University of Minnesota

Frequently Asked Questions

Here are the answers to some of the questions we get asked most frequently. Please read through them carefully, and and iff you can't find the answer to your question, you can ask a question of your own to one of our experts!

Monarch Biology
Rearing monarchs
Butterfly Gardening
Natural Enemies and Other Invertebrates

Monarch Biology

What is the function of the gold dots on the chrysalis? Are they really made of gold?

Fred Urquhart first studied the gold spots on monarchs in the 1970s. He felt that the spots were involved in the distribution or formation wing scale coloration. However, the experiments that he did involved cauterizing the gold spots on the pupa, and it is possible that this process may have damaged the underlying tissue and affected the color patterns. Interestingly, all danaine butterflies (monarchs and their relatives) have metallic spots on them. A group of researchers in Germany did a careful study of the properties of these spots. They are not metallic (so they aren't really gold), but the cells reflect light like metals do, giving them the appearance of being metallic. Other danaids have silver, copper, or gold spots.

Here are some hypotheses for the reasons that these butterflies have metallic-looking spots on their pupae:

  1. Camouflage -- they could reflect colors of the surroundings and break up the shape of the pupa; they might also look like dew droplets.
  2. Warning coloration
  3. Filtering particular wavelengths of light which might be harmful to the monarchs
  4. They might not have any function but just be the result of something else in the cuticle of the insect.
How many wings do monarch butterflies have?

Monarchs have four wings. 

What do monarch caterpillars (larvae) eat? Can they eat anything other than milkweed?

In North America, monarch caterpillars rely on milkweed (Asclepias)  and a few closely related genera to grow and develop.  Female monarchs use a series of cues to find milkweed and lay their eggs on the leaves of this plant.  After the egg hatches, the caterpillar feeds on milkweed exclusively, and does not leave the host plant until it is ready to pupate.  Milkweed is known as a “host plant” for monarchs.  An example of a plant closely related to milkweed that is an appropriate monarch host plant is Cynanchum laeve (common names sandvine, honeyvine, bluevine milkweed, and smooth swallow-wort).  It is native to eastern and central U.S. and Ontario, Canada.  

Monarchs have been known to lay eggs on species other than milkweed, such as the invasive swallow-worts (Cynanchum louisea, formerly C. nigrum or Vincetoxicum nigrum) and pale swallow-wort (Cynanchum rossicum, formerly Vincetoxicum rossicum), which are members of the milkweed family (Asclepiadacea) native to Europe.  However, monarch caterpillars cannot feed on these plants and do not survive.

How many legs does a monarch caterpillar have?

Monarch caterpillars have 6 true legs (3 sets) and 10 prolegs or false legs (5 sets).

Does a monarch butterfly have 6 or 4 legs?

All insects, including monarchs, have 6 legs! The front set of legs appear much smaller on the butterfly and are sometimes hard to see, but they are there! 

How big is a monarch egg and what does it look like?

Monarch eggs are about 1.2 mm tall and 0.9 mm wide (about the size of a pinhead). They are an off-white to pale yellow color and have longitudinal ridges. 

How big can monarch caterpillars get? How many times do they shed their skin?

Monarchs go through five caterpillar growth stages, that we call instars. Each time, they shed their skin (molt) allowing them to grow larger. 1st instars are generally 2-6mm, 2nd instars are 6-9mm, 3rd instars are 10-14mm, 4th instars are 13-25mm, and 5th instars are 25-45mm.

How does the color and pattern of the butterfly wings help them to survive?

Monarch adults and larvae have aposematic coloration. This means that the bright colors of either an adult monarch or a monarch larva signal to potential predators that they may be dangerous to eat. The bright and contrasting colors of monarchs warn predators to keep away because they are distasteful or unpleasant.

How long does it take for monarchs to develop from egg to adult?

This process generally takes about one month. Once an egg is laid, it will hatch in about 3-5 days. The caterpillar stage (1st-5th instar) lasts for about 2 weeks. Finally, the pupa or chrysalis stage in monarchs lasts 9-10 days.

Why do new butterflies hang upside down?

Butterflies hang upside-down when they emerge from their chrysalis so that gravity can help them pump the fluid from their abdomen into their wings. This allows the wings to expand and dry so that the monarch can use them to fly!

How many eggs can a female monarch lay in her lifetime?

A female butterfly only lays eggs over a 2-5 week period. During this time, she probably lays an average of 300-400 eggs in the wild, although numbers in captivity are higher - about 500-700 depending on things like temperature and the conditions under which the female is kept. The largest number we've ever observed in our experiments was about 1100! Of course, most monarchs in the wild don't survive the egg and larva stage, but are eaten by predators.

