Bt Infection in Caterpillars

Do you know what a caterpillar infected with Bacillus thuringiensis (Bt) looks like? Bt is the most commonly used microbial insecticide, yet you may not really be aware of what happens to the caterpillar in the 2-3 days between ingestion of the bacterium and death.

Collect some small (less than 1/2 inch long) caterpillars from cabbage plants. Bt does not work well against caterpillars that are more than 1/2 grown, so try to get young ones about a week old. (You could try to capture cabbage looper moths or imported cabbageworm butterflies and keep them in containers until they lay eggs and use the baby caterpillars a few days after they hatch.) Cut out 15 equal sized pieces of leaves from unsprayed cabbage, broccoli or cauliflower plants. If you use leaves that have been sprayed with an insecticide there may be enough pesticide residue to kill your test insects. Dip 5 of the leaf pieces in water; 5 in a Bt solution (mixed according to label directions); and 5 in a Bt solution that also contains a spreader/sticker, such as Bond or X-77. The spreader/sticker should help the solution adhere better on the waxy leaves. Let the leaves dry, then place them individually in containers. Put 1-5 small caterpillars on each leaf piece (depending on how many caterpillars you have and the size of the leaf pieces).

Let the caterpillars feed for at least 72 hours before assessing mortality, but look at them every 24 hours. Are the ones on the Bt-dipped leaves still feeding? How much of the leaf did they eat compared with the ones on the water-dipped leaves? At 72 or 96 hours (3 or 4 days) record the number of caterpillars that are active and dead on each leaf piece. Was the Bt effective against your caterpillars? Was there much mortality in the water control? If so, maybe your caterpillars were naturally infected with a disease. Did the spreader/sticker make any difference in the efficacy of the Bt?

- Dan Mahr, University of Wisconsin - Madison

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