Show the Importance of Natural Enemy Conservation with the Insecticide Check Method

Many landscape pests such as spider mites, aphids and scales are kept under control by resident natural enemies. You can demonstrate the importance of these natural enemies in the landscape by using the "insecticide check method."

Spider mites are probably the best for demonstration purposes because their effects on plants are dramatic. Find a planting of burning bush, thornless honeylocust, or an apple orchard. Apply a broad spectrum pyrethroid insecticide, such as cyfluthrin or resmethrin, in early June to some plants, insecticidal soap to others, and leave the rest alone. Examine plants in each of the three groups every week to look for spider mites, natural enemies and leaf discoloration.

To sample the mites and natural enemies, place an 8.5x11" sheet of white paper beneath a branch and tap the branch twice. Count the number of mobile mites you see, along with any lacewing or lady beetle larvae (Stethorus punctum). Sampling 4 branches per tree or shrub should be enough to detect natural enemies. Small black adult lady beetles (probably S. punctum) can also be monitored by counting all you can see during a 3 minute walk around a plant. If you track the number of mites and predators you find per sample on treated and untreated plants, you should see that predators will be detected much later in the season on plants treated with pyrethroids.

Compare the quality of the foliage in August. Heavily infested trees will have bronzed leaves or may even be defoliated. You can take photos of the plant leaves, and plot graphs of mite and predator populations to use at winter meetings when discussing the merits and disadvantages of using pyrethroids, soap, and refraining from pesticide use.

- Cliff Sadof, Purdue University


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