Why does an egg turn black/dark just before it hatches?

Just before a monarch egg hatches, the dark pigmentation of the monarch's head capsule develops. This is visible through the translucent egg shell and gives the egg its characteristic grey dot!

How do you tell male from female in each of the life stages of monarchs?

In adults, male monarchs have a dark spot on each of their hind wings, called androconial pouches. These are thought to be significant in the courtship/mating of some species, but the function in monarchs is unknown. Females do not have these dark spots on their hind wings.

You cannot tell male from female in the egg and larva stages of monarchs.

There is a very tiny line on the female pupa that isn't present on the male. This line is at the top of the pupa, in the abdominal segment second from the cremaster. It's difficult to describe without showing a picture or diagram. You usually need a magnifying glass to see it, unless you have super vision!

How big is an adult monarch?

Monarchs have bodies that are about 10 cm wide (including their wings). Monarchs weigh, on average, about half a gram, this is about the weight of a paperclip. This can vary from about .27 grams (a very small monarch!) to about .75 grams (a very big monarch!).

How many times can a monarch's wings flap in a minute?

About 5 to 12 times a second, so about 300 to 720 times a minute. (This is actually quite slow compared to many other butterflies.)

How do monarchs breathe?

Monarchs breathe through tiny openings on the sides of their bodies called spiracles. (The spiracles are in their cuticle, like our skin). The holes open into a system of tubes in their body (called trachea) that carry the oxygen all over their bodies. They don't have lungs. This is different from the system that we and other mammals have. We breathe air into our lungs. Special cells in our blood pick up the oxygen, and the arteries of our circulatory system carry oxygen to the rest of our body.

Where can I find a monarch pupa in the wild?

Monarchs pupate on many different kinds of plants and other structures. We’ve found them on blades of grass or other plants, spruce trees, park benches, fences, milkweed plants, and garage eaves.  They rarely stay on the plant on which they were feeding, probably to avoid parasitic wasps that can use chewed leaves and frass as cues to locate their hosts.  They can crawl several meters from their host plants (milkweed). We really don't know if there is a "preferred" plant species or other surface for pupating. 

How long do adult butterflies live?

This depends on when they live (summer or winter). It also varies a lot among individuals (just like it does it humans). In the summer, adults live from 2 to 6 weeks in captivity, and probably about that long in the wild. The ones that migrate live longer, from August or September to about April (although a lot die before this). When people hear this, they say they'd rather be a migratory monarchs, but these butterflies probably face many more risks, and are likely to have a smaller chance of getting offspring into the next generation.

Do monarchs live in other parts of the world besides North America?

Yes, monarchs are found in many places throughout the world, but they probably originated in the Americas, and were spread either with the help of humans or on their own to other places. They are found in Australia and New Zealand, and many islands east of these countries (most islands between Australia and Tahiti have monarchs). They are also found in Hawaii, most islands in the Caribbean, and even sometimes in western Europe.

How are monarchs toxic to predators?

Monarchs become toxic to predators by sequestering or storing toxins from the milkweed plants that they eat. Milkweed contains toxins called cardenolides, or cardiac glycosides, which are toxic to predators. This makes monarchs very distasteful or unpleasant to predators. Some predators have evolved ways to avoid or tolerate these toxins, such as the bird predators found in the Mexican overwintering colonies. 

Rearing monarchs

Should I buy monarchs to release in my garden?

We DO NOT recommend purchasing monarchs from commercial butterfly growers to release into the wild. While some breeders may be cautious of disease and test for parasites, many monarch researchers believe that the practice of mass-rearing and selling of monarchs is unlikely to benefit the population, and could actually hurt it. For more information about these concerns, visit Journey North's Raise and Release Safely page.

How much milkweed does it take to feed one caterpillar?

We generally estimate that it takes about one fairly mature milkweed plant to feed a monarch caterpillar. The early instar caterpillars eat very little, but a 5th instar could eat 4-5 large common milkweed leaves. This of course depends on the species of milkweed that the caterpillar is feeding on because some species have much narrower leaves. 

How do I raise monarchs safely in my home or classroom?

See the Monarch Joint Venture fact sheet on Rearing Monarchs

How do I clean or sterilize my rearing containers?

Clean your rearing containers often with a 20% bleach solution. Ideally, soak containers in the bleach solution for ~20 minutes and then rinse thoroughly. This should be done each time a new monarch is introduced to the container, or whenever you suspect signs of disease. 

