before being mixed with corn syrup to prepare the poisoned bait, so you should test all three ingredients.
Suppose you conducted a feeding trial to test each ingredient. In the trial, a set of four agar dishes, three containing all except one ingredient and one with all three ingredients (the control), were weighed and placed 3 cm apart on the kitchen floor in an infested apartment. After two days, the dishes were weighed again to measure food consumption. You then repeated the trial 4 times and averaged the results.
A diagram showing the setup of the feeding experiment
Here are the results from your experiment.
Tested (absent) diet component Average food consumed (mg/day)
Hydramethylnon 16
Oleic acid 12
Corn syrup 71
Control (all three ingredients present) 15
Adapted from Silverman and Bieman. 1993. Glucose Aversion in the German Cockroach,
Blattella germanica, Journal of Insect Physiology 39: 925-933.
Based on the results of the feeding experiment, what conclusions can you draw? Select all that apply. (For help interpreting the results, see the hints.)
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