Respect Your Alders
Nature is full of hidden worlds, waiting to be discovered. Some are visible to the naked eye—like the lichen clinging to a rock or the bark of a tree. Others, too small or just out of sight, require special tools to be seen. Still, the influence of these seemingly small or invisible realms is big: trees thrive through partnerships with mycorrhizal fungi—the mushrooms we see at the surface signaling a vast, underground network—a world beneath the soil.
This is a story about one of these hidden worlds. It’s a compact world that I was happily immersed in for two full days in July… my life will never be the same. And it’s not over—this same drama will play out again (can you believe it!) in late August and September. Keep your eyes open.
Ready Player #1: Woolly Alder Aphids (Prociphilus tessellatus)
Woolly alder aphids are the central characters here (and I already know—no one likes aphids in their gardens). But this story might just shift the way you think about them—probably not, but maybe.
“…aphids have to process huge amounts of sap to survive, which means they also excrete a lot of excess liquid.”
Woolly aphids are found on alders—a shrub generally found in wetlands.
At the risk of oversimplifying their life cycle: when you see these insects, they’re likely busy sucking juices from alder stems and leaves. Why? Sucking sap is how they get the sugars and water they need for growth and reproduction. Fortunately for them, alder sap flows all summer long.
But there’s a catch: alder sap is high in sugar but low in nitrogen and other key nutrients. So, aphids have to process huge amounts of sap to survive, which means they also excrete a lot of excess liquid.
They rid themselves of this surplus in two ways. First, they produce a waxy, cotton-like coating during each molt that covers their bodies. The fuzz not only gives them their “woolly” look (that they think makes them look cool)—but it also helps protect them from predators and environmental stress. (And, conveniently for me, it makes them easy to spot.)
Second, they excrete the extra sugars in droplets called honeydew—a sweet, sticky liquid that’s irresistible to many other insects.
Player #2: Ants (The Protectors)
Ants… I don’t know about you, but I can’t stand still if I feel a single ant on me—forget dozens of crawling all over me. Not with the tolerance woolly aphids seem to have—but then again, I don’t have a symbiotic relationship with ants.
If you stumble upon ants swarming woolly aphids, it may look like they’re just chasing a sugar high, but there’s more going on here. Ants actively defend aphids from predators—a behavior known as “tending to.” From the outside, this looks like ants patrolling the aphids like bodyguards and occasionally tapping them gently with their little ant antennas. And that’s essentially what’s happening.
But here’s the trick: those little antenna taps are believed to stimulate the aphids to release honeydew—it’s more of a belly rub to encourage the production of honeydew, rather than a gut punch to get the bowels moving so to speak.
So, the ants get a sugar reward, and the aphids get bodyguards. But there’s one predator ants can’t fend off…
Player #3: Harvester Butterfly (The Predator)
When you hear “butterfly,” you might picture a delicate flying insect—big colorful wings, long proboscis and all—effortlessly dancing from flower to flower. But not all butterflies feed this way. Some drink sap or salt (some even feed on scat).
And there is one adult butterfly in North America that has a proboscis so short (don’t make fun!) that it’s functionally useless for retrieving nectar from flowers. So instead, it feeds exclusively on aphid honeydew. That butterfly is called the harvester (Feniseca tarquinius).
The harvester is the only butterfly in North America whose caterpillars are carnivorous. The larvae feed directly on woolly aphids, making them the only predatory butterfly caterpillar on the continent. The species belongs to the subfamily Miletinae, which includes around 50 mostly predatory species worldwide, but the harvester is the only one found here.
“The larvae feed directly on woolly aphids, making them the only predatory butterfly caterpillar on the continent.”
Player #4: Alders
The native shrub. Alders are important here, they are key to the story! The alders are what get sucked! And in Midcoast Maine, there’s no shortage of places to find Speckled alder (Alnus incana). Whether along stream banks, pond edges, or damp thickets, they are there for the finding!
Speckled alders enrich the soil around them which helps other plants grow (an entirely other story of an entirely different hidden world). Unfortunately for them, this enriched nitrogen also makes their nutrient-rich sap appealing to aphids—as we’ve seen.
Player #5: Me
The observer and briefly the intruder when I accidentally stepped on an ant mound below some aphids. It’s as if I became part of the story for a moment or two… as if the ants had to protecting the aphids from me… their #1 fan! Can you believe it?
The story
For the last couple of years, I spent a lot of time visiting an area frequented by woolly aphids. Visiting and revisiting the area with the hope of coming across a harvester caterpillar. This has been on my bucket list for five years—just read my story from April 2020 and you’ll see!
So finally, after five years, my repeated visits have paid off. Because early this July as I made another routine visit to the patch of alders, there right smack in the middle of the aphids was a harvester larva—as if that’s exactly where it belonged.
“I returned with a coworker and found the caterpillar almost entirely covered by the carcasses of the aphids it had eaten…”
Now, I was hoping to hear the crunching of an aphid being devoured by a caterpillar, but what I saw instead was quite a surreal scene. The ants—the protectors—weren’t actually doing a whole lot to protect the aphids. And this isn’t laziness, but due to a chemical the caterpillar produces which mimics aphids. By mimicking the aphids, the caterpillar tricks the ants into going about their business—leaving the aphids vulnerable to predate on.
Now the next day, I returned with a coworker and found the caterpillar almost entirely covered by the carcasses of the aphids it had eaten—just another attempt to disguise itself. And then the day after that it was gone and that was it.
Epilogue – “You Can Take It Off Your Bucket List”
This was a moment I’d been preparing for—one I could have easily missed had I shown up just two days later. Sometimes in nature, you find the drama unfolding; other times, you find only the aftermath, and in those instances, the story takes a little more time to be pieced together.
This was my first gall sighting, and it was proof that there’s always more to discover!
Just when I started thinking that nothing could top this experience, I found something new: alder tongue galls. These are bright red, tongue-shaped growths caused by the fungus Taphrina alni, which infects alder flowers and can impact their growth and populations. This was my first gall sighting, and it was proof that there’s always more to discover!
Epilogue to a Prelude
The woolly aphid adventure isn’t over. Another generation of harvesters is due soon, and I plan to be there.
If you’d like to try, now’s the time. Look for patches of woolly alder aphids in late August. If you’re lucky, you might spot adult harvesters feeding on honeydew. By early to mid-September, check for harvester caterpillars among the aphids. Be patient—harvesters develop quickly, so daily visits give you the best chance.
And if you do spot them, I’d love to hear from you. Send sightings or photos to kgentalen@mcht.org. Your observations help piece together the story of these tiny, remarkable worlds.