Why Caterpillar Cannot Pupate? [7 Causes & Solutions]

Those who grow butterfly eggs indoors notice when into caterpillar pupas lie still, shrivel, turn black, turn into goo, are misshapen, or develop in incomplete ways. Why do these things happen?

When caterpillar pupas fail, it may be caused by:

  1. injury or damage
  2. malnutrition
  3. developmental abnormalities
  4. predators
  5. pathogens
  6. chemical, genetic, or organic disruptors
  7. meteorological issues

Without us knowing, caterpillars and butterflies play critical roles in our lives. For the many who look after them at home, it is important to know how to keep them safe and healthy. Here’s a quick review of what we need to know.

What Happens In a Cocoon?

Stop. That doesn’t sound right.

Only a moth makes a cocoon. A butterfly makes a chrysalis. The term “chrysalis” can mean a hard case, a soup, or a process. The right question is: What happens in a chrysalis?

First, a caterpillar begins its pupa stage by building a hard case. This case is called a chrysalis. Then the caterpillar enters and seals the entrance of the chrysalis.

Second, in the chrysalis, the caterpillar turns into soup. This soup is also called chrysalis.

  • In the soup, parts, and organs begin to form and organize themselves into an adult butterfly: antenna, body, legs, wings, and so on.
  • This can complete itself from 5 up to 21 days, depending on the type of butterfly, the weather, and the temperature.

Third, the process of transitioning from caterpillar to butterfly in that soup is also called a chrysalis.

PRO TIP: If a chrysalis falls and it’s not injured too badly, use a bead of glue to reattach the chrysalis so that it can continue to develop and a butterfly can hatch and dry its wings properly.

Inside the Cocoon: 3-D Views of a Butterfly's Metamorphosis

Why Caterpillar’s Pupas Fail

When pupas fail, it could be due to injury, damage, weakness from malnutrition, genetic defect, predators, pathogens, chemicals, or the weather.

Caterpillar Cannot Pupate – Infographic

But sometimes, there’s no reason to worry. If you can’t see the pupas, it’s because caterpillars disappear elsewhere to find a good place to pupate.

And if pupas aren’t moving, it could be because they’re playing dead due to stress or they’re disturbed, or they’re just naturally dormant due to delayed eclosion (remember “eclode”?).

PRO TIP: Many people enjoy bringing caterpillars from their gardens indoors to pupate and emerge as adult butterflies. But occasionally, these caterpillars harbor infectious parasites including bacteria, viruses, or protozoa.

The pupa stage is a complex and delicate process that can, for various reasons, fail at any point. In fact, I’m receiving reports from different places about pupas dying, turning into goo, developing incomplete parts, or ecloding improperly. What’s going on?

Pupation of a butterfly caterpillar

Here’s a summary of answers to why these are happening, and what you can do.

1. Injury or Damage? Euthanize

Touch can deform, damage, or injure a new chrysalis or a pupating larva. The damage can either injure the chrysalis or later show as deformed butterflies.

The issue: In the first hour after pupating, the soft chrysalis can be damaged easily by the touch of twigs, leaves, or even an insect walking on it. The same damage can also happen to a caterpillar that is still forming a “J” when it starts pupating

For instance, when the caterpillar’s rear end is injured, it is unable to properly hang its chrysalis. It’s also unable to shed its old skin. These can cause pupation failure.

What you can do: If you’re sure that pupa failure is due to injury or damage, you can quickly euthanize them. Seal them in a plastic bag and place them in the freezer where they die within minutes. This works for defective butterflies, their eggs, caterpillars, and chrysalises.

PRO TIP: When a chrysalis or larva is severely damaged, try and give it a chance to heal itself. Leave it alone for a while. Maybe it’s just dormant or playing dead.

Raising Monarchs - Chrysalis Repair (Help The Monarch Butterfly)

2. Malnutrition? Provide Enough Food

The issue: A lack of food supply or overcrowding of larvas (caterpillars) can result in premature pupas, or chrysalises that are poorly anchored or are too weak to molt. In some cases, lack of food can increase fighting and aggression – even cannibalism – among caterpillars.

What you can do: If you believe that caterpillar pupas are failing due to lack of food, increase their food supply. At the same time, space them out to prevent overcrowding.

Caterpillars eat flowers, grass, honeycomb, and leaves. The most common caterpillar food includes dandelion, clover, apple, alder, oak, willow, birch, cherry, and poplar leaves. You can also raise milkweed from seeds such as this one from Amazon.

3. Developmental Abnormalities? Euthanize

The issue: Genetic inbreeding in butterfly colonies can result in mutations and defects such as juvenile hormone disorder that prevent pupas from developing into adult butterflies.

Another disorder is called anal prolapse, where the rectum forms a green ball at the rear end of the caterpillar, is a fatal condition caused by a faulty gene inherited from a parent butterfly or by a pesticide or herbicide.

What you can do: If you believe that caterpillar pupas are failing due to developmental abnormalities, you can euthanize them to prevent the genetic defects from spreading.

🦋 I Rescued An Injured Butterfly - "Wrinkles" 🦋

PRO TIP: When a caterpillar stops moving, don’t touch it. It’s probably getting ready to molt, grow, or change instars. Leave it alone.

