Rearing diamondback moth (DBM) larvae to look for parasitoids

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

Introduction

Larvae of the Diamondback Moth (DBM) cause very serious damage to cabbage, Chinese cabbage, Chinese kale, mustard, Kwangtung, cauliflower, broccoli, and others. DBM are resistant to many insecticides and are difficult to control. The usual symptoms of DBM damage are small transparent, papery, "windows" on the surface of leaves; these windows do not go all the way through the leaf (Figure 1). There are several species of parasitoids which help reduce DBM populations in the field. Overuse of some types of insecticides, however, can kill DBM parasitoids and make control more difficult. Farmers can help conserve parasitoids by using microbial pesticides such as Bt or natural products such as neem. Farmers can learn whether DBM parasitoids are present in their field by conducting the following exercise. This can be part of the Insect Zoo in a Farmers Field School.

1. Collect the larvae

You can collect DBM larvae (Fig.1) from your own field or from an old field that has been abandoned. Make sure you know which larvae are DBM; they are very small, only 5 to 12 mm long. Look carefully for them on the undersides of the leaves. Remove them from the leaves using a small soft-bristle paintbrush or tear off a piece of the leaf with the larva. Put an unsprayed leaf of cabbage or other crucifer crop into a small container (any kind of container will work for field collection). Brush the worms onto leaves in the container. Keep your collection box in the shade until you move the larvae to a rearing container (step 4). Collect 25-50 larvae. Remember, the more larvae you collect, the greater chance you have of finding DBM parasitoids.

Figure 1 Diamondback moth (DBM) larvae
Figure 1 Diamondback moth (DBM) larvae

2. Prepare a rearing container 

Many types of containers can be used for rearing DBM but the cheapest and easiest is probably a plastic wide-mouth bottle (Figure 2). The bottle should be covered using mosquito-netting or similar cloth and a rubber band (Figure 3). Clear plastic boxes can also be used if available.

Figure 2 Preparing a rearing container
Figure 2 Preparing a rearing container


Figure 3 Rearing container for diamondback moth
Figure 3 Rearing container for diamondback moth

3. Prepare DBM food (cabbage or other crucifer crop leaves)

Pick a few unsprayed leaves of cabbage or other crucifer crop. Trim the leaf petioles with a knife or cutter (Figure 4). Soak some cotton balls in plain water or water mixed with a soluble fertilizer (Figure 5,6 and 7) 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. Wrap the cotton and leaf base with a small plastic bag and seal this using a rubber band (Figures 8 and 9). The leaf should now live for 2-3 days without wilting or drying out.

Figure 4 Preparing leaves for use in rearing container
Figure 4 Preparing leaves for use in rearing container


Figure 5 Preparing rearing container
Figure 5 Preparing rearing container


Figure 6 Soak cotton in water
Figure 6 Soak cotton in water


Figure 7 Preparing leaves for use in rearing container
Figure 7 Preparing leaves for use in rearing container


Figure 8 Wrap plastic around the wet cotton
Figure 8 Wrap plastic around the wet cotton


Figure 9 Leaves ready for use in rearing container
Figure 9 Leaves ready for use in rearing container

4. Transfer food and larvae to the rearing container

Put 1-2 prepared leaves in the rearing bottle (Figure 10). One large leaf should be food enough for 15-20 larvae for 2-3 days. Transfer the larvae to the new leaves in the rearing container using a paintbrush. Put in fresh leaves after 2-3 days-or whenever you see they have been eaten or have dried out.

Make sure to protect the rearing bottle from ants.

Figure 10 Rearing container with leaves
Figure 10 Rearing container with leaves

5. Look for DBM adults or DBM parasitoids

If DBM larvae are not parasitized, they will develop normally into light green cocoons (Figure 11). Depending on how large the larvae were when collected, this could happen within a few hours or up to a week after collection. DBM adults (Figure 12) will emerge from the cocoons a few days later. Some DBM larvae will die and you may see a very small white cocoon next to the dead DBM larva (Figure 13). This is the cocoon of a parasitoid of DBM. Wait a few more days and you will see a very small wasp (about 2.5-3 mm long, Figure 14) emerge from this cocoon.

This species of wasp is the most common parasitoid of DBM in Thailand and Southeast Asia. Several other species of DBM parasitoids have been found in Thailand (see: Parasitoids of the Diamondback Moth in Thailand).

Parasitoids of DBM pupae

You can also check for parasitoids of DBM pupae by collecting only DBM cocoons from the field. You can keep these in any kind of small container without food. Just keep them for 1-2 weeks to see if DBM moths or parasitoids emerge from the cocoons. Parasitoids of DBM pupae are described in more detail in: "Parasitoids of the Diamondback Moth in Thailand" 

Figure 11 Diamondback moth (DBM) pupa in cocoon
Figure 11 Diamondback moth (DBM) pupa in cocoon


Figure 12 Diamondback moth (DBM) adult moth
Figure 12 Diamondback moth (DBM) adult moth


Figure 13 The pupa of a parasitoid (Cotesia plutellae) and a dead larva of the diamondback moth (DBM)
Figure 13 The pupa of a parasitoid (Cotesia plutellae) and a dead larva of the diamondback moth (DBM)


Figure 14 Pupa and adult of a parasitoid (Cotesia plutellae)
Figure 14 Pupa and adult of a parasitoid (Cotesia plutellae)

OPTIONAL

If you want to determine the percent of DBM that are parasitized, you may have to wait until all the larvae have changed to either adult moths or parasitoids. This can take up to two weeks. Wait until all the moths and parasitoids have died in the container. Count the total number of adult moths (a) and parasitoids (b). Divide the number of parasitoids (a) by the total number of adult moths and parasitoids (a + b) and multiply this number by 100 to get the percentage.

Example: 

After two weeks we take out the dead moths and parasitoids and count them: here are the results:
Dead moths: 15
Dead parasitoids (tiny wasps): 5
Total moths + parasitoids = 15 + 5 = 20
Percentage of parasitoids: 5/20 = 0.25 x 100 = 25%

 

Brent Rowell
3/4/2004

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