Clean or remove frass (caterpillar poop) from the container daily and make sure that the milkweed in the container is always fresh. Do not leave milkweed leaves in the container long enough for them to get moldy. 

Can I keep adult butterflies with caterpillars?

No! Adult monarchs can spread OE parasite spores to monarch larvae if they are infected, so it is best to be cautious and never keep adult monarchs in the same cages as your growing larvae. 

What makes a good rearing container?

Monarchs can be raised in a variety of different containers. Mason jars, clear plastic containers, aquariums, plastic shoebox tubs, and wooden/mesh cages can all be good rearing containers. Just make sure that they are somewhat transparent and have adequate ventilation for the monarch(s) to breathe. Containers should NEVER be kept in direct sunlight, as this can be very dangerous and often fatal to the monarch inside. For more information about rearing containers, see the rearing page on our website. 

How many caterpillars can I rear in one container?

Whenever possible, we recommend that you raise only one monarch per container. This helps to prevent the spread of disease, and allows you to track individual monarchs and report your observations to projects like the MLMP ( However, many people choose to rear multiple monarchs per container. How many monarchs to keep in one container will depend on the size of the container. For example, in a 1 quart deli container, we would recommend raising no more than 3-5 monarchs. In a plastic shoebox container, you could safely raise about 10. In any situation, it is important to avoid raising monarchs of different sizes (instars) together. Keeping larger 4th or 5th instars with eggs or 1st instar larvae could cause the larger caterpillars to eat the eggs or outcompete the smaller caterpillars for food. 

What should I do if my monarchs are dying and appear diseased?

There are many things to be cautious of when rearing monarchs, including disease. In general, monarchs in the wild are solitary, meaning that they do not usually come into contact with other monarch caterpillars. This helps to minimize the spread of disease, so when monarchs are raised in close proximity to one another, disease can spread quickly. Signs of disease vary greatly. Some monarchs become pale and lethargic and eat very little, sometimes not at all. Others appear healthy one day and are dead the next day, often turning completely black or brown in color. We've also found that an early sign of disease in monarchs is when a caterpillar appears dark (thicker dark stripes on the caterpillar's body). Any caterpillars with symptoms of disease should be isolated from other monarchs as soon as possible. Do NOT handle any monarchs suspected of being diseased if you are planning to also handle healthy ones. Make sure to properly wash your hands before handling. 

Another symptom to watch out for is if caterpillars appear to be vomiting green liquid. This is a typical symptom of poisoining, and chances are the milkweed your caterpillars are feeding on has been treated with pesticide. If it is not too late, find a new source of milkweed to feed them! 

When is a good time to release a newly emerged adult monarch taking into account weather and the migration?

When monarchs emerge (eclose) from their chrysalis, they will hang with their wings down for several hours as they pump them with fluid from their abdomen and dry off. It is important that they be able to hang like this with plenty of room for their wings to be straight. If the wings do not dry properly the monarch may not be able to fly.

Once the wings are dry the monarch is able to fly - if the monarch is beginning to fly around in its enclosure you are safe to handle it carefully and gently to release it. We usually tell people to wait about 4-5 hours after the adult emerges before touching it. 

People often ask us what to do if the weather is bad and when it's safe to release monarchs or how late is too late for them to join the migration. Here are some general tips:

1. Monarchs cannot fly unless their flight muscles are about 55 degrees F. If the weather is cool, cloudy and either very windy or rainy, it's best to wait for a warmer or sunnier day. If it's cooler and sunny, releasing monarchs should be fine as they will absorb as much heat from the sun as they can. 

2. These temperature and weather concerns are usually only a concern as the migration nears and the weather begins to change near the end of summer (particularly in the Midwest). 

3. If you're wondering when the migration in your area will be starting, look at the maps on the Journey North website for monarch migration, and see what the Monarch Watch website says on estimated peak migration this year. Keep in mind these won't be exact but they'll give you a general idea. 

4. If you have late monarchs (monarchs that have eclosed past the time when most monarchs have already left your area) and you still are looking to release them, wait for a string of warmer days if you can. The monarch has a better chance of making it to Mexico if you release it than if you do not. 

