Five days before the meeting, place 200, 500, and 1000 live house fly pupae in one of three clear quart mason jars with screw-on bands. The number of fly pupae represent various levels of sanitation: clean farms (200 pupae), moderate (500), or poor (1000). To each jar add 20 female parasitoids to represent a fixed number of parasitoids as determined by the number of cows the farmer owns. Place a piece of fine mesh screen or organdy cloth over each jar, and put on the lid to hold the cloth tightly. Give the female parasitoids 24 hours to attack the fly pupae and then remove them from the container to stop parasitism. Dump the contents of the jars into a number 12 sieve (if you have one; otherwise, just put the contents into a shallow pan and carefully pick out the adult parasites); then put the fly pupae back into their jars and replace the lids.
At the meeting show the farmers the 3 containers. In the 200 pupae container (clean farm) no adult flies will have emerged; the parasitoids should have killed them all. In the 500 pupae container about 10-20 adult flies will have survived, and in the 1000 pupae container about 300 flies will have survived. This clearly demonstrates that on a farm with a poor sanitation program the parasitoids are not able to keep up with the flies, but on clean farms they will kill most of the flies. At this point most farmers begin re-evaluating their manure management programs.
- Wes Watson, Cornell University
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