Diseased and contaminated host plant leaves are a threat to caterpillars. As caterpillars eat disease pathogens on contaminated leaves, they contract the disease. But what can we do when we are raising caterpillars indoors?


Simple! Disinfect the leaves with a bleach solution before feeding them to caterpillars. (If you prefer to keep the leaves on the plant, placing the entire plant with the caterpillars, there are instructions at the bottom of page.

Milkweed is often contaminated with Ophryocystis elektroscirrha (OE) spores. There isn’t a state in the US that is free of OE. Contrary to popular opinion, OE isn’t often fatal. Most butterflies with OE fly and act normal.

Passion vine, later in summer and fall months, is often contaminated with Nuclear Polyhedrosis Virus (NPV) particles in the south. In the north, it will be on passion vine a bit later in the year. NPV is deadly. The caterpillar turns to liquid.

NOTE: Leaves are disinfected for many diseases, not only to kill OE spores.

If possible, disinfect leaves outdoors. It’s simple.

There are many recipes for this purpose. On this webpage, we share our recipes in these instructions.

Outdoors: 90% water and 10% bleach for 5 minutes. This is 9 cups of water and 1 cup of bleach. If more solution is needed, it is 18 cups of water to 2 cups of bleach.
Indoors: 95% water and 5% bleach for 10 minutes. This is 9 1/2 cups of water and 1/2 cup of bleach.
(Because the fumes are a bit stronger with 10% bleach, we recommend using 90/10 solution outdoors.)

Items necessary:
~ Gloves that reach the elbow
~ Three containers large enough to hold the cut plant material
~ Measuring cup
~ Bleach
~ Water
~ Leaves or stems

1. Measure the correct percentage of bleach and water in one container.
2. Fill the second and third containers with water.
3. Wearing gloves, insert the leaves/stems into the bleach solution.
4. Agitate gently every minute for 10 seconds.
6. At the correct time (depending on your recipe), remove the leaves and place them in the first container of water, again agitating every minute for 3 minutes.
7. Move the leaves to the second container of water and again agitate every minute for 3 minutes.
8. Remove the leaves and pat some of the excess water from the leaves.
9. Stand stem cuttings in a container with two or so inches of water in the bottom. If only individual leaves were disinfected, lay them out between dry paper towels or pat totally dry.
10. After leaves have dried, they are ready to feed to caterpillars.
11. If extra leaves were disinfected, they can be placed in a plastic bag in the refrigerator. We recommend disinfecting several days to a weeks worth of leaves at a time, storing the extra in the refrigerator. Treat it like lettuce and other vegetables we keep in the refrigerator.

If you have too many milkweed leaves and don’t need them, remember that many species of milkweed can be frozen for use when milkweed is unavailable outdoors.

Agitation is recommended because surface tension prevents the solution from touching all parts of the leaves. Passion vine has a coating that repels water. It is most important to agitate when disinfecting passion vine leaves.

~ Tropical milkweed wilts quickly. It may be necessary to wrap the stems (above the container) with paper towels to keep them from wilting as fast.
~ Hospital disinfectant can be used instead of bleach. Hospital disinfectant is much more expensive. Bleach is effective and the insect pathologists I’ve talked with all recommend bleach for those who are not allergic to it.
~ A few drops of dish detergent can be added to help break surface tension. All soap must be removed from the leaves. Soap is a deadly meal for caterpillars.
~ Leaves are not infected, they are contaminated. Pathogens are on the outside of the leaf. The plant is simply a surface that the pathogens rest upon.
~ Contaminated leaves look like any other leaf in appearance. The pathogens are too small to see without a microscope. Some pathogens must have a strong microscope and the leaf treated with oil immersion for the pathogens to be visible.

How do we disinfect a living plant when we want to use it to feed caterpillars? Some of us like to keep the leaves and stems on the plant.


1. Place the well-watered plant, pot only, in a plastic bag and tape it shut around the stem.
2. Turn the plant upside down and lower the plant (but not the pot) into the solution (outlined in directions for leaves/stems, above).
3. Place two sticks across the top of the bucket, spaced to hold the pot above the solution.
4. Gently agitate for 10 seconds every minute. To ‘agitate’ a living plant, simply move it around a bit in the solution. 5. At the correct time (depending on your recipe), remove the plant and place it upside down in the first container of water, again agitating every minute for 3 minutes.
6. Repeat step 5.
7. Remove the plant from the water and place another plastic bag over the one on the pot. (This prevents caterpillars from crawling over any pathogens that may have been transferred to the bag while we were handling the plant. Taking the bag off and replacing it would add to the danger of pathogens on the soil and pot from being transferred to the plant or new bag.)
8. Place the plant where butterflies can’t touch it until the leaves have dried.
9. Place the plant, bag and all, in the rearing container. The plastic bag keeps caterpillars from crawling over pathogens on the soil and pot. As they crawl over pathogens, picking them up on their bodies, they transfer pathogens to leaves as they crawl to leaves and begin to eat.
10. Because the plant was watered before it was disinfected, it should not need water before the caterpillars eat all the leaves.