Three monarch caterpillars feed on milkweed plant.

Monarch Caterpillar Predators | Beneficial Insects Aren’t Always Beneficial

If you haven’t seen it, you might be interested in watching our video chronicling the life cycle of the monarch– from caterpillar to chrysalis to butterfly.  We also went down to the Saint Marks National Wildlife Refuge last fall for a little monarch tagging, and to learn about their Monarch Milkweed Initiative.

Rob Diaz de Villegas WFSU Media

Our second monarch raising season started much earlier this year.  At this point last year, we had just released our first six monarchs; as of Monday, we have released twenty six (we released one on August 21, just after the eclipse).  

We also have a handful of chysalides and a few caterpillars working on what’s left of our milkweed.  It’s growing back after twice having been mostly decimated, and even so, I spotted eggs earlier this week (which, since I started this draft, have hatched).

Over the last four months, we’ve had some successes and some learning experiences.  The learning experiences came when we had too many caterpillars to bring in and protect.  I learned last year that if you bring too many in, smaller caterpillars get crowded out and don’t eat.  So I’ve been trying to only bring in larger caterpillars I feel are going to soon spin their chrysalides.  This means that I’ve been leaving more of them to deal with the many predators of the backyard garden ecosystem.

Now, there’s not much I can do about lizards and, as I saw with one of my black eastern swallowtail caterpillars, wasps can fly in at any time and pick them off.  I do routinely brush spider webs off of my milkweed plants, and try to move spiders elsewhere.

As for the predators I can control?  Some of those were kind of surprising.

Beneficial Insects Aren’t Always Beneficial

The more I spend time growing milkweed and observing monarch caterpillars, the more I notice the many other insects that associate with the plant.  And that’s the cool thing about observing any one plant for long enough- it’s an ecosystem unto itself.

The one insect I see the most are little orange pests called milkweed, or oleander, aphids (Aphis nerii).  They have no direct effect on monarchs, though they can damage milkweed by ingesting its sap and weakening the plant.  Last year, they weren’t much of a problem.  This year, they had been much thicker on the plants.  Before I gave any thought to managing them, nature stepped in.  At the time, I thought the results were kind of amazing.

White swamp milkweed with a milkweed aphid infestation.

White swamp milkweed (Asclepias perennis) with a milkweed aphid infestation.

These great beneficial insect species started showing up and eating the aphids.  My favorites are the syrphid larvae.  The syrphid fly is a bee mimic.  Last year, I saw one of its larva on a milkweed plant and looked it up to see what it was.  This year, I was pleased to see many more feasting on the throng of little orange pests.

Syrphid larva eating a milkweed aphid on a milkweed plant.

Syrphid larva eating a milkweed aphid on a milkweed plant.

I also started seeing more ladybugs, always a welcome sight in a garden.

A ladybug stalks milkweed aphids.

A ladybug stalks milkweed aphids.

By mid June, I was fairly pleased with our milkweed/ monarch situation.  We had a dozen or two caterpillars, and the garden ecosystem was checking our pests.  But then, we left town over one weekend.  During the one night we were gone, many of our caterpillars vanished.  We didn’t have many large caterpillars ready to make chrysalides, and there was plenty of milkweed left to eat.  Something bad happened to them.

I gathered up the survivors and brought them in.  Then I found one of them dead in the enclosure.  Another caterpillar was sitting on the netting, with a stowaway syrphid larva sitting a few inches away.  That’s when I turned to Google for some additional information.

To Milkweed, a Monarch Caterpillar is a Pest

We like insects like ladybugs because they kill garden pests.  Those pests tear up our food plants and pretty flowers.  But those ladybugs don’t know that we planted some of those flowers so that insects would eat them.

It was quite a surprise.  Ladybugs kill monarch caterpillars.  I couldn’t find anything about syrphids eating monarchs, but it makes sense that they would.  They eat insects that eat milkweed.

Now I clean milkweed aphids off of the plants.  They squish easily and will yellow up your fingers, so I use a wet paper towel.  Since I started doing this, I haven’t seen any syrphid larvae or ladybugs on our milkweed.  And our caterpillars have done much better overall.

Other Monarch Predators

There are a couple of other predators that you can manage.  One is this guy:

Milkweed bug on white swamp milkweed.

Different articles about milkweed bugs have different bits of information regarding monarch caterpillars.  They are not specifically monarch predators; instead, they eat leaves and especially milkweed seed pods.  Reading a little more, I found that they will indiscriminately eat monarch eggs and small caterpillars along with the leaves.  We get rid of these guys.

The final monarch caterpillar predator I’ll mention is the most surprising- the monarch caterpillar.

Monarch eggs on tropical milkweed (Asclepias curassavica).

Monarch eggs on tropical milkweed (Asclepias curassavica).

Large caterpillars will cannibalize smaller ones.  And, they too will eat an egg on the leaf they’re grazing.

If I Was a Scientist…

Leaving more of the caterpillars outside this year has been educational.  Observing them awakens a scientific interest within me, and I kind of wish I had more time to observe them and make notes.

For instance, as in the photo above, I see that some caterpillars lay eggs over top of the leaves (or even on flowers), instead of underneath where they are less visible.  I would think that the more visible eggs were more vulnerable to predation.  But how would I test that?

I’ve also noticed that caterpillars I bring inside seem to grow faster than their counterparts on the outside.  Sometimes, a plant has a few caterpillars of similar size, and so I lighten its load by bringing one or two in.  Do the inside caterpillars grow faster because they aren’t hindered by a fear of predators?

I remember following Dr. David Kimbro and Dr. Randall Hughes’s oyster reef research.  Their caged oyster spat tiles excluded predators; the exposed tiles allowed predators access.  In some experiments, a set of cages were exposed to predator cues, and some weren’t.  When predators are near, oysters stop eating to make themselves less conspicuous, even when safely in a cage.  This can slow their growth.

If I left an enclosure outside, would caterpillars still sense their predators?  Would they grow more slowly than the caterpillars inside, where predators are completely absent?

And, like Randall and David, I would need a control cage with an opening to allow predators access, to determine if being in the cage is what was affecting results.  With unlimited time, space, and resources, I could go crazy placing monarchs in different enclosures and settings, and observing all sorts of different behaviors.  Maybe when Max and Xavi are older, I can push that as a science project idea.

Of course, for the results to be meaningful, we’d have to repeat them multiple times over a course of years…

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About Rob Diaz de Villegas

Rob Diaz de Villegas is a senior producer for WFSU-TV, covering environment and the outdoors. Rob is in the process of completing Roaming the Red Hills, an exploration of north Florida/ south Georgia ecology funded by Tall Timbers Research Station and Land Conservancy. Rob’s previous ecology projects include EcoShakespeare, which was funded by PBS member station WNET and the National Endowment for the Humanities, and In the Grass, On the Reef, a collaboration with the Florida State University Coastal and Marine Lab funded by the National Science Foundation. Rob’s EcoAdventure segments air on WFSU’s Local Routes and can be found on the WFSU Ecology Blog.