What Happened to my Monarch Caterpillars? by Val Frankoski

You had a bunch…but now they are gone? What happened?

With strong winds from the south, Monarch butterflies appeared in the Joplin area in mid-April, with only a few weeks to live, and females desperate to unload their eggs. People with milkweed in their yards were ecstatic and reported multiple eggs on many of their plants, and later, a bunch of small caterpillars as eggs hatched. Small holes appeared on leaves as caterpillars chowed down on the ONLY food they will eat. (And we thought our own kids were picky eaters!)

Today, many who earlier were delighted to see nature happening up close are sad to report their caterpillars as missing in action! While each case may differ, there can be “negative consequences” to both early arrival, and dependence on a toxic food source!

First of all, mid-April milkweed plants in our area are still quite small. They do grow relatively fast, and young plants are more tender and preferred by monarchs, but many eggs on one plant is considered “egg dumping” and presents potential problems.  

Later into May, plants are larger, and monarch females normally lay only one egg on a plant and avoid plants where other eggs are already in place. The reason for this is because of the tendency for the first-emerging caterpillar to eat his/her own eggshell, and then chow down on other available eggs. Never fear…once hatched, caterpillars are no longer subject to this cannibalism.

When plants are small and there are multiple eggs on each leaf, the opportunity for this egg-eating behavior increases, and probably improves chances of survival for the eater. The egg-case and any other eggs eaten are able to supply a newly hatched caterpillar with a decent first meal and extra energy to attack the milkweed leaf.

Yes, attack! Energy as well as strategy is needed to tackle a milkweed plant, because the plant attempts to defend itself from being eaten. First, a “lawn” of hairs or a waxy layer must be mowed or scraped…the first layer of defense. Next caterpillars must deal with the latex “milk” which has no nutritional value. Latex is under pressure, and one bite into the leaf presents a flow of latex that is like drinking from a fire hydrant! Many newly-hatched (estimated at 60%) are engulfed by this sticky goo and die. Others are able to release the pressure, wipe off their faces, and come back to consume the leaf-section drained of this defense.

Currently unstudied is the ability of a caterpillar in this stressed state to absorb milkweed plant toxins without ill effects. Certain milkweeds have much greater levels of the toxins the monarch needs for protection from bird and mammal predators. Most newly hatched caterpillars will die attempting to consume a plant with an overly toxic “cocktail” of chemicals.

Another consequence of early egg-laying is the possibility of late frost or low temperatures. Larval development is slowed due to lower temperatures giving insect predators more time to rustle up a tasty meal of monarch eggs!

Under the best of circumstances, only 1 or 2 out of 100 monarch eggs survive to become a butterfly. The first day of a caterpillar’s life is often its last, and eggs laid in Missouri in mid-April have more challenges than those arriving in May. Do not despair! If you plant milkweed, the Monarch butterflies are sure to come! Watch and learn.


Posted on May 4, 2017 .