5. If you do need to hold on to an adult for a day or so because conditions are not ideal for release, you may need to feed it. Cut flowers are best if you have them, but a solution of honey or sugar and water will also work. Just mix up 20% honey or sugar with water and soak a cotton ball or clean sponge in it. The monarch may not recognize this as food, but they can learn! If they don't find the food on their own, just gently hold them between two fingers (making sure all 4 wings are secured between them) and place them so their feet touch the cotton ball with the mixture. Sometimes this is enough to get their proboscis to uncurl and begin feeding. If it does not uncurl, you may need to gently take a toothpick and uncurl the proboscis until it touches the food. Then the monarch will know this is food and hopefully begin to drink. If it does not drink it's probably not hungry yet. When it curls it's proboscis back up and begins to move away from the food you'll know it's done. After learning that the cotton ball contains food you shouldn't have to feed them by hand again. Food should be offered every day, but they may not eat.

Is there a way to tell when a chrysalis is about to open?

Sort of! The very last thing to form in this stage is the pigment, which is what you see when the chrysalis "turns dark" as some people say. It's not really "going clear" - the pupa casing has always been clear, but the organism underneath is changing colors.

Once you can see the black and orange pigments of the adult's wings beneath the pupa casing, you'll likely have an adult within the next 24 hours.

Beyond that there are no other tells that a monarch is ready to emerge from the chrysalis. You'll just have to watch it carefully. If you do see it start to crack open, you'll know emergence is immenent. Definitley stick around to watch! Look away and you might miss it!

Does rearing late summer monarchs indoors affect whether or not they will be able to migrate to Mexico?

Great question - it would be best to try to mimic outdoor conditions as much as possible. We don't really know exactly what combination of environmental cues triggers reproductive diapause, and when that response is triggered during development. We do know the cues that trigger it, but can’t say for sure when those cues are most important for inducing diapause.

That said, yes, keeping them indoors where temperatures don't fluctuate and they may experience different night/day regimes from artificial light could potentially impact their ability to go into diapause (prepare for migrating). If you keep them indoors, it is probably better to keep them in ambient light/dark cycles.

Our work on temperature (Goehring and Oberhauser) showed that it temperature does have an effect although the effect of daylength is stronger.  A large day/night difference is more likely to induce diapause.  Temp, milkweed quality, and daylength seem to have additive effects.

What do I do if I run out of milkweed while rearing my caterpillars?

Monarch caterpillars eat a LOT and people are often surprised by how much food one caterpillar can go through in a day. If you do end up running out of milkweed - don't despair! While it's best to have a constant food source available, research has shown that a caterpillar can go 24 hours or so without food and not be negatively affected. This does not mean that you should only feed them every other day - they still need food every day for the best chances of survival.

If you've been collecting from your yard, you'll have to go elsewhere for the milkweed. Check local public places, but be sure it's okay to take plants from there! Sometimes you'll find milkweed along walking paths or public roads, particularly in suburban or semi-rural areas. In Minnesota you cannot take plants from State Forests without a permit (excepting fruits and mushrooms). Check the rules on taking plants with your local DNR or county government. 

Once you find a source for milkweed, you'll want to rinse it thoroughly before feeding it to your larvae. This will increase the chances that it's not covered in pesticides or other harmful chemicals.  

Butterfly Gardening

How do I know which species of milkweed to plant in my area?

See the milkweed fact sheet on the Monarch Joint Venture website ( They have prioritized milkweed species known to be important for monarchs in each region of the U.S. 

Other than milkweed, what are other important features to consider when gardening for butterflies?

The Monarch Joint Venture has a great flyer on "Gardening for Monarchs" that can be downloaded from their website. 

Where can I find milkweed seeds or plants to add to my site?

Check out the Milkweed Seed Finder managed by the Xerces Society for Invertebrate Conservation. 

The Monarch Watch Milkweed Market may also have milkweed plugs available for your area. 

Consult with a local gardening group, such as Master Gardeners, Master Naturalists, or a local Wild Ones chapter for more information about where to find native plants. Native plant nurseries are a good place to start looking for native milkweeds and nectar plants. 

What are some symptoms of diseased milkweed plants, and how do I treat plant disease?

We certainly aren't plant experts, but diseased plants is something that does come up on occasion. Some symptoms of diseased or unhealthy milkweed plants could include unusual spots on the leaves, misshapen or asymmetrical leaves or pods, or sometimes plants can appear wilted or weakened. These things can be caused by a number of different fungi or bacteria, but it is usually a good first step to look for tiny insects on the undersides of the leaves that may be eating the plant (not aphids). If these tiny thrips or mites are present on the undersides of the leaves, you can try to spray the undersides of the leaves with a strong dose of water. If you suspect a fungus or bacteria in your plants, you can try cutting back anything that looks diseased. If this doesn't work, consult with a local garden club or native plant nursery to see what they recommend. 

