o Why does a Monarch caterpillar or chrysalis turn black?

The monarch chrysalis normally goes through a dark phase prior to the butterfly emerging. If it seems like it is taking too long or there is a foul odor, continue reading, otherwise skip to the normal dark phase section of this FAQ. Photos of the black death syndrome are at the bottom.

Causes and Prevention of Monarch "Black Death"

Caterpillar: Pseudomonas Bacteria
Chrysalis: Nuclear Polyhedrosis Virus
Text and Photos by Forum Member tdogmom

The interesting thing is this: The main way to help prevent either disease is to keep the rearing containers DRY. Warm, moist environments promote the growth and spread of the bacterial and viral 'predators' and cause unclean conditions. People can spread these diseases to caterpillars through contact with their hands. The bacteria are usually not spread person-to-person,
ALTHOUGH Pseudomonas bacteria CAN be a problem particularly if it infects the human eye; it can ulcerate the cornea and lead to blindness. (NOTE: There are LOADS of different types of Pseudomonas bacteria—it is one of the bacteria that causes "swimmer's ear" aka otitis and is commonly the bacteria found in hot tubs.)

One way to notice if a caterpillar (aka "cat") has been infected by the Nuclear Polyhedrosis Virus (NPV) is if the caterpillar has the following symptoms: Runny, wet, or moist frass (poop), regurgitating goo, shriveling filaments at either end, sluggishness, and discolouration of the skin.
These "cats" should be removed from the other cats and destroyed since the virus is deadly. Period. It is spread from one caterpillar to another through the excretions (the runny frass or regurgitations) the infected cat has left on leaves that are then eaten by another cat.

How to destroy the NPV? Wash and dry the leaves the cats are fed. Ultra-violet light has also been found to destroy the virus so it can be used to "disinfect" habitats and rearing containers. A bleach solution also works.

Pseudomonas bacteria is found in warm, moist areas and is common in the soil and on plants. This is why it is CRITICAL to keep all rearing containers dry. Since it is found on plants, it is practically impossible to really eliminate this particular bacteria.

One thing I do is wipe the leaves I feed to my Monarch cats with those anti-bacterial Kleenex-brand tissues.
Does this help? I don't know. :P

Normal dark phase information follows:

Posted by jrcagle: I almost threw out a monarch chrysalis that had turned black and stayed that way for 36 hours without ever turning clear to reveal the monarch inside. Then it hatched into a healthy female. Are there any diagnostics on the pupae that can help distinguish the sick from the healthy? Or perhaps, is there a suggested time frame for observation?

Posted by msrpaul: Was it firm or soft? I presume firm. I have always let them go for a few days...and if soft I really check them hard.

Posted by tdogmom: Usually, when the Monarch chrysalis turns 'black' you can sort of see the wings of the butterfly inside if the pupae is indeed ready to eclose into an imago. At roughly day 30 (the day the egg was laid) you may see a bit of colouration through the chrysalis. At roughly day 31 the chrysalis begins to get dark (dark colouration from wings & body). At this point, every hour, the colouring becomes darker so that within 48 hours an adult butterfly ecloses. The distinguishing feature is whether or not you see any orange and whether or not there is an odour. If you smell anything, then chances are you have a diseased pupa. Healthy pupae have no odour to them.

Posted by susanlynne48: I had a couple that were several hours late eclosing after they began to darken. At first I could see the color of the wings, but then they turned so dark I couldn't detect anything within the chrysalid at all. I so thought it was horrible black death, but I remembered that everyone said to just be patient. I was, and indeed they emerged just fine, without any problems, or any sign of infection or disease. Other forum members have told me that often weather factors will delay emergence, such as if you've been in a rainy pattern.


Image by: tdogmom 2005
Examples of black death

Entered by larry_gene

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