4. Predators? Attack and Protect

The issue: Predators such as Chalcid wasps, ambush bugs, Tachinid fly maggots, and Trichogramma wasps can attack and cause pupa failure.

What you can do: If you believe that caterpillar pupas are failing due to predators, you can try out wasp trap sticks such as this one on Amazon, or a nontoxic wasp deterrent such as this one, also on Amazon.

5. Pathogens? Isolate and Euthanize

The issues: The two most common pathogens are (a) the Ophryocystis elektroscirrha parasite, also known as OE, that infects milkweed butterflies, and (b) the nuclear polyhedrosis virus, also known as Black Death or NPV virus. It can be used as an eco-friendly bio-pesticide.

What you can do: If you believe that caterpillar pupas are failing due to pathogens, you can isolate and euthanize the infected ones and move survivors to fresh leaves. To kill the OE spores, you can also soak the infected eggs, leaves, or larva in a solution of.90% water and 10% bleach.

Prevention strategies include: avoiding handling of eggs or larvae after handling adults; keeping hands very clean; and sterilizing tools and equipment with a solution of 20% bleach and 80% water.

6. Chemical, Genetic, or Organic Disruptors? Keep It Clean

The issues: GMO milkweed, anti-flea medication on pets, and pesticides such as insect growth regulators (IGR), as well as BT bacteria (Bacillus thuringiensis) can kill 99% of caterpillars. Organic pesticides such as neem oil can also cause pupation failure.

What you can do: If you believe that caterpillar pupas are failing due to chemical, genetic, or organic disruptors, you can grow your own milkweed from uncontaminated sources, avoid transmitting anti-flea medication by keeping your hands clean (even after touching a pet), and stay away from IGR pesticides upwind.

Raising Monarchs - Sanitation (Help The Monarch Butterfly)

PRO TIP: To protect themselves against caterpillars that eat their leaves, some plants produce defensive chemicals that reduce larval feeding and growth, affect development of larva, and increase caterpillar deaths.

7. Meteorological Issues? Fix It Fast

Research shows that higher temperature increases juvenile hormones, triggers premature metamorphosis, as well as body size and color of caterpillars. Research also indicates that disruption of circadian behavior can affect caterpillar feeding, growth, as well as result in pupation failure.

The issues: Caterpillar pupas should not be exposed to direct sunlight as they don’t have the means to protect themselves from ultraviolet radiation and dehydration. In addition, poor airflow, as well as temperature/humidity swings, can also affect pupation.

What you can do: Among the many causes of pupation failure, weather and temperature are the easiest to control n, as well as temperature and humidity indicators (this is a good one from Amazon). No direct sunlight. Keep temperatures between 70 and 80F with humidity between 65 and 80%.

From Caterpillars to Butterflies

Butterflies are gentle creatures that evolved on earth when dinosaurs existed between 65 million and 135 million years ago. A butterfly develops from a larva, which is called a caterpillar. How does this happen? Let me simplify the process.

First, a butterfly lays eggs. Then, after some weeks, each egg hatches a caterpillar. In short, caterpillars are baby butterflies.

Butterfly Life Cycle - Egg to Chrysalis

When the caterpillars become pupas, they change from a worm-like appearance into flying petals or color fairies – yes, they become butterflies. In that process of change, we receive many questions about things that are going wrong.

However, many of us confuse pupas with cocoons and chrysalises. So let’s clarify six (6) words:

  • Eclode – (verb) 1. A pupa becomes an adult insect; 2. An insect’s egg becomes a larva or caterpillar.
  • Molt: To shed skin or outer covering and take on a new skin or covering.
  • Instar – Each instar is a phase of development from egg to adult butterfly. The first instar hatches from the egg. This turns into the second instar. For instance, Monarch butterflies complete five instars.
  • Pupa – A pupa is also called a chrysalis. This is when the caterpillar (larva) changes into a butterfly.
  • Cocoon – A layer of silk that a moth spins to wrap and protect the chrysalis. Butterflies don’t make cocoons, only moths do. A butterfly only makes a chrysalis.
  • Chrysalis – A hard, green casing that holds the caterpillar soup as it turns into a butterfly. At this stage, the butterfly pupa (caterpillar) is also called a chrysalis.
Pupa, cocoon, chrysalis - what's what in insect development?

Now that’s clear, let’s talk about the good stuff. For instance, although it may seem that caterpillars and butterflies don’t do anything about world hunger, they actually help to feed the world.

And, although they don’t seem to be a part of big stuff like a business, trade, commerce, or profit-making, science, research, culture, and religion, they are, actually.

Why Are Caterpillars & Butterflies Important?

Caterpillars and butterflies are very important to the well-being of the earth, to theoretical and practical sciences, to food chains, to global food production, as well as to culture and religion.

