Sunday, June 22, 2014

Wheel Bugs Growing Up....

Preying mantises and ladybugs have great reputations as good predators in the garden, but I've found a species that I think is even more awesome:  wheel bugs (Arilus cristatus), in the assassin bug family.  Like preying mantises, wheel bugs are very mobile and eat almost anything they can catch.  I've observed them eating a wide variety of insects, ranging from grasshoppers to beetles to caterpillars.

Wheel bugs have only one generation per year so, as with most predators, it takes them longer than plant-eating insects to ramp up their numbers after a population crash.   This is why, when you spray insecticides (which indiscriminately kill ALL insects), predator populations take so long to catch up that the plant-eaters/prey insects end up having huge population explosions in the meantime.  This is great for insecticide companies, but not so good if you're trying to use fewer chemicals in your yard or garden.

Overwintering as eggs laid in clusters on the underside of tree branches, the wheel bug life cycle begins when the tiny new nymphs hatch out in late April or early May, just about the same time that the honeylocusts bloom.  Although I occasionally find wheel bug eggs on other trees, most of them seem to be laid on honeylocust.  I doubt this is coincidence.  The egg cluster above, about the size of a quarter overall and located, like usual, underneath a honeylocust branch, was just starting to hatch on April 21st this spring.

When the tiny wheel bug nymphs hatch out in late April or early May, they look very different from their parents.  For starters, they have a bright red abdomen.  (Actually, when they first hatch out, they are completely yellowish-orange, until their exoskeleton hardens the first time.)  There is no "wheel" structure on their back and they, of course, have no wings.

Even at this tender stage, they are fearsome predators.  On May 11 this spring, I photographed this 1/4" wheel bug nymph carrying (and sipping on) the pupa of another insect, firmly pierced by its proboscis.

With each succeeding molt, the wheel bug nymphs' appearance changes just a little.  They get a little more gray and black, a little less red.  However, they never lose their fierce abilities as predators.  On June 7, this wheel bug nymph had moved from the tree where it was hatched to a clump of Jerusalem artichoke about 30' away, where it enjoyed a tasty meal of grasshopper nymph.

The photo above is, unfortunately, rather blurry, but I was lucky enough one day to spy a wheel bug nymph molting under a leaf of American plum.  The exoskeleton of the newly emerging nymph is, again, orange until it hardens.

That same day, June 8, I found this nymph hiding in daisy fleabane by its discarded, old exoskeleton.  It had obviously also emerged recently; the new, larger exoskeleton was not yet hardened and darkened to the normal colors of gray and red.

Also on June 8, I found this wheel bug nymph eating what looks to be a small caterpillar while posing on a spent bud of lanceleaf coreopsis and sporting a classy red and black abdominal outfit.

Right now, on June 22nd, I'm seeing nymphs that have turned almost completely gray, but that still don't sport the adult's wheel structures on their thorax or their wings.

They are still busily eating their way through the insect population of my yard.

Here in south central Kansas, the earliest I've seen a mature adult is July 21st... or at least that's the earliest I've photographed an adult.  The above photo was actually taken on September 9th in 2008.  In it you can clearly see the wheel on the back of the thorax, as well as the wings folded neatly over the abdomen.  All red coloration is gone from the body, although a little remains on the legs and antennae.

The adults continue to feed, essentially indiscriminately.  Here an adult female is feeding on a tiphiid wasp that was herself feeding on goldenrod nectar.  Actually, in this case the wheel bug's choice is rather unfortunate, since tiphiid wasp larvae parasitize white beetle grubs, the kind that eat grass roots.  Judging from this female wheel bug's abdomen, though, she has eggs of her own that she needs to lay.

By October, the new egg clusters are being laid and the year's adults are beginning to die off.

This female was still around on November 15th last fall, but her will to live seemed to be ebbing.  I watched her for a long time as she sat right next to this tachinid fly, but she never made the move to pounce on it and eat.  I figured that she must have laid her quota of eggs and be feeling worn out and ready to go. 

As cold weather hits, all the adult wheel bugs are gone and the future of the species lies safely - but solely - protected within the years' eggs, glued firmly to the underside of tree branches.  It's a system that's worked for thousands of years.  I can only hope and trust that nothing disrupts it in the future....


greggo said...

Spotted a few in the garden last night making a meal out of potato beetles. Unfortunately I had to remove a lot of the potato beetles as they were eating all my plants. It seems there are a lot of non-pollinator insects this year. Beetles ate my Baptisia blooms, I have cats on Echinacea, cats on my native Delphinium and larva on my apple trees. I should take a picture for Id?

Gardener on Sherlock Street said...

That was a great post. My hubby is most impressed with you documenting each stage. Good photos and I will know what to watch for better when they're little. Will they eat squash bugs? Seems like nothing eats squash bus so I must hunt them myself. Ha!

Gaia Gardener: said...

Greggo, don't worry about the wheel bugs (unless you removed the potato beetles by spraying with insecticide) - they are indiscriminate feeders and will find plenty of other food! Including, perhaps, some of those cats!

I'm guessing that the cats on the Echinacea are pearl crescent caterpillars - they feed "en masse" for the first couple stages. About the time you see them, they will disband to finish growing and start pupating. The Echinacea will be fine, although it will have a good case of the "uglies" for a while.

Most perennials - especially prairie plants, truthfully - can take total defoliation at least once without dying.

I'd love to see photos of what's eating your plants...although I have no idea if I'll know what I'm looking at! I'm a great fan of They are phenomenally helpful!

Just hope you don't get the grasshopper hordes that we've got again this year.....

Gaia Gardener: said...

GonSS, thanks for the compliment! I think wheel bugs would eat squash bugs, but I doubt that they'd eat enough of them to keep them in check. I'd keep up the hunt myself! (By the way, I like to carry a peanut butter jar filled with soapy water when I hunt squash bugs AND potato beetles. I just drop the bugs into the water, then flush the mess down the toilet later. The soap makes the insects drown quickly...and I'm not having to squash bugs (pun somewhat intended!), which grosses me out.)

Oh, I also saw a robber fly eating a LARGE stink bug yesterday, too, so there are predators that will get these plant nasties.

Melanie said...

Hmm. .will have to watch for those!! I will take all the predator insects I could get!!

Gardener on Sherlock Street said...

My hubby gets great satisfaction in squashing the squash bugs and I don't mind setting them on the concrete and stepping on them. The ants love the feast. If I ever see a wheel bug with a squash bug, I will grab the camera!

Gaia Gardener: said...

Melanie, the more I watch the insect life in my gardens and yard, the more amazed I am at how many predators there are! It's really fascinating stuff.

Hope you're getting some of the rains we've been having lately.

Gaia Gardener: said...

GonSS, I'm looking forward to the photos!