To demonstrate this, set up side by side plots. Plot size can vary, but plots at least 10 rows wide will help minimize insect movement between plots. Treat one with a Bt product (granular formulations of Dipel, M-Peril, Condor, or several others) and the other with a conventional insecticide (granular formulations of Counter, Dyfonate II, Pounce, or others). Be sure to read and follow label instructions of whatever products are used.
If you can return to the plots regularly, make counts of common predators (lady beetles, green lacewings, spiders, etc.) at various intervals after treatment (be sure not to enter the field until the restricted entry interval has expired; see the pesticide label). Are there more natural enemies in the Bt-treated plot? If so, which ones are most common?
Three or four weeks after the treatment compare the efficacy of the two materials used against corn borer. Select 25 corn plants from each of the two plots and split the stalks lengthwise with a knife. Count the number of tunnels present and the number of live corn borers. How do the counts differ in the two treatments? You can also look at what happened to other pests that were present at the time of application (fall armyworm, armyworm, aphids, etc.). How much armyworm damage is in the two plots? Are aphid populations different? Do you think these differences might be due to naturally occurring predatory and parasitic insects?
In some areas, reductions in predator numbers by use of conventional insecticides in whorl stage corn may lead to increased populations of spider mites later in the summer. If you are at a location where spider mites are common, compare spider mite damage in the two plots in the late summer. Did differences in the number of natural enemies seem to make any difference in spider mite populations?
- Bob Wright, University of Nebraska
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