How and when do I collect milkweed seeds to plant next season?

This depends on the species you are collecting and where you live! First, we recommend only using native milkweed species for monarchs, so make sure to collect only species that are native to your area. You should harvest the entire seed pod when it has split open, but has not disbursed seeds. The seeds inside the pod should be brown in color. You should NOT collect all of the milkweed seed pods if you are collecting them from a natural area. Leaving some milkweed seeds will help milkweeds to spread in that area the following year. 

Once you have collected the pods, you will want to remove the seeds from the pods and the fluff that they are attached to. There are many different methods for doing this, so you can try different things to see what works best for you. We like to split open the pod, keeping one end pinched tightly, then you can slide your finger over the ripened seeds scraping them off into a container. Make sure to label the species, location, and date that you collected them. 

Species native to areas that experience extended periods of freezing temperatures usually require cold stratification, or a period of exposure to cold/freezing temperatures, in order to germinate the following season. Some people prefer to sow the seeds outside in the fall or later winter/early spring to let the seeds undergo this process naturally. Another option is to keep the seeds in an airtight container with just a slight amount of moisture (using a medium such as sand or a damp paper towel) in your refrigerator or freezer. They should remain their for about 3 weeks, but it doesn't hurt them to stay cold longer. When you are ready to plant, remove the seeds allowing them to warm up, plant, water, and watch them grow! 

Can I transplant milkweed from the wild to my garden?

It is possible, but not always easy or successful. It is best to transplant milkweed when plant shoots are very small. Milkweed has a deep tap root and if you cut off too much the transplant is less likely to succeed. If you find small plants that are likely growing from seeds disbursed the year before, these will be much easier to tranplant to your garden! If you do try to transplant milkweed, make sure that you get as much of the taproot as possible, and do not take all of the plants from the site you are transplanting from. 

What host plants can I plant to bring other butterflies to my garden?

Just as monarchs need milkweed, other Lepidoptera (butterflies and moths) species require host plants for development. Some are specialists, meaning that they require a particular type of plant, but others can eat a variety of different plants. Depending on the region that you live in, you can attract different kinds of butterflies by growing the plants that their larvae need to develop, in addition to nectar sources that the adults use. Check out the North American Butterfly Association's Butterfly Garden and Habitat webpage for more information and regional butterfly gardening guides for more ideas about what plants to use in your area.


Why do monarchs migrate and not hibernate?

This is an excellent question! For some reason, monarchs have just not evolved to hibernate. Iit's a hard question to answer, because we need to know the evolutionary history of monarchs. We are quite sure that the ancestors of monarchs were tropical butterflies that could not survive long periods of very cold weather. When monarchs moved into areas that had cold winters, they never evolved the ability to tolerate these winters, and need to migrate to warmer locations. Many people think that monarchs evolved in the tropics, and just move north each spring to take advantage of all the milkweed we have in the summertime. Most other temperate insects can withstand the freezing temperatures of winter by entering a state called "diapause." Some do this as eggs, others as larvae, pupae or adults.

How do they travel such far distances?

They are able to travel such far distances by flying very efficiently. They take advantage of air currents and actually soar, like many birds do. This takes much less energy than flapping their wings all the time.

They choose altitudes at which they can take advantage of the wind to help them on their long migratory flights. And they don't fly when there's a strong wind blowing in the wrong direction.

They also store up a lot of energy for these long trips. This energy comes from the food they eat as caterpillars, and also from the nectar they get from flowers.

How do monarchs know where to go to overwinter?

We don't know for sure. We do know that they don't learn where to go, but instead are genetically programmed to go to the right place at the right time. We also know that they use the sun as a cue to tell them which way is south.

How many miles can monarchs travel in one day?

In general, about 25-30 miles. When weather conditions are favorable, they may be able to go further, but poor weather conditions may also prevent them from traveling at all! Many factors influence how far monarchs can travel in one day, so it is quite variable. 

Can butterflies fly in the rain?

No, at least not for long! 

How far can one monarch migrate?

It depends on where the migrating monarch starts! Monarchs that develop in areas west of the Rocky Mountains usually do not migrate as far as those east of the Rockies. Rather, these monarchs travel a shorter distance to the Pacific coast of California to overwinter in many sites along the coast. 

Monarchs east of the Rocky Mountains can travel up to 3000 miles south to Mexico in order to overwinter. However, only one generation of monarchs undergoes this long journey - those that emerge in late summer and fall. Only those from the northernmost reaches of the breeding grounds travel this far. Others that emerge at lower latitudes during the fall have a shorter distance to travel to reach their overwintering grounds! 

