Velcro bug, el Torrito, Mesquite Twig Girdler, or Oncideres rhodosticta: the most abundant beetle in Arizona after the monsoon

It’s Velcro-bug time again!

Anywhere between Green Valley and Sierra Vista, Arizona, and probably all over New Mexico and far into Texas, border patrol agents, gas station and supermarket employees, and everyone else who leaves his porch lights on at night is familiar with those half-inch long beetles that cling to every surface as if glued to it. They hold on with little hooks on their feet and even with their mouth parts. For a good reason, as in nature being picked up usually means being eaten by a bird, a grasshopper mouse or a lizard.

Male (left) and female (right) Mesquite Girdler

During the day the beetles rest motionless clinging to the bark of a tree, mostly well hidden by their cryptic wing pattern. Observed closely, the shades of dark-brown and silver-grey and the raised reddish dots are very attractive. The scientific species name Oncideres rhodosticta refers to those markings which distinguish this very common species from the related rarer Oncideres quercus and the much larger cousin Lochmaeocles marmoratus, all found in Arizona.

Arizona members of the tribe Onciderini. Mesquite Girdler on the right

Like most adult longhorn beetles, adult O. rhodosticta feed on plant material. The chew leaf buds and the green bark of fresh mesquite twigs. Sometimes the nightly feeding frenzy of the beetles leaves the ground under a tree littered with chewed-off leaf-matter and twigs.

Life cycle of the Mesquite Twig Girdler Oncideres rhodosticta :
Adult twig girdlers eclose from late August to early November. 

Towards the end of our summer monsoons, the female beetles create the most well-recognized sign of O. rhodosticta 'infestation' when they prepare a nursery for their off-spring: Dead or dying finger-thick mesquite (or sometimes Acacia) branches that stay connected to the tree and usually carry the wilted, bleached leaves like flags into the winter months.

The common name, Mesquite Twig Girdler, hints at the story: Before she lays her eggs, the female chews a precise, complete circle around a finger-thick twigs. This task can take up to two days. She bites all the way through the Xylem and Phloem of the bark and thus disconnects the branch from its water source. Then she chews a separate shallow grove for each of about 8 eggs in the distal, dying part of the branch. The larvae will hatch and live in the wood until they are grown, pupate, go through their metamorphosis and hatch as adult beetles by the end of the next monsoon season to restart the cycle.

Female Mesquite Girdler at her girdling site

Most wood boring insects attack sick, injured or dead wood rather than a living tree. This is partly because of the ability of healthy trees to fight intruders by ‘gumming’ them up, that is by drowning and encapsulating them in sticky resin rich tree sap. So the female Mesquite Girdler protects her eggs by cutting off the tree’s defense lines. A glob of fresh tree sap often hangs from the girdling cut: the trees attempt to fight the parasite which didn't reach its target. The nursery that the longhorn beetle creates is so attractive that several other insect species infest the girdled branches. I have raised more wasps, buprestids, anobiids, bostrichids and dermestids from collected girdled branches than O. rhodosticta adults.

Of course, the pruning-activity of  the beetles comes as a cost to the trees. They lose the carbohydrates stored in the girdled twigs and a part of their photosynthesis-machinery.  However, a study of Texas Tech. University showed no conclusive results concerning the use of the beetles to control the mesquite tree overpopulation of the grass lands.  Natural girdler infestation can cause an over 30% reduction of the canopy, which does not seem to harm the mesquite trees but instead to induce healthy re-growth in the following season.

Living and dead Mesquite Girdlers under the lights of the I 19 border patrol check point
Photo by Joyce Gross Sep. 2008 

Mass occurrences of members of a single species that last for more than a couple of vegetation cycles, as observed in the Mesquite Girdlers in southern Arizona since 2007, usually indicate a disturbance of the natural balance. In this case the overabundance of mesquite trees on former grass lands is certainly a contributing factor. The mesquite tree was introduced into the grass lands through the widespread practice of feeding mesquite beans to cattle. While the nutritious pods are digested, the seeds pass the bovine digestive system not only intact, but with an increased capacity to germinate. But the trees have been with us at least for several decades and the Girdlers seem to proliferate more than ever since 2007. A climate changing to drier and hotter summers seems to have favored this species, while its predators, parasites and diseases are still lagging behind. But there is no doubt among entomologists that these factors will eventually catch up to collapse the population explosion of the girdlers.

