The Location of Eggs on Milkweed Plants

Where do we find Monarch eggs on milkweed plants?  Why do females lay their eggs in certain locations?  These are important questions.

Monarch eggs are always laid on milkweed plants.  The reason has been known for decades.  All species of milkweeds have toxins in their sap, known as cardenolides.  Although these are a bit toxic to the caterpillar itself, feeding on milkweed has great advantages--the caterpillar (and then the butterfly) becomes toxic and distasteful.  As a warning to predators "do not eat me," both caterpillar and butterfly have highly visible warning coloration.  Most (but not all) predators quickly learn to avoid these distasteful morsels.

Usually a female lays only one egg per plant, probably because there's more food for the caterpillar if he's alone on a plant.  And small caterpillars sometimes eat one another or eat unhatched eggs.  But occasionally you can find more than one egg per plant, presumably due to several females randomly visiting the same plant.  And I have seen the same female laying multiple eggs on a plant.

Location of eggs on the plant

Eggs are nearly always laid on the undersurface of a leaf.  This provides shelter from the sun--tiny eggs in the sun on a hot day could quickly heat to lethal temperatures.  The downside also shelters an egg from rain drops--giant wrecking balls in comparison to tiny eggs.  And eggs under leaves are much less visible to visual predators like birds.

I have long noticed that when looking for eggs, you tend to find them on the same part of a plant.  Usually, they are on the top third of the milkweed.  The advantages: easier for the female to reach, and probably more tender.  Occasionally you can find an egg on the main stem or on parts of a flower, such as the bud or flower stem.  A possible advantage of laying on flowers is that flowers are very complex in all their minute surfaces, so it would be hard for predators to find an egg there.

Given the very high predation on eggs, there must be tremendous selective pressure for females to lay eggs in the best possible place.  And by place, I don't mean just location on the plant.  Where the plant itself is located may also be important.

Location of eggs in the landscape

I've noticed that eggs are often found on plants located along borders, such as a lake shore or edge of a prairie.  This might not have specific advantages--it could just be caused by the channeling effect of  borders.  For example, a butterfly is flying along and comes to a lake.  It avoids crossing the lake and so ends up flying along the shore.  If it finds a milkweed there, it deposits an egg.  Other animals like indigo buntings are specifically attracted to borders between forests and fields.

There are always lots of monarch adults in a small prairie near my home--just east of the Odana Golf Course.  But when I capture butterflies there, most of them are male.  It's likely that males hang out here because there are lots of flowers that may attract passing females.  It's like a pick-up place.  You can find eggs in this prairie, but there are good reasons why females may not linger here to lay eggs. 

For one thing, many of the milkweeds here are infested with aphids--and aphids attract (and are tended by) ants.  Ants are predators that may eat monarch eggs.  I don't find eggs on plants infested with aphids.  I suspect female monarchs can smell the ants or aphids and avoid those plants.

There's another reason to avoid laying in a place like Odana with a concentration of milkweeds:  Paper wasps are predators of caterpillars.  It would be easier for them to search through a large number of milkweeds in once place, than to find an isolated milkweed plant with a single caterpillar.

So, I suspect that once female monarchs mate, they disperse across the landscape to seek out more isolated small patches of milkweed, or even isolated plants.  It's as if the predators (spiders, beetles, wasps) and the prey (monarchs) are engaged in a complex mathematical game, each trying to increase their odds of success by selecting just the right spot on the landscape for their activities.

Mammals produce a few young, then actively protect them from predators.  Insects have an entirely different strategy, producing hundreds or thousands of eggs and then abandoning them to chance.  But monarchs seem to improve the odds to by choosing the best place for their eggs.  It's a kind of parental care before the young arrive.

Laying style of individual females

When looking for eggs, I've often noticed that you tend to find eggs on the same part of a milkweed plant.  Usually they are on the top third of the plant.  But sometimes you find an egg on the bottom third, or even the very top, the growing shoot.  You might find several eggs in the same area but on different plants--but all in the same location of the plant.  I interpret this pattern as "laying style."  I hypothesize that each female has her own instinctive or learned preference for where to lay.

If different females lay in different places, it's more difficult for a predator to find eggs.  They can't learn to always look in the same place.

Yesterday, Ana Bernal and I were looking for eggs to start a breeding colony at Aldo Leopold Nature Center.  Within 20 minutes, she quickly found three eggs, each near the top of the plant on the third tier of leaves.  In the photo, you can see the eggs are even on the same right side of each leaf's underside.

My interpretation: All three eggs found in the same part of the prairie, along a border, on the same part of the plant (even the same right side): they must have been laid by the same female.  Given that it's early in the season, there aren't likely to be many females around.

Implications for breeding butterflies in captivity

We were collecting eggs so the nature center can start breeding butterflies.  A huge criticism of captive breeding of any animal, over generations, is that inbreeding results.  Harmful traits come to the surface.  So we want to ensure we have as much genetic diversity as possible.

If we raise these three eggs to maturity, get a male and a female, and mate them, then we are mating a brother and sister.  A very bad practice--leading to birth defects and reduced survivability of any butterflies we eventually release. 

To avoid this outcome, we need to take eggs from two different locations separated by at least 10 miles and raise the caterpillars in two separate batches.  Presumably, these two batches represent different females.  When the butterflies emerge, we give them numbers as individuals, but included in the number is an "A" or "B," indicating which genetic line (location) they came from.  For breeding, we will mate A males with B females (and vice versa).