What dangers do they face during the migration?

The long journey of the migration comes with many dangers. They rely on good weather to help them make progress in the right direction; strong winds and rains may prevent them from moving, holding them up for days on end. If winds are too strong in one direction, monarchs may not be able to travel as far, if at all. Another weather factor that can impact monarch travel is temperature. If late season temperatures get too cold, monarchs' flight muscles can't warm up enough, making it physically impossible for them to fly. Extreme weather events can blow monarchs off course, prevent them from moving forward, or kill them. 

Monarchs also require nectar to fuel their flight during the migration. If they are unable to find suitable nectar sources throughout their migratory path, they may die of starvation. They may make it to overwintering grounds, but sometimes they are undernourished when they arrive and could not store enough lipids/fats to survive the winter. It is important that they have access to abundant nectar sources along the way. 

Disease can impact the monarch migration. The OE parasite can have a culling effect, meaning that monarchs infected with the parasite may not be strong enough to complete the migration and die on their way. 

Other anthropogenic factors, like collisions with vehicles, also pose dangers for migrating monarchs.  

Where do they stop along the way?

Monarchs do stop at various locations during their migration. Sometimes they are forced to make pit stops due to adverse weather conditions, but they also stop to rest and refuel at locations with nectar plants available. Monarchs can be sighted on trees in small or large groups as the migration progresses. These roosting sites are where monarchs rest during the migraiton and can be seen throughout the migratory corridors, though it is hard to predict exactly where they can be seen and when they will be seen there. A few locations in the US, like Cape May, New Jersey, and Peninsula Point, Michigan see more consistent monarch migration movements, since monarchs coming from regions north of those locations funnel through a much smaller area. Monarchs funnel through Texas in large numbers as well, so reports of large migratory movements also come from Texas.

You can follow real-time overnight roost sightings on the Journey North website each fall. See this map from the fall migration of 2013:


Do monarchs need to eat during the winter?

Monarchs that migrate to Mexico or the coast of California do not rely on nectar to be available during the overwintering months. They consume as much as possible during their migrations, and actually gain weight along the way. This allows them to store enough lipids, or fat reserves, to survive the winter without eating. They can do this because the cooler temperatures at the overwintering sites allow them to stay relatively inactive during the winter, which helps them to save energy. They do need water during the winter, however, and visit streams or other water sources near the overwintering colonies. 

What kinds of trees do monarchs overwinter on in California?

If you've ever been to the coast of California, you'll have noticed that most of the trees there are eucalyptus trees, and this is the main tree that the monarchs use. They also use pine trees where these are growing near the coast, and just about any other kind of tree that is growing in the right locations.

When do monarchs arrive in Mexico and when do they leave?

Typically, monarchs arrive in Mexico around the same time as the Day of the Dead in early November. They spend the winter in the Mexican overwintering sites and then the colonies start to break up and migrate back towards the U.S. starting in March. 

How many overwintering sites are there in Mexico and California?

This numer varies from one year to the next, usually depending on the size of the monarch population and the quality of the habitat. 

In Mexico, monarchs return to the Monarch Butterfly Biosphere Reserve, a large area of land where the overwintering sites are located. Within this reserve there are typically between 5 and 14 different sties known to be used by monarchs. Not every site is occupied by monarchs every year, but monarchs are able to find the same specific sites each year, even though they have never been there. 

In California, there are many more overwintering sites than are in Mexico. Hundreds of sites along the California coast have historically been the home to overwintering monarchs, but these sites are not as consistent as those in Mexico, nor are they as large. Some of the larger sites, such as Pismo Beach Grove, or Pacific Grove typically have monarchs each year. Some sites in California are considered transitional sites, as they do not have monarchs present for the entire overwintering period. There is more movement between sites in California, whereas this does not occur in Mexico that we know of. In 2013-14, 162 sites were visited and monitored during the overwintering season. 

How many monarchs are in one overwintering colony in Mexico?

Each of the site that monarchs occupy in the Monarch Butterfly Biosphere Reserve are different in size. Scientists measure the size of each colony or overwintering site by measuring the area of the trees/land that is occupied by monarchs. With these measurements, they use an estimate of between 10-50 million monarchs per hectare of land occupied. 

How do they count the butterflies at overwintering sites?