PS: The numbers of Mesquite Girdlers did indeed collapse in 2011. Since then I have not seen more than 3 to 5 on my nightly light trap, even during the prime season and in mesquite areas. I was wrong about one thing though: the summers from 2005 to 2009 were not particularly hot or dry in hindsight. It only got hotter and drier from then on.

Literature: The Ecology of the Mesquite Twig Girdler Oncideres Rhodosticta and its Evaluation as a Biologial Control Agent. Thesis in Entomology by K.L. Polk

A 'Young Beetle', Complete Metamorphosis

Seven-spot and Convergent Lady Beetle, both adult

"Is he getting any bigger than this?" "Is this a young Lady Bug?" These are questions that I hear very often. The answer is of course that adult beetles do not grow, and the lady bugs are two different species.

Yucca Plantbugs Halticotoma valida, adult and nymphs

I also often hear the description that "Big beetles were surrounded by their little ones". In this case the omni-present term 'Bug' would have been a better fit, because this person had most likely seen a group of True Bugs. Bugs do indeed often care for their off-spring, and those kids do resemble their parents. True Bugs undergo an incomplete metamorphosis as described in an earlier blog chapter. Visit also Ashley Wood's photo stream who followed the life cycle of many European bug species in beautifully detailed drawings

Larval stages of a Stink Bug by A. Wood

Young beetles, however, bear no similarity to their parents. Insects that have the full, hard wing covers that identify them as beetles, are adults and do not grow. If they are small, and some are barely a millimeter long, they stay that way, and make the life of a beetle photographer very difficult. Beetles, like butterflies, flies, and bees and wasps, undergo a complete metamorphosis.

Ashy-gray Lady Beetle, Olla-v nigra, adults and larva

Like all insects, beetles lay eggs. They are usually deposited on or close to the food that the hatching larvae will eat and that's all the care most beetle off-spring will receive. After hatching, beetle larvae feed voraciously. Growing and storing energy is their only purpose in life. Most are not very invested in locomotion, their legs are short and their bodies are huge. Well developed mouth-parts deliver food to the intestinal system that seems to take up most of the segmented, elongated body.
Beetle larvae live in almost all biological niches imaginable. Some live as predators in leaf-litter, under tree bark, and in fresh water, and even on flowers.


Predators, of course, move well and have well developed sensory organs to find their prey.

Larvae of Dermestes marmoratus on cow carcass

Many beetle larvae are invaluable for the decomposition of dead animals, feces, and dead plant material.

Pattern left by Bark Beetle females and their larvae, outer bark removed

Some live protected in the nests of other insect species as brood parasites, devouring provisions and off-spring alike. Others live off living plant parts, boring or mining inside wood, fruits or leaves

Leaf beetle larvae: cryptic (left), aposematic (middle), actively disgusting holding excrement package (right)

The ones that are chewing away openly on the outside are usually protected by cryptic, mimetic or aposematic colors, shapes, and or behaviors. Some even build their own protective cases.

Scarab grub

Cellulose feeders like many scarab larvae tend to be especially big: they not only eat very bulky food, they also house a whole world of cellulose digesting symbiotic organisms in their guts.

Beetle larvae may be storage pests and raid organic material horded by squirrels, pack-rats, and humans. This includes starchy products as well as fur coats and insect collections. But as far as I know, no beetle species, larvae or adults, directly parasitises any living warm-blooded animal.

Examples of Erotylidae larvae (left) and Curculionidae (right) by A. Zaitsev

In adaptation to the demands of so many different environments and ways of life, beetle larvae come in many very different shapes. Artem Zaitsev's exceptional work, shown on his flickr photostream, gives you an impression of the multitude of morphological adaptations.

