Attack of the Huisache Girdlers
A couple of years ago, as we were working on adding some local color to our
backyard, we went on a shopping spree at a native plant nursery. We bought
several trees that day, and all are doing nicely in our garden. But the one
that's happiest of all is our huisache (Acacia farnesiana),
which quickly grew to several times its original size, and has since treated
us to yellow pom pom flowers in spring. It is "very native" – not just
to the state of Texas, but to our area, where most wouldn't consider it a
desirable garden tree because of its fierce spines. I'm quite pleased with it
though, and hope it continues to grace our garden for many years to come. And
until recently, I had little reason to doubt it would – it's cold-hardy, drought-tolerant,
and pests didn't seem to bother it. A rock of a tree, in other words.
The first attack
So I was a little surprised to see, when I walked out into the backyard
one day a few weeks ago, that one of the sizeable upright branches of our
tree had collapsed. We'd had no strong wind, or anything else that would
explain the sudden break, so I investigated closely. The branch, a little
more than an inch in diameter, had fallen forward, and was still partly
attached to the tree. Its heartwood had splintered, making the irregular
jagged break that I would expect to see. But the wood along the back of the
break was curiously smooth-cut. At that time, I didn't think too much more
about it; I cut the branch off and chipped it for the compost pile, and the
tree wasn't much worse for the wear. I didn't even take a picture.
The second attack
Just a few days later, I was out walking in the garden again when I noticed
that a second upright, a larger branch (actually, one of the trunks of the
multi-trunked tree), was completely girdled. The damage area was close to where
the first branch had broken, at almost exactly the same height. The cut was
fairly clean-looking and regular, and there were bits of sawdust-like wood debris
on the ground below. In the photo at right, you can see the place where the
first branch succumbed in the back, and the newly damaged one in the foreground.
Now I realized there was something more going on. Were my neighbors,
unhappy with such a thorny tree growing near our fenceline, sneaking into the
backyard and undoing the tree a little bit at a time? Nonsense, of course. How
about our new puppy, who loves to chew on everything? No way she has the dental
dexterity to cut smoothly around a branch. But then what could it be?
I did as I usually do on these occasions: I consulted the internet. By
force of habit, my first stopping place is the Houzz gardening forums, current
home of the old Gardenweb forums. I posted a picture of the damage in the Texas
Gardening and Trees forums, and sure enough, before too long I had an answer:
it appears that my tree was under attack by a huisache girdler!
Huisache girdlers (Oncideres pustulata), I learned, are
long-horned beetles. The genus Oncideres is home to several
twig-girdling species, but the species O. pustulata apparently
specializes in huisache trees and close relatives tepeguaje and mesquite.
Its natural range extends from Texas to northern Mexico. Adults come out to
mate in fall (late August through October), after which the females deposit
their eggs underneath the bark of one of their host trees. Apparently it
would be inconvenient for the developing larva to have to contend with tree
sap, so mama then proceeds to girdle the branch underneath (that is, closer
to the roots) the oviposition point, effectively stopping the flow of sap.
It also effectively (over time, in any case) kills the branch, which falls
to the ground when it's dislodged by wind. The larva proceeds to feast on
the dead wood, eventually emerging as an adult the following summer. From
what I've found in online reading, branches attacked are usually smaller in
diameter (9-12 mm) than the ones that were selected by my girdling visitors,
but girdlers are known to take on quite large branches as well. While I'm not averse
to letting native insect species do their thing in my garden, I wouldn't
mind if they picked on some smaller limbs in their future child-rearing
Return of the girdlers
Several days after I noticed the second attack, I got a text from my son,
a woodworker and tree enthusiast who had been following the unfolding story.
He had caught two of the beetles in the act, at least one of them chewing
away at the same notch – apparently, the cut wasn't deep enough for
their tastes. He took the picture at left, and captured one of the
girdlers; the other ran off (surprisingly fast, he reports) and is still at
large. The notch was noticeably deeper as a result of these renewed efforts,
and by the next day, the limb had started to sag under its own weight, even
though the girdling hadn't done any damage yet to the foliage.
A photo session
Showing off its long horns
The captured girdler was subjected to an extended photo shoot in a couple
of indoor and outdoor locations: I find that my camera with close-up lens
allows me to see detail that I cannot hope to see with my (aging) bare eyes,
and these are interesting-looking creatures. The specimen may be a male
(Ben captured the slightly smaller one that wasn't actively chewing in the photo above, and
chewing is the mother's job among girdlers). Its body is close to an inch
long (the antennae are of course much longer, as befits a long-horned
beetle), and its wing casings are covered in shiny black spots (the
"pustules" or pimples to which its species name refers), the underlying color
divided into three cross-wise bands of different shades of grayish-brown.
Beefy mandibles. This photo also shows why this species is classified
among the flat-faced longhorns.
The mandibles on female twig girdlers are extended, to make for heartier
chewing. Even the chewing pieces on the one that sat
for my shoot (again, likely male) look pretty fierce. I was happy with this
picture – in full resolution, it shows the faceted eyes, as well as the
furry hair covering its face and legs.
Shiny black dots cover the subtly banded wing covers
A few more photos for good measure. This one shows the
characteristic markings and color bands typical of the species. I've seen
a good bit of variability in coloration in online photos; in some, the
banding isn't nearly as regular or prominent.
My temporary guest was fairly docile during the photo
shoot, thanks no doubt to some time spent cooling off in the family fridge.
Once he started walking around on our table, he did scoot to the underside,
which allowed me to take a nice upside-down side shot. A little later, out on
the patio table where I took some of the other pictures, he demonstrated that
he can fly, even though it seems they don't fly much when they're in a happy
place. But that's undoubtedly how they found the only huisache growing in my
entire neighborhood – scanning for a happy place from overhead!
A few days later, the second limb came crashing down
under its own weight. As a result, the tree has lost about a third of its
crown, but since it's otherwise healthy and fast-growing, I'm not too concerned
over the long-term effects of the girdler attack. Still, I think I'll be monitoring
the tree more closely in early to mid autumn from here on out.
The incubation time for girdler eggs is reportedly about
7 days; since more than a week had passed between the initial attack on limb number
two and its collapse, I decided to strip the bark off the section of the branch
closest to the notch to see if I could find any larvae. I didn't find any,
but did see this feature, which looks like an egg, about an inch up from the
notch. Since we'd been having some colder weather, I figured the egg may need a
little longer to hatch. So I covered the stripped section with some tape to
maintain a protected environment. Thus far, no further sign of life...
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October 30, 2019