The best time to sample for parasites is at the end of each STLM generation, after the leafminers are mostly pupated but before adult moths start to fly. Use a pair of fine-pointed forceps to carefully open the leaf mines by tearing back the lower leaf epidermis. Within the distinct cavity of the mine you will usually see one of the following: (1) a mostly mature unparasitized STLM larva (look carefully to be sure there are no eulophid parasite larvae attached), (2) a brown STLM pupa (does not have a cocoon), (3) a parasitized STLM larva with eulophid larva attached, (4) the brown cocoon of a braconid parasite, (5) a naked brown or black eulophid pupa, or (6) a "collapsed" STLM larva without noticeable parasites, usually a sign of predation or "host feeding" by adult eulophid wasps. If the STLM was parasitized, the shrunken cadaver should be present as well as the parasite. The small black specks are STLM feces.
Sample 20 current-generation mines per tree on each of 5 trees and keep track of your findings based on the above 6 categories. Occasionally a mine may be completely empty; if so, move on to another. Calculate the total percent STLM mortality and that portion contributed by each of the different agents listed. Try doing this for each of the three STLM generations. Do you see any differences? Also, compare parasitism in a conventionally-sprayed orchard vs. an abandoned orchard or other unsprayed trees. Do you see differences in numbers of mines? Or differences in amount of parasitism? If so, can you conclude what effects orchard insecticides might have on leafminer parasites and leafminer damage? Can you think of ways pesticide interference might be reduced?
- Dan Mahr, University of Wisconsin-Madison
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