Leaf Disk Test: Bioassay of Effect of Tannins on Insect Feeding Behavior
The feeding behavior of herbivorous insects is guided by plant chemistry. There are specialists and generalists with regard to the range of plant species they attack. Nutrients such as sugars and proteins, and secondary plant compounds such as phenolics, terpenoids, or alkaloids, determine whether or not an insect will feed on a plant and to what extent.
To investigate food preferences by insects and the compounds responsible for their choices, especially reduced consumption, these compounds can be added to their diet and tested in feeding bioassays. Here we will add a commercially available mixture of phenolic compounds (gallic acid and several galloyl glucoses), known as “tannic acid,” to the diet of hornworm caterpillars. The tobacco hornworm, Manduca sexta, normally feeds on Solanaceae such as tomato or potato plants. For more on the natural history of this insect, consult the information sheet prepared by the biological supply company that ships these caterpillars.
We will perform the Leaf disk test, also known as leaf disk assay or leaf disk choice test, the second of two bioassays of tannins in the diet of insects in this book. In this often used bioassay, leaf sections of a standard size are treated with the compound(s) in question. Several circular leaf sections (“leaf disks”) (Ali et al. 1999, Filho and Mazzafera 2000, Shields et al. 2008, Wheeler and Isman 2000) or cellulose membrane filters (Hollister and Mullin 1999, Larocque et al. 1999) are presented to a caterpillar in a choice experiment. We measure how much chow the caterpillar has consumed and whether any feeding inhibition is concentration dependent. Regardless of what compounds are being tested, leaf disk tests serve as an important tool in bioassaying feeding inhibitors and stimulants in insects. The cited references are examples of such studies. (In the first tannic acid experiment – Chap. 18 – the tannic acid was mixed into diet in varying concentrations.)