Why Do Monarch Caterpillars Turn Black?

Monarch caterpillars thrive in their natural habitats. Their distinctive colors warn their predators or potential threats to stay away from them. In addition, they can change their color to black in certain situations.

Why Do Monarch Caterpillars Turn Black? Monarch caterpillars can turn black because of Nuclear Polyhedrosis Virus (NPV), molting, bacterial infection, feeding on toxic plants, parasitic infection, and dehydration.

They go through various life stages before becoming adult monarch butterflies. They feed on their specific plant leaves, such as milkweed, to fulfill their dietary needs. You should provide them with their favorite host plants so that they can survive effectively.

Nuclear Polyhedrosis Virus (NPV)

Monarch caterpillars are sensitive and small creatures. They thrive on their specific host plants, such as milkweed plants.

They can get the pathogen due to living in contaminated host plants, which can cause them to change their color to black.

Furthermore, the nuclear polyhedrosis virus NPV is microscopic and infects monarch caterpillars. Once a caterpillar becomes infected with NPV, the virus starts to multiply inside its body.

You can notice changes in its appearance as the virus spreads in its body, and its color might start to change and turn black.

It spreads through contact with contaminated surfaces in their environment. Once inside a caterpillar, the virus multiplies quickly and weakens their immune system, which can lead to death.

This is why NPV is referred to as the black death for caterpillars. NPV is a serious condition for these tiny creatures. They cannot live longer after being infected with this deadly virus.


Molting is an essential phase in the growth of monarch caterpillars. Their bodies undergo significant changes when they grow and shed their old skin.

This new skin is initially soft and vulnerable. The appearance and color of these creatures can turn black or darker when they shed old skin.

It is not rare for a newly molted caterpillar to appear darker in color. This change is primarily due to the freshly molted skin being softer and more pliable than the older, harder exoskeleton.

Molting is a natural and necessary part of their growth process, enabling them to accommodate their increasing size and continue their journey towards becoming magnificent adult butterflies.

It is essential to note that the change in their physical appearance can be a sign that they are stepping into the next stage of life and ready to become a butterfly.

However, the abnormality in their behavior can be due to several reasons, and they can turn black.

 It goes through several changes in its body before becoming an adult butterfly.

It can be pretty normal for them to transform, while in certain conditions, it can be a cause of disease or virus, which can lead them to death.

Bacterial Infection

Monarch caterpillars are sensitive to change and can attract several pathogens if they live in moist environments.

The water bodies are home to several pathogens. They can get an infection if exposed to contaminated and moist places.

One of the causes of their black bodies is pseudomonas. It is an infection that can lead to the death of them.

They can experience a change in color, including a color change if infected by this bacteria.

This color change can indicate a sign of illness in these caterpillars.

Additionally, this bacterial infection leads to reduced vitality and a decline in their survival chances. This phenomenon is also referred to as black death in these creatures.

They change their color into black after being infected by bacterial infections and eventually die if not treated properly. It is essential to monitor the unusual signs to tackle the problem immediately.

Feeding on Toxic Plants

Monarch caterpillars rely on milkweed plants as their primary source of food. However, certain species of milkweed contain toxins that can change the color of caterpillars if they consume them.

They can turn black after consuming the leaves of certain toxic plants. This color shift can benefit them as they confuse their predators by mixing up in the environment and deter potential threats or dangers.

They become less evident due to changes in their color. This is a survival tactic for these creatures to increase the chances of their survival.

They lack self-defense due to their small size and delicate body appearance, so turning into black color can give them an advantage in deterring the potential threat or danger.

My uncle had several monarch caterpillars in captivity. He shared that they feed from milkweed plants, and they change their color after feeding from various leaves of specific plants.

I asked him how they changed their color to black. He explained that the toxins present in the milkweed plants can affect the physical appearance of their bodies because of the potent consistency of toxic plants.

Parasitic Infection

Parasitic infection in monarch caterpillars is a phenomenon in which they become hosts to various parasites.

One of the most common parasitic infections in these caterpillars is caused by a group of parasitoid wasps known as braconid wasps.

These wasps target these caterpillars as hosts for their developing larvae. Female braconid wasps locate them and lay their eggs on or near their bodies.

 As the wasp larvae grow, they can cause significant harm, and they turn black due to this infection, hindering their growth and overall well-being.

 Furthermore, infected caterpillars can display abnormal behaviors, such as reduced feeding, sluggish movement, or erratic behavior. The difficulty in their growth can lead them to death.


They need water to stay hydrated and can get water from raindrops or moisture from leaves.

However, if no water is available in their natural habitats, they can get illnesses, which can lead them to change their color.

They can turn black due to dehydration, which significantly changes their appearance. This transformation highlights the severity of dehydration’s impact on their overall health.

One of my friends told me that he had various monarch caterpillars in captivity. He shared that they turned black after a few days. He decided to seek advice from an entomologist about the condition of his caterpillars. The entomologist explained that they changed color due to a deficiency of water, a condition that could lead to death.

The entomologist advised him to monitor their behavior and provide them with enough food and water for their well-being and survival.

Following this advice, my friend ensured proper care. After some days, the caterpillars returned to their colors and became healthy again.

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