Nature’s notebook

The mystery of the missing garden cats

Black, yellow and white striped monarch caterpillars with aphids on swamp milkweed. Photo: Ruth Ann Grissom

As I left town for a quick trip over Labor Day weekend, a dozen hungry monarch caterpillars were munching away on my swamp milkweed. When I returned 32 hours later, they had vanished. Many stems had been denuded, so the cats had apparently eaten a lot of leaves before they disappeared. Most had been fifth instar, the final larval stage before they pupate. I searched the surrounding plants for a chrysalis. Nothing on the swamp rose, the swamp sunflower, the native hibiscus. No sign of a jade green, jewel-encrusted cone. The foliage is dense, I told myself.  They’re camouflaged. I tried to remain hopeful, but a chance encounter the previous week unnerved me.

The European paper wasp. Photo: Ruth Ann Grissom

When the first monarch of the season fluttered into my garden in late July, I monitored its behavior. It seemed to favor the Joe Pye weed for nectar, and it clearly preferred swamp milkweed to orange butterfly weed as a host plant. I inspected the swamp milkweed daily, searching for tattered leaves and eggs. Instead, I found legions of oleander aphids.

Milkweed is vulnerable to these nonnative pests. They suck the sap like tiny yellow vampires then secrete a sticky substance known as honeydew which enables the growth of a sooty black mold. The milkweed suffers aesthetically, but an otherwise healthy plant can withstand all but the worst infestations.

Still, I wondered if aphids were problematic for monarch larvae or eggs. Online, I found other monarch aficionados who shared my concern. Someone claimed aphids eat the eggs, but since they’re herbivores, that seemed unlikely. (I worried more about the ants and ladybugs attracted to the aphids, but that’s another story.) Another source asserted that monarchs won’t deposit eggs on an aphid-covered leaf. That seemed plausible, but aphids tend to congregate only on the young and tender growth. 

Suggestions for dealing with a plague of aphids ranged from wacky to sensible, but most involved some risk to the monarchs or an inordinate amount of effort. So I fretted. I demurred. And in the days that followed, I spotted the monarch’s perfect, pearly eggs amid a smattering of yellow aphids.

I plucked a chosen leaf and studied it through a magnifying glass. The aphids prowled the mid-vein, indifferent to the egg. I convinced myself the concerns about aphids were unfounded. I kept an eye on my swamp milkweed, inspecting it multiple times each day. Soon, tiny black, white and yellow caterpillars were feeding on the aphid-ridden leaves. All seemed right with the world.

Until, that is, I saw a milkweed bug standing over the carcass of a monarch cat. Part of its body had decomposed, turning the color and texture of tar. Unlike those aphids, milkweed bugs are native, but they also feed on plants. The respected website, Monarch Watch, lists ambush bugs and stink bugs as possible monarch predators. I debated whether the insect in question could be the similar-looking milkweed assassin bug, but it didn’t quite match the description.  My plants have always hosted scores of milkweed bugs, and I’ve never seen evidence of predation, but how to explain one on caterpillar remains?  Coincidence, I thought. I wasn’t looking for trouble in my garden. 

But trouble found me there. A day or two later, I happened on a grisly scene. A strange and hateful wasp was attacking one of my cats! They were tangled on the ground, the wasp stabbing, the caterpillar writhing in pain. I resisted the urge to shoo away the wasp until I memorized its markings – a cross between a yellowjacket and a paper wasp – then I dashed inside to my computer.

Native paper wasps (Polistes fuscatus) are common across the Piedmont, responsible for the familiar gray, honeycombed nests often placed under a deck or the eaves of a house. They are also notorious predators of caterpillars. This is a good thing if the cats in question are hornworms or cabbage loopers devastating your vegetable garden, but it’s a disaster if you’re growing monarch butterflies. The species that attacked my cat, the European paper wasp (Polistes dominula), is even worse.

Milkweed bug on monarch caterpillar carcass.  Photo: Ruth Ann Grissom

First documented in the Boston area in the late 1970s, the EPW has since spread across much of the U.S. According to Donald Lewis, entomologist at Iowa State University, it has completely displaced native wasps in some areas. He told me numerous reports indicate the EPW is primarily “an urban pest.” He also cited Colorado State University entomologist Whitney Cranshaw as saying the EPW is “devastating to essentially all species of yard and garden Lepidoptera [butterflies and moths].”  

“Far too often,” Cranshaw writes, “I have seen butterfly larvae enticed to develop in my garden get turned into bug burger by Polistes dominula, and I can't take it anymore; breaks my heart.” I fear the cats on my swamp milkweed suffered this fate over Labor Day weekend. According to Soni Holladay, a horticulturalist with the Houston Museum of Natural Science, “Once a wasp finds a host plant with caterpillars, she will come back regularly to check for more.”

This presents a harsh dilemma for urban gardeners committed to providing milkweed and nectar sources for the monarch. Are our efforts, on balance, helping monarch populations or are we luring them into a sink habitat that will ultimately lead to further declines? If we attract monarchs to our yard, should we also take steps to deter the wasps by placing mesh or screen over milkweed plants after eggs are laid? 

If butterfly populations in rural areas are less affected by European paper wasp predation, this underscores the urgent need for more land conservation and better habitat management in resilient ecosystems like the Uwharries and other natural areas in North Carolina.