Palo Verde Beetles

It’s that time of year again!  Time for the palo verde beetles to descend on Tucson and fill the night sky with giant beetles flying around drunkenly looking for mates.  I already wrote a post about how much I love these beetles and promised to write more about their biology.  Today I am making good on that promise!

Meet the palo verde beetle (or palo verde root borer beetle), Derobrachus hovorei:

palo verde beetle top view

Palo verde beetle

I think these beetles are one of the best parts of living in the Sonoran Desert.  Each summer when the monsoons begin, these beetles start to appear.  They’re large, dark brown nocturnal beetles, 3 – 3.5 inches long.  Check out the long, luxurious antennae:

palo verde beetle antenna

Palo verde beetle antenna

Palo verde beetles belong to the beetle family Cerambycidae, the longhorn beetles.  You can see how the family got its common name!  Nearly all members of the group have these long antennae, including several important wood pest species (such as the Asian longhorn beetles).  The palo verde beetle is no exception.

I think the palo verde beetles look rather fierce.  Check out the spikes on the thorax:

palo verde beetle thorax

Palo verde beetle thorax

And the big pinching mouthparts (called mandibles):

palo verde beetle jaws

Palo verde beetle jaws

In spite of their size, their armor, and the powerful jaws, these beetles are largely harmless.  That’s not to say that they won’t flail about wildly and try to bite you if you pick them up, and they can deliver a strong, painful pinch if you’re not careful.  (That’s never stopped me from picking them up!)  Mostly though, the beetles use those impressive mandibles for fighting and/or mating.  I posted a photo of a male and a female palo verde beetle struggling with each other before they mated a while back and jaws were used extensively as the male subdued the female.  In fact, she lost a leg and both antennae in the struggle.  Those strong jaws are also used by males in battles with one another to win females.  The better fighter a male is, the more females he has a chance to mate with.

There’s one thing the jaws aren’t used for though: feeding.  Adult palo verde beetles don’t feed at all and rely on nutrient reserves they ingested as larvae to fuel their adult activities.  As result, their adult lifespans are pretty short, less than a month.  During that month, they fly around (not very well and in the dark – there’s nothing quite like seeing one of these flying toward your head at night!), fight, mate, and lay eggs.  That’s a lot to do for a large flying animal that doesn’t eat!

Once a male finds and mates with a female, the female will burrow into the soil at the base of trees and lay her eggs about a foot down.  When the eggs hatch, the larvae feed on the roots of the trees, focusing on the starches within the roots.  After 2-3 years of feeding and growing, the larvae are enormous and look like this:

palo verde beetle larva

Palo verde beetle larva! This one was over 3 inches long.

The larvae have strong and powerful mouthparts too, essential for cutting trees roots open so they can eat.  When they’ve grown large enough, they pupate underground.  The adult emerges when the monsoons arrive and dig their way up to the surface, leaving large round holes around the base of the tree where they grew up.  Then they go about the serious business of flying around in the dark (scaring a lot of people in the process), looking for mates, and starting the whole process all over again.

Palo verde beetles get their name from the palo verde tree, a gorgeous desert tree with green bark native to the Sonoran Desert.  If you dig up palo verde trees, you will supposedly nearly always find several palo verde beetle larvae happily munching away on the roots.  Because they are root borers and root borers are commonly associated with dead, dying, or unhealthy trees, palo verde beetles are often considered pests.  If you search the internet, you’ll find all sorts of crazy ideas for how to rid your yard of these “dangerous” beetles so that they don’t kill your trees.  It all a bit sensationalistic though!  Palo verde beetles DO eat roots of trees, but consider this: there are millions of palo verde trees in the Sonoran Desert and nearly all of them have several palo verde beetle larvae gnawing on their roots.  If the beetles are really destroying tress, wouldn’t there be fewer palo verde trees around?  Palo verde beetles can cause some damage to trees, especially non-native ornamentals, but usually only in trees that are already having problems.  The best defense against palo verde beetle damage is taking care of your trees!  If you keep young trees healthy by watering them regularly and fertilizing, they will usually be able to withstand palo verde beetle larvae eating their roots quite well.

While I completely understand why people might be scared of these lumbering, giant beetles – they are VERY large after all – I can’t help but love them!  I associate them with lazy, hot summers and the arrival of the much-needed rains.  They’re hilarious to watch flying around.  And they’re stunning!  As proof, I leave you with this last image:

Palo verde beetle side

Palo verde beetle, side view

Love ’em!

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