  • Food chain & pest control: At the same time, they’re a critical part of the food chain ecosystem for bats, birds, lizards, spiders, and other animals. Some provide natural pest control and eat aphids while they’re in caterpillar form.
  • Global food production: They’re also important pollinators of flowers and other crops. How important? More than 30 percent of the world’s food production depends on these pollinators.
  • Culture and religion: In Christianity, the butterfly represents a resurrection of the soul. Many cultures around the world consider the butterfly as a symbol of beauty, of change in life, and hope in adversity.
  • Planetary health & science: Butterflies and caterpillars are critical indicators of the health of our planet. They’re also important in science, quantum mechanics, atmospheric prediction, and chaos theory.


How do I know when a caterpillar is about to pupate? A Monarch caterpillar gets ready to pupate by spinning silk, attaching and hanging in a “J” shape (with the head down). After around 24 hours, the caterpillar will straighten a bit (final molt) and their antennas hang down their lip. This indicates that their larval skin will shed in a few minutes.

Should a chrysalis hang? Yes. A butterfly must hang upside down immediately after emerging from its chrysalis as a butterfly. The butterfly needs space to hang and dry their wings so that they can fly.

Is a dark chrysalis alive? When a butterfly is about to come out, the cocoon will turn very dark or very clear. Too dark cocoons may be dead. Try and gently bend the abdominal region. If the cocoon stays bent, the caterpillar is probably dead.

Why is the cocoon bleeding? That red goo is not blood but meconium, the leftover parts of the caterpillar that are not needed to form a butterfly.

Is a black chrysalis dead or alive? The pupa may be dead. However, if you try and can straighten out the abdomen (gently), then it’s still alive.


There are many reasons why butterfly pupas can fail. Here’s a summary:

  • Injury, or Damage: A pupa may be injured by touch. If the injury is at the rear end, the chrysalis may fall or show anal prolapse. If the cocoon is severely damaged, the butterfly won’t form. Euthanasia is recommended to end the suffering.
  • Malnutrition: Lack of food supply (maybe due to overpopulation) can result in pupas that are formed prematurely or poorly anchored, or too weak to molt. Provision of sufficient food supply is recommended to prevent malnutrition.
  • Developmental abnormalities: When pupas develop abnormally or with defects due to colony inbreeding or genetic mutation (e.g., juvenile hormone disorder), euthanasia is recommended to prevent the spread of the mutation.
  • Predators: When Tachinid fly maggots, ants, Chalcid or Trichogramma wasps, and ambush bugs attack butterfly pupas, predator protection is recommended.
  • Pathogens: If pupas fail due to pathogens such as NPV (Nuclear polyhedrosis) infection, OE parasite, or the Pseudomonas bacteria, isolation and extermination are recommended.
  • Chemical, genetic, or organic disruptors: If caterpillar pupas are failing due to pesticides (IGR), herbicides, anti-flea pet medication, GMO milkweed, or Bacillus thuringiensis (BT), identify source points and protect your populations.
  • Meteorological Issues: If caterpillar pupas are failing due to direct sunlight, ultraviolet radiation, dehydration, temperature and/or humidity swings, poor airflow, or disruption of circadian programming, shading, temperature control, misting, ventilation, and shelter adjustments are recommended.

Congratulations! You’ve just completed a thorough review of the causes of pupation failure.

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“Butterfly Evolution” by the American Museum of Natural History

“Butterfly Pea (Clitoria ternatea), a Cyclotide-Bearing Plant With Applications in Agriculture and Medicine” by G. K. Oguis, et al in Frontiers in Plant Science

“Butterflies Choose Plants for Medicinal Qualities” by c. Graber in Scientific American

“When Butterflies Get Bugs: The ABCs of Lepidopteran Disease” by S. M. Altizer & J. de Roode in American Butterflies

“Are Butterflies Two Different Animals in One? The Death And Resurrection Theory” by R. Krulwich in NPR

“Caterpillar Life-cycle” by Wildlife Insight

“3-D Scans Reveal Caterpillars Turning Into Butterflies” by E. Yong in National Geographic

“A Chrysalis Is Not A Cocoon” by Science NetLinks in AAAS

“Rearing Caterpillars” by the Amateur Entomologists’ Society

“Hidden genetic variation yields caterpillar of a different color” by E. Pennisi in Science

“The effects of environmental variation on a mechanism that controls insect body size” by G. Davidowitz, et al in Evolutionary Ecology Research

“Synthesis and transport of juvenile hormones in insects” by S. J. Kramer & J. H. Law in Accounts of Chemical Research

“Circadian regulation of caterpillar feeding and growth” by A. Suszczynska, et al in Journal of Insect Physiology

“Timely plant defenses protect against caterpillar herbivory” by G. Jander in Proceedings of the National Academy of Sciences

“Jasmonic acid-induced plant defenses delay caterpillar developmental resistance to a baculovirus: Slow-growth, high-mortality hypothesis in plant–insect–pathogen interactions” by I. Shikano, et al in Journal of Invertebrate Pathology

“Effects of Foliar-Applied Salicylic Acid, Flax and Caraway Oils on Caterpillar Biometrics on Lima Beans” by H. A. Eldoksch et al in Alexandria Science Exchange Journal

“A role for isothiocyanates in plant resistance against the specialist herbivore Pieris rapae” by A. A. Agrawal & N. S. Kurashige in Journal of Chemical Ecology

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