Farmer Field School exercise: How Bt controls vegetable pests

One of the greatest advantages of using Bt is that it does not kill insect natural enemies which help control many pests. But Bt works differently from most insecticides and farmers often don’t understand this.

Pest caterpillars must eat very small amounts of the Bt-sprayed leaves before they will die. After eating Bt, the insects will not die quickly but will get sick and will stop feeding almost immediately. It may take 1-2 days for the larvae to die after eating Bt. It will take even longer for larger larvae. Very large larvae and larvae already inside cabbage heads are not likely to be killed by Bt.

Farmers can learn how Bt works and how to use Bt properly by conducting the following simple exercise.

Step 1: Collect the pest larvae

Collect small (less than 2 cm long) cabbage loopers or other caterpillars that feed on crucifer crops from your own field or from an old abandoned field. Look for feeding holes and small larvae on both sides of the leaves. Remove larvae from the leaves by hand or with a small soft-bristle paintbrush. Put an unsprayed leaf of cabbage or other crucifer crop into a container (any kind of container will work for field collection) (Fig 22). Transfer the caterpillars from the plants to the leaves in the container. Keep your collection container in the shade until you move the larvae to rearing containers (see step 5).

Figure 22. Farmers in an FFS collect caterpillars in a collection container.

Step 2: Prepare rearing containers

Many types of containers can be used for rearing caterpillars but one of the cheapest and easiest is a plastic wide-mouth bottle. The bottle should be covered using mosquito-netting or similar cloth and a rubber band. Clear plastic boxes or even open boxes or trays can also be used. For this exercise you will need at least two rearing containers.

Figure 23. Rearing container for caterpillars.

Step 3: Prepare the food for caterpillars

Pick a few unsprayed leaves of Chinese kale, cabbage or other crucifer crop. Trim the leaf petioles with a knife or cutter (Fig 24). Soak some cotton balls in plain water or water mixed with a soluble fertilizer (Figures 25, 26) or urea dissolved in water. Squeeze some of the water (but not all!) out of a cotton ball and wrap it tightly around the cut base of the leaf petiole (Fig 27). Wrap the cotton and leaf base with a small plastic bag and seal this using a rubber band (Figure 28). Alternatively, wrap the end of the petiole with water-soaked cotton and push the stem and cotton into a small glass or plastic vial. In both cases make sure the cut end of the leaf petiole is in close contact with the water-soaked cotton. The leaf should now last 2-3 days without wilting. Check the cotton every day and add water if it begins to dry out.

Figure 24. Preparing leaves for use in rearing container

Figure 25. Preparing leaves for use in rearing container

Figure 26. Soak cotton in water

Figure 27. Preparing leaves for use in rearing container

Figure 28. Wrap plastic around the wet cotton

Step 4: Prepare and spray the Bt solution

Mix a Bt product with water according to the instructions on the product label (Fig 29). Make sure you are using fresh Bt from a reputable dealer and not old Bt that has been in the store for a year or more. It is important to also mix in a small amount of spreader-sticker (Fig 30) when Bt is to be used on crops in the cabbage family. Pour this solution into a plastic hand sprayer. Put on gloves and spray both sides of the leaves (Figure 31) with a fine mist. Allow the spray to dry on the leaves before introducing the caterpillars. Do not spray all of the leaves; save some of them to use as “controls”. Also, if Bt is going to be sprayed again on another day, make sure a fresh solution is mixed for each day.

Figure 29. Two types of Bt commonly found in Thailand: trade names Florbac (Bt aizawai) and Bactospeine (Bt kurstaki).

Figure 30. Mix Bt with a spreader/sticker

Figure 31. Spray both sides of the leaves with Bt solution

Step 5: Transfer caterpillars to Bt-sprayed and unsprayed leaves

You will need at least two rearing bottles or containers. Put one or two Bt-sprayed leaves in one container and one or two unsprayed leaves in the second container. Mark the containers with a label. Now transfer the caterpillars you have collected to leaves (2-3 larvae for each leaf) in the rearing containers.

Step 6: Observe caterpillars and their feeding damage

Observe the caterpillars after they have been transferred to the Bt-sprayed and unsprayed leaves. Are they actively feeding? (Fig 32). Observe them again 10-15 minutes after they begin feeding--are they still feeding or have any stopped feeding and/or moved away from the feeding site? After about an hour, check again to see which caterpillars are still feeding and compare the sizes of the holes in the Bt-sprayed and unsprayed leaves. Check the caterpillars and their feeding damage again the next day--have some of the feeding holes become larger? Do any of the larvae appear sluggish or have stopped moving? Check and make the same comparisons again the following day--have some of the larvae died, changed color, or shriveled? (Fig 33, 34). Check the bottom of the containers for larvae that may have died and fallen from the leaves.

Figure 32. Observe if the caterpillars are actively feeding.

Figure 33. Larvae killed by Bt may become discolored or turn black (left is healthy looper, right is sick looper)

Figure 34. Dead larvae often become shriveled

More learning exercises

There are several other activities that can be done using similar procedures. These offer excellent learning opportunities in Training of Trainers courses and in Farmer Field Schools

	Compare Bt treatments with and without stickers.
	Compare effects of Bt on small and large larvae.
	Compare Bt aizawai and Bt kurstaki products.
	Compare different brands of Bt products.
	Wash Bt-treated leaves with water and compare them with unwashed Bt-treated leaves.
	Release natural enemies (e.g. spiders, ladybird beetles, hoverfly larvae) on the Bt sprayed leaves and observe that they survive.

Acknowledgements

Most of this page by: Brent Rowell, Dept. of Horticulture, University of Kentucky, email: browell@uky.edu