In California, volunteers help to count the butterflies at each location by participating in the Western Monarch Thanksgiving Count. Groups of volunteers go out with a leader and estimate the number of monarchs in each cluster by counting individual butterflies. This is possible because there are many fewer butterflies at the California sites (as opposed to the Mexico sites). 

In Mexico, it would be impossible to count each and every butterfly at a particular overwintering site. Researchers actually measure the area occupied by monarchs and then using this measurement, estimate the number of individuals based on an estimate of between 10 and 50 million monarchs per hectare. 

What are the characteristics of an overwintering site in Mexico?

Monarchs in Mexico overwinter in high elevation oyamel fir forests in the Transvolcanic mountain range of central Mexico. Typically the locations that they find favorable are about 2 miles above sea level, or about 3,000 meters. These sites are typically found on moderately steep south-soutwest facing slopes and are usually in close proximity to a stream or source of water. The temperature at these locations is fairly cool, typically just a bit above freezing. It can get warmer during nice days, but nights remain fairly cool. 

What do monarchs do at the overwintering sites?

Monarchs need to consume enough milkweed as caterpillars and nectar as adults before reaching the overwintering sites so that they do not need to eat to survive while they are overwintering. There is not enough nectar available at the overwintering sites to sustain that many monarchs for the entire winter, but there is typically some nectaring behavior during the winter months. 

Monarchs do need water during the winter, so on warm days they find streams or water from dew or fog/clouds on the mountain. They need moisture so that they don't become dehydrated. 

They remain fairly inactive clustering in trees to conserve energy/lipid reserves, but do fly some on warm days and warm their wings in the sun. Towards the end of the season, monarchs may also begin mating in Mexico as they prepare for the journey northward.  

Predation by birds also happens at overwintering sites, but monarchs cannot do much to avoid this. 

Are there predators in Mexico?

Yes, there are two bird species that can eat monarchs in overwintering sites in Mexico. They are the black-headed grosbeak and the black-backed oriole. There are also about 5 species of mice that prey on the monarchs on the ground. 

Natural Enemies and Other Invertebrates

Are aphids on milkweeds harmful to monarchs?

No, aphids eat the milkweed plant, not monarchs! Ants that may be tending the aphids can sometimes attack small caterpillars on the plant, but the aphids themselves do not harm monarchs. 

How are monarchs toxic to predators?

Monarchs become toxic to predators by sequestering or storing toxins from the milkweed plants that they eat. Milkweed contains toxins called cardenolides, or cardiac glycosides, which are toxic to predators. This makes monarchs very distasteful or unpleasant to predators. Some predators have evolved ways to avoid or tolerate these toxins, such as the bird predators found in the Mexican overwintering colonies. 

What invertebrate predators do monarchs have?

Invertebrate predators such as ants, spiders, and wasps attack monarch larvae on milkweed plants. Wasps have been observed feeding on monarch abdomens at a California overwintering site, and fire ants have been suggested as a major predator of monarch larvae in Texas. Other research suggests that wasp predators may be sensitive to the chemical defenses of monarch larvae, and that wasps fed monarch larvae with high cardenolide concentrations had lower reproductive potential and more deformities in their nests than wasps that preyed upon less toxic caterpillars. Some invertebrate predators, such as ladybug larvae or lacewing larvae prey on monarch eggs. 

Small white worms (maggots) emerged from my monarch pupa, what are they?

These are tachinid fly larvae. Soon after they emerge from the monarch caterpillar or pupa, they will turn to dark brown pupae. An adult tachinid fly lays her eggs on a monarch caterpillar, then the tachinid larvae burrow inside the monarch caterpillar and develop within it. They emerge from late stage monarch caterpillars or early pupae, killing the monarch. 

Are there parasitoids that affect monarchs?

Yes, there are a few. Tachinid flies are the most commonly known parasitoid of monarchs. Adult flies lay fly eggs on monarch caterpillars, which then burrow into the caterpillar and develop inside. After feeding on the growing monarch, they emerge from the large monarch caterpillar or early pupa as larvae and then pupate soon after, killing the monarch. Research in the Monarch Lab suggests that the species Lespesia archippivora (La) is the most important monarch tachinid parasitoid. 

Less is known about the extent to which other parasitoids attack monarchs, but at least one wasp in the family Braconidae has been reported in monarchs. The closely-related queen, Danaus gilippus is parasitized by two Chalcid wasps,Brachymeria annulata and B. ovata, as well as L. archippivora. Current research in the Monarch Lab demonstrates that a wasp in the family Pteromalidae and the same superfamily, Chalcidoidea, as the two Chalcid wasps found in queens, could be an important pupal parasitoid.

