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Monarchs and other butterflies are very susceptible to diseases. The best news is that these diseases are not transferable to people.

When you have problems rearing caterpillars, often the problem is disease. Don’t give up!

An essential step in preventing disease is keeping your rearing containers and utensils sterile. Never reuse a container without sterilizing it first. If your container is small enough, a good wash in the dishwasher may be enough. Ten percent bleach is also a method of sterilizing your containers and utensils. Be sure to wash your hands before touching their food or reaching into their containers.

Do NOT overcrowd. Keep only a few caterpillars in a container.

If possible do not allow them to run out of food.

Do not allow their containers to become filthy or wet. Excess moisture is not healthy for caterpillars.

It is believed that various diseases are latent in some butterflies and these diseases become active when caterpillars are stressed. Overcrowding, hunger, and other factors causes stress on their bodies which causes these diseases to become active.

One sign of almost all disease problems in caterpillars is a decreased appetite. If they suddenly start eating less, take notice and watch your caterpillars closely. Often they also show signs of diarrhea.

If you suspect that butterflies and caterpillars in your area may be diseased, wash all leaves which you feed to your caterpillars with a mixture of 90% water and 10% bleach. Rinse well and allow it to fully dry before feeding them to your caterpillars. If your host plants are being raised in a greenhouse where wild butterflies cannot touch the plants, you do not need to bleach the leaves.

This disease is also known as ‘melt’. Caterpillars and chrysalises with melt literally melt. The caterpillar turns into liquid which drips and splashes across leaves to be eaten by other caterpillars. Each caterpillar which ‘melts’ contains literally a billion virus particles. When a caterpillar eats these particles, it will ‘get’ the disease. The stench of a ‘melted’ caterpillar is horrible.

Nuclear polyhedrosis virus is characterized by a caterpillar dying and hanging by its middle legs. C polyhedrosis virus is characterized by a caterpillar dying and hanging by its rear legs (prolegs).

Six hours of full sunlight will kill the virus particles. Remember, however, that the underside of leaves or plants is not exposed to six hours of direct sunlight.

This disease is so effective as a caterpillar killer that pesticide companies create sprays to use in gardens, fields, or forests to control undesirable caterpillars.

Polyhedrosis virus occurs as CPV cytoplasmic polyhedrosis virus or as NPV nuclear polyhedrosis virus.

Nosema is a protozoan disease which affects many insects.

Although nosema may not kill them directly, holes will be created in their abdomens and bacteria will enter their abdomens causing death. Nosema is inside butterfly eggs before they hatch and cannot be sterilized ‘out’. If a farmers or your breeding stock is nosema infected, the quickest method to healthy caterpillars is to bring in new uninfected stock.

Ophryocystis elektroskirrha - Oe

Oe is a protozoan disease which affects butterflies which eat milkweed. Monarchs are notorious for Oe.

Caterpillars with Oe often pupate and emerge as apparently healthy butterflies. A check with a microscope can reveal heavy Oe infestation in these same butterflies. Butterflies are often checked for Oe before breeding when they are raised by conscientious butterfly farmers. Spores of these protozoa are visible on scales under a strong microscope. A piece of clear tape is touched to the adult butterflies’ abdomen and placed on a slide before viewing. Monarchs are not damaged by a check for Oe.

Very often, though, caterpillars with heavy Oe become sluggish or die. If the live to pupate, they often die as pupae. If they live to emerge, they often cannot leave their pupa, emerging all except for their abdomen which stays stuck in the pupa. If they fully emerge, often their wings are crippled, their abdomens are swollen, green is visible on their abdomens, and/or they die within a few days.

Be sure to purchase your Monarch, Queen, and/or Soldier stock from a farm which checks for Oe. Many butterfly farmers use a bleach solution to sterilize the outside of butterfly eggs before allowing them to hatch. Because most caterpillars eat their egg shells upon emerging, if protozoa spores or virus particles are on these shells, the caterpillar will become sick.

Bacillus thuringiensis is the most widely used bacterium for caterpillar control. These bacteria will also kill your caterpillars if your plants have been treated with BT. Be sure to wash your hands thoroughly after using any insecticide or garden spray.