Conserving Natural Enemies of Aphids in the Landscape

Aphids are common on many ornamental plants, but natural enemies often control them if they aren't sprayed with insecticides. How much of an impact do naturally occuring parasites and predators have on aphid populations in your area?

Find two trees or shrubs that are infested with aphids. Mark four branches on each plant with tags so that you can readily identify them. Woody plants likely to be infested with aphids are cotoneaster, crabapple, hawthorn, rose, and spirea. Count and record for each branch the number of aphid-infested terminals, the number of deformed terminals on each branch, the number of terminals with natural enemy activity, and the types of natural enemies you see (consult a reference book if you're unsure).

After these initial counts, take a picture of one of your study branches on each tree. Spray one tree or shrub with malathion at the labeled rate. Leave the other unsprayed. Photograph branches and examine terminals weekly for the next 8 weeks, counting the same things as before. At the end of your study you can calculate the average of these counts for the sprayed and unsprayed trees for each sample date.

If you want, graph the results and pair the weekly photos of branches to compare their appearance. You can use the photos and graphs at winter meetings to show the advantages and disadvantages of sprays for maintaining plant appearance and discuss the effects on pest and natural enemy abundance. When did the number of aphid-infested terminals decline on sprayed plants, and how does that compare with the number on unsprayed plants? How does the number of deformed terminals compare on sprayed and unsprayed plants over time? Are there any noticeable differences in plant appearance at the end of the study on sprayed and unsprayed plants? Were there any differences in plant appearance at any time during the study? Which changes in appearance would be acceptable or unacceptable in your back yard? What kinds of natural enemies did you see? Which natural enemies were most common? How does the number of natural enemies change through time on sprayed and unsprayed plants?

This same demonstration can also be done using eight bedding plants instead of tree branches. Though aphids can be found on most bedding plants, you are likely to find them on chrysanthemums, nasturtiums, primrose, and vinca.

- Cliff Sadof, Purdue University


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