Are all milkweeds toxic?

Yes, all milkweed species contain cardenolides, or cardiac glycosides, making them toxic. Species do, however, differ in level of toxicity. For example, common milkweed, Asclepias syriaca,  has a much lower cardenolide concentration than tropical milkweed, Asclepias curassavica


What do you think is the greatest threat to monarchs?

Because the monarchs are concentrated in a very small area in the winter, they are more vulnerable during this stage of their life. However, breeding sites in Canada and the US are also crucial to their survival, and additional losses of these sites also pose threats. I don't think that we should focus so much on what the greatest threat is, but more on what we can do as individuals to permit monarchs and other species to live alongside humans.

Why are monarchs important?

Monarchs are important for many reasons! They may not be as good of pollinators as bees, but they are what we call a flagship species for conservation. This means that monarchs are well-known and very likeable, so people are more likely to get involved in working to protect them. By promoting habitat restoration for monarchs, other pollinators and wildlife species are also benefiting. Milkweed is a great nectar source for pollinators and provides habitat and plant diversity in a number of different landscapes. 

Is non-native tropical milkweed harmful to monarchs?

In some geographic locations, it may be. In parts of the southern U.S. and California where they do not experience freezing winters, tropical milkweed is able to persist year round. Research shows that this may be increasing the prevalence of the OE parasite in monarchs, which can be very harmful for the migratory population. Read more about the risks of planting non-native milkweed for monarchs on this Monarch Joint Venture fact sheet

What is the most important thing I can do for monarchs in the U.S.?

One of the biggest concerns that we have for monarchs in the U.S. is loss of breeding habitat. The most important thing that you can do for monarchs to help create breeding habitat is to plant milkweed! Visit for more information.  

What are the top five things I can do to help monarchs?

It's hard to pick a top 5, but here are some things that you can do (in no particular order): 

  • Plant native milkweeds and nectar plants
  • Avoid pesticides to minimize impact on pollinators
  • Support monarch conservation organizations like the Monarch Lab, Monarch Joint Venture, and Monarch Butterfly Fund, among others
  • Become a citizen scientist volunteer and monitor monarchs in your area
  • Create an outdoor learning space or garden to educate youth about pollinators and conservation
  • Advocate for monarch/pollinator conservation by reaching out to politicians in your area
  • Educate others by providing outreach programs or reaching out to local media

Help spread the word! 

Why is the monarch population declining?

The monarch population decline is a problem with many causes, as commonly stated by Dr. Karen Oberhauser. There are many factors that can influence the population. One of the major concerns is habitat loss in breeding, migrating, and overwintering areas. In the U.S. and Canada we are concerned about the loss of quality breeding habitat due to development, agricultural expansion, and the increasing use of herbicide tolerant crops (which have essentially eliminated milkweeds from the agricultural landscape where they were once very abundant). Additional habitat loss can be attributed to land management practices, such as increased pesticide use and mowing to control unwanted species. Logging in Mexico is still somewhat of a concern, although the problem was reduced when a decree was set in place to protect the Monarch Butterfly Biosphere Reserve. Other concerns in overwintering colonies include diversion of water for agriculture, predation, and to some extent, tourism (high traffic) at the overwintering sites. 

Monarchs also face risks from decreasing migratory habitat; migratory habitat means abundant nectar and resting spaces available for spring and fall migrations. Climate change may be impacting monarchs, though less has been studied regarding this. In addition, monarchs face risks from natural enemies and disease and potential risks from non-native plant species like tropical milkweed and swallow-wort. Tropical milkweed, if grown in locations in the southern US and California, may persist year round. Research has shown that this is increasing the prevalence of the OE parasite in monarchs. Black and pale swallow-worts can act as a sink for monarchs, because females may lay eggs on the plant thinking that it is milkweed, but their offspring cannot feed on it and will die.  

Will the monarch population recover?

We hope so, but it is hard to predict how a population will resond to environmental changes, especially when there are so many factors to take into account. By providing suitable breeding and migratory habitat in the U.S. and Canada, and continuing to protect and restore overwintering sites in Mexico and California, we hope that monarchs will recover from the record-low population numbers we have been seeing recently. 

Are monarchs endangered?

No, monarchs are not listed as an endangered species. Their migration, however, was listed as an endangered phenomenon by the International Union for Conservation of Nature in 1983. This does not mean that monarchs are not in dire need of protection. It is important that we take action now to help the monarch migration continue for future generations to enjoy. 

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