Finding Milkweed

Monarchs travel hundreds of miles across the landscape in search of the milkweed they need for egg-laying. How do they find it?

Image of a faded female laying eggs on newly emerged milkweed. — “A faded female laying eggs on newly emerged milkweed - A. syriaca and A. incarnata.” Sighted April 27, 2017 in Lititz, PA by Kim Frey.

Waiting for the Surge

The lull between generations continues. The first half of May is always the slowest time of year, and this year is no exception.

Observers are seeing the last of the monarchs from Mexico and their physical descriptions are increasingly dire. These “torn, tattered, dull, faded, pale” butterflies may be 9 months old!

Hot and Cold

Monarchs responded to this week’s temperature extremes:

First sightings were reported as far north as New York City during a heat wave in the eastern U.S., where northern regions were as warm as Texas.
The migration’s northward advance came to a halt in the Midwest where freezing temperatures and snow exposed monarchs to the risks of frost.

Finding Milkweed

Monarchs have an uncanny ability to find milkweed. This week they found some under a deck, beside a pool, in potted plants, wildlife parks, backyards and schoolyards, on milkweed that was just emerging and on plants in full bloom.

Milkweed isn’t easy for us to find, so how do monarchs do it?

Image of First monarch butterfly sighting for New York

First for New York

Hot and Cold

Risk of Frost

When a monarch emerges from its chrysalis, it's born with vivid orange wings in pristine condition.

Monarch Wings Over Time

How Do Monarchs Find Milkweed?

Monarchs find milkweed in places and ways that astonish observers. Kathryn Cole could not believe her eyes. “Milkweed just broke the ground yesterday and grew 3 inches in one day. We began checking for eggs and, sure enough, a monarch had already deposited her eggs on the newly emerging plants.”

An Everyday Miracle
Monarchs find milkweed in places and ways that astonish observers. Kathryn Cole could not believe her eyes. “Milkweed just broke the ground yesterday and grew 3 inches in one day. We began checking for eggs and, sure enough, a monarch had already deposited her eggs on the newly emerging plants.”

When monarchs arrive from Mexico in the early spring, milkweed is beginning to emerge. Female monarchs must find milkweed quickly. They are at the end of their lives and have eggs to lay. Milkweed is the only food monarch larvae will eat, so finding it is of the utmost importance.

An Urgent Hunt
When monarchs arrive from Mexico in the early spring, milkweed is beginning to emerge. Female monarchs must find milkweed quickly. They are at the end of their lives and have eggs to lay. Milkweed is the only food monarch larvae will eat, so finding it is of the utmost importance.

Dr. Bill Calvert describes what he observes in the field: “The female flits over the field and, when she stops on a plant, she drums the surface with her forelegs.” The forelegs are tiny, close to the head, and difficult to see. How do the forelegs help the female find milkweed?

A Touch of Milkweed
Dr. Bill Calvert describes what he observes in the field: “The female flits over the field and, when she stops on a plant, she drums the surface with her forelegs.” The forelegs are tiny, close to the head, and difficult to see. How do the forelegs help the female find milkweed?

“Monarchs use a combination of visual and chemical cues to find milkweed,” says monarch scientist Dr. Karen Oberhauser. “Once they land on a plant, they use sensory organs on their feet and heads to tell them if it is a milkweed, and probably the quality of the milkweed.”

A Combination of Senses
“Monarchs use a combination of visual and chemical cues to find milkweed,” says monarch scientist Dr. Karen Oberhauser. “Once they land on a plant, they use sensory organs on their feet and heads to tell them if it is a milkweed, and probably the quality of the milkweed.”

It’s difficult for scientists to study monarch’s senses because an insect’s senses are so different from ours. Yet even our own senses are mysterious. When we smell and taste we are actually sensing chemicals in our environment. People and monarchs do this in a similar way.

A Look at Senses
It’s difficult for scientists to study monarch’s senses because an insect’s senses are so different from ours. Yet even our own senses are mysterious. When we smell and taste we are actually sensing chemicals in our environment. People and monarchs do this in a similar way.

People and monarchs sense chemicals in the environment with special structures called chemoreceptors. They come in different shapes and sizes, but they work in the same way. When a chemical fits a receptor, the way a key fits a lock, the brain identifies the chemical.

Chemoreceptors (“KEE mo ree CEP terz”)
People and monarchs sense chemicals in the environment with special structures called chemoreceptors. They come in different shapes and sizes, but they work in the same way. When a chemical fits a receptor, the way a key fits a lock, the brain identifies the chemical.

Monarchs use their antennae to detect plant chemicals. The antennae are covered with chemoreceptors, especially at the tips.

Antennae Have Chemoreceptors
Monarchs use their antennae to detect plant chemicals. The antennae are covered with chemoreceptors, especially at the tips.

All six of the female monarch’s legs have chemoreceptors. Here you can see the midlegs. Look closely and you’ll see the sharp spines monarchs can use to cut the plant and release the chemicals that tell the monarchs it’s milkweed.

Legs Have Chemoreceptors
All six of the female monarch’s legs have chemoreceptors. Here you can see the midlegs. Look closely and you’ll see the sharp spines monarchs can use to cut the plant and release the chemicals that tell the monarchs it’s milkweed.

Eyes sense light, not chemicals. Monarchs have compound eyes with thousands of lenses. They see thousands of single images at the same time. Monarchs can see in all directions at the same time, too. Monarchs do not see the world in a single image the way we do. The monarch’s vision is specialized for seeing colors, direction and movement.

Eyes
Eyes sense light, not chemicals. Monarchs have compound eyes with thousands of lenses. They see thousands of single images at the same time. Monarchs can see in all directions at the same time, too. Monarchs do not see the world in a single image the way we do. The monarch’s vision is specialized for seeing colors, direction and movement.

Some plant species grow together in groups called ‘guilds.’ Together, the plants defend themselves by producing a mixture of plant odors. Herbivores, like monarchs, can’t find the correct host because there are too many confusing odors. This would explain why monarchs need to use a combination of senses to find milkweed.

Confusing the Senses
Some plant species grow together in groups called ‘guilds.’ Together, the plants defend themselves by producing a mixture of plant odors. Herbivores, like monarchs, can’t find the correct host because there are too many confusing odors. This would explain why monarchs need to use a combination of senses to find milkweed.

Picture a monarch floating through the air, following invisible scents to find the milkweed it urgently needs. Watch how a butterfly can locate milkweed that’s hidden in a sea of green plants.

Finding Milkweed
Picture a monarch floating through the air, following invisible scents to find the milkweed it urgently needs. Watch how a butterfly can locate milkweed that’s hidden in a sea of green plants.