Endocrinological control of larval development:

To grow, beetle larvae have to molt out of their rigid chitinous skin to a 'larger size'. This happens several times until finally the larvae changes into the stiff, immobile pupa from which eventually the adult beetle emerges.
Delicate shifts in the balance of three hormones determine whether a molt leads from one larval stage to just another, larger one, or to pupation and thus metamorphosis.
In short, Prothoracicotropic Hormone (PTTH) is released from the brain and activates the prothoracic glands to release Ecdysone. Two endocrine glands (corpora allata) in the head of the larva are producing Juvenile Hormone (JH). An increase in Ecdysone while the JH levels in the heamolymph of the larva are high will cause molting into another, larger larva. An increase in Ecdysone when the levels of JH are low will cause the larva to molt and undergo pupation and metamorphosis instead. PTTH release and corpora allata activty (JH production) are controlled by the central nervous system that is able to integrate endogen and exogen inputs (body size, age, food availability, day length etc.)

Pupa of the Palo Verde Root Borer (Derobrachus hovorei)

The beetle pupa is not as tightly enclosed as a Butterfly chrysalis. Appendices like legs, antennae or wings are already clearly recognizable. But the pupa is covered by a dense chitinous layer that protects against dehydration. While the pupa rests without moving or feeding, radical changes take place inside, the metamorphosis. Tissues are actually dissolved to be reshaped into completely different organs. Compound eyes, chemically sensitive antennae, wings and sexual organs develop while the feeding apparatus is reduced. At the end of the pupal rest the sexually mature adult beetle will hatch (eclose) from the pupal skin.

Mating Soldier Beetles, Chauliognathus profundus

The adult beetles do not grow. Many species don't eat at all or just replenish their resources with easily digested sugars. Their only purpose is now the propagation of the species. Most are able to fly and are thus highly mobile. This allows them to disperse and find unrelated mating partners for a healthy exchange of gens and possibly fresh unexploited living-space for their off-spring. Since most adults either don't eat, or have a different diet from the larvae, there is no competition for resources among the generations.

Adults and larvae of Altica ambiens completely defoliate small alders

The advantage of this arrangement is easy to understand after observing one of the exemptions: Alder Flea Beetle (Altica ambiens) larvae and adults both eat the leaves of Alders and often totally denude the little trees. I'm not quite sure how the next generation survives. Maybe there is some built-in prolonged pause in the sequence of generations that allows the trees to recover.

But back to our 'Young Beetle':
On a Mount Lemmon trip of the Sabino Canyon Naturalists led by Ned Harris, we actually found what could really be called a 'young beetle'. It was Mid-August an it had been raining. Even the mountain climate was humid and warm enough to be called muggy. The area along the Oracle Ridge trail was heavily burnt several years ago. The dead trees are still upright and soot and carbonized parts have been eroded away. These are ideal conditions for many different fungi and the mushroom connoisseur among beetles, Gibbifer californicus, the Pleasing Fungus Beetle.

First we found some adults and last instar larvae, and then a whole gallery of hanging pupae. About 60 of them were lined up under the protective overhang of a leaning tree. Some where already empty.
Perched on top of pupae and empty cases was a ghostly white beetle with his under-wings partly unfolded.
This very freshly eclosed Fungus beetle showed nothing yet of the characteristic blue coloration or any of the black accents. This was a teneral specimen, a true young beetle. His final colors became apparent only hours later when his new exoskeleton had completely dried.

The mature color of Gibbifer californicus ranges from gray to deep blue (left) and purple. However, the beautiful color does not survive the death of the beetle. Gibbifer californicus specimen in collections are of a sickly yellow hue, only the black extremities remain dark.
Incidentally, the illustration in the Peterson Field guide (right) depicts a beetle with those non-colors to represent the species. That image was one of the triggers that started my collection of life-images of Arizona beetle species.