A magnificent Longhorn Beetle

About every three years the beetle collecting community - that does not include me, as a mere photographer - buzzes with new about the emergence of the spectacular longhorn beetle Megapurpuricenus magnificus   (Texas Canyon Longhorn). It's the beetle that used to be called  Crioprosopus magnificus, but when I had just learned that name, it was changed.

Many big beetles go through several years of larval development. But members of most species still emerge every year. But like the 17 year cicadas, the Texas Canyon Longhorns emergences appear to be rather synchronized. The beetles are very rare for a couple of years, then they appear in numbers, then again only a few individuals show up. Despite their common name, they do not only occur in Texas Canyon but in several sky island areas of Arizona and Mexico. The three year rhythm seems consistent everywhere, but the different populations all seem to be on their own clocks, so in a particular year they may fly in one mountain range, but not in the other.

Their host trees are several of the local live oaks, Arizona white oak, Emory oak, Silverleaf oak (Hovore 1983). In Arizona, the beetles seem to like smaller trees (or the host trees are stunted due to the beetles activity?) - the diameter of the stem  between knee and hip height is usually only about 10 inches. From Mexico there are damages reported, and those mostly occurred in pure Quercus potosina stands at 2500-2642 m elevations.  Under trees with fresh emergence holes you can often find piles of old saw dust that former generations of beetles pushed out when they emerged.

From Fred Skillman I learned a little about the beetle's biology. I'm not sure in which layers of the tree trunk the wood boring larvae feed, but when they are ready to pupate, they chew and exit hole to the outside. This is important because the adult beetle lacks the mouth-parts to chew its own way out. The hole is closed with a double plug of  wood pulp, then the larva pupates. When the imago (adult beetle) sheds the pupal skin in early summer it removes the first plug, but leaves the second in place. Behind it, the beetle is ready for the first heave rain of the monsoon.  At that time it emerges, neatly timed with its neighbors. Now males and females will be active for a short time, usually on warm mornings between 9 and 11 am and preferably after rainy days or nights, so they can find each other to start a new generation which will take flight after another three years.

If they don't get caught by collectors first. Luckily, the beetles behavior protects them to a degree: to find each other, they climb to the top of the tree canopy. The females may just sit up there and send out their pheromone signals, but the males can be seen cruising from tree top to tree top. In Texas Canyon, the oaks are fairly low and loosely spaced, so the beetles are quite visible once you develop a search image for them. Flying up there, the beetles are out of reach of most butterfly nets.

But there are other ways. Once I came upon a group of collectors on a friend's private property who were lounging on the soft carpet of leaves, leisurely watching the top of an oak.  In the morning they had found a female emerging from her pupal chamber in a nearby tree trunk.

They put her into a little cage and with a rope pulled her up into the tree canopy.  There she obviously did her chemical calling, because eventually we saw a male flying in.

 He circled, found the cage, landed on it. The collectors lowered it, reached impatiently with their nets, lost him. Warned, he disappeared. But soon another one showed up, and this one was caught.

 It took a lot of convincing to make the guys set him on a tree trunk for  me to take pictures. Believe me, their nets hovered right out of sight. They got three or four males that day, no harm was done to the population.

The female was eventually released, and after my photo shoot, she made her way up to the tree tops, this time without a lift and free to seriously enter the mating game.

It is not easy to harm insect species by collecting. The number of eggs that each female produces is enormous. So gaps left by the ones collected should normally be closed by the offspring of those that remain. None-the-less,  older collectors tell me that this particular species used to be much more numerous, if never common. Due to its popularity and its insular distribution, it may have been over-collected. But Fred Skillman also tells of occasional floods in Texas Canyon that may have drowned big numbers of beetles waiting to emerge - remember, most of their holes are only knee-high in the trees. And besides such local catastrophic events, there seems to be an ongoing decline of insect numbers not only in the Southwest, but all over the world may it be due to habit loss, insecticides or climate change. 

References:

Hovore F.T. (1983) 1984. Taxonomic and biological observations on southwestern Cerambycidae (Coleoptera). Col. Bull. 37: 379–387 (Full text)

Sánchez-Martínez G., Moreno-Rico O., Siqueiros-Delgado M.E. (2010) Crioprosopus magnificus LeConte (Coleoptera: Cerambycidae) in Aguascalientes, Mexico: Biological observations and geographical distribution. Col. Bull. 64: 319-328 (Full text)

Longhorn Season

Longhorn Beetles (Cerambycidae) spend a great part of their lives as larvae, chewing and feeding on the inside of branches and twigs, under bark and even in the core wood of trees. Some live in the soil, feeding on roots and other plant matter. Even though a thumb-sized larva of the Palo Verde Rootborer  at the Arizona Desert Museum that accepted apple slices as surrogate food pupated and metamorphed into a big beetle just fine, most Cerambycids are specialists when it comes to their host plants and foods. Special requirements reach from the stalks of perennials to the living cambium of tree trunks, to freshly dead branches, and even to freshly burned wood for some species. Old dead wood is usually left to other groups like the Powderpost Beetles (Bostrichids).

 The feeding behavior within the plant is genetically programmed, and the shape and location of tunnels and holes in the wood, and whether those passage ways are filled with digested wood pulp or not, can tell the specialist a lot about the species and age of the larvae. (I can tell the family. That's something, too)

Cerambycid Larva, ready to chew

Insects that feed and pupate in the trunk of  hardwood trees like oaks are well protected. Tunneling within the wood is no problem for the strong mandibles of the larva whose only purpose is to accumulate nutrients and grow.  But the adult beetle is much more geared towards mobility, finding a partner, and propagation. After the metamorphosis there are wings, antennae, long legs, the ovipositor if it's a female, but the strong, wood chewing mandibles are gone. So how do the beetles get out of the wood?

Longhorn Beetle Pupa

It's all taken care of by the larva before pupation. It chews out a pupal chamber close to the surface, but under enough wood to protect the vulnerable pupa. The larva also prepares and exit way to the surface, but then carefully closes it with a plug of chewed wood pulp. In some species, like our beautiful three year oak cerambicyd Crioprosopus magnificus there are even two plugs. Thus protected in its chamber, the larva pupates, goes through the metamorphosis and finally ecloses as a finished beetle - and then stays in the puparium and waits. In different ecosystems there are different signals that will trigger the final emergence of the beetles. The signal may be a rise in temperature, or in day length if that can be perceived within the wood, signals coming from the tree, even the smell of a near-by fire that promises food for the next generation. Here in Arizona, most beetles wait for the monsoon rains. The triggering signals will synchronize the emergence of males and females. All the waiting adult beetles become active, push out those plugs and emerge simultaneously, immediately ready to find partners, mate, disperse and reproduce.

Crioprosopus magnificus pair

 For Crioprosopus magnificus who develops in small oak trees on rocky slopes in Cochise County,  this great event usually happens only every third year. Knowledgeable collectors have told me that the double plugs in the emergence hole probably fine-tune the timing: the beetles respond to an increase in humidity by removing the first plug and  push out the second just after the first heavy monsoon down pour. After that, if you are very lucky, you can see them flying over the crowns of the low-growing oaks.

Mallodon dasystomus, Hartwood Stump Borer

 In 2010 I was at Steward Campground in the Chiricahuas in early July just after the first rains. My black light, my dog and I were overrun by  Hardwood Stump Borers, big guys that my dog did not like very much. A month later I went back and couldn't find any.

Monochamus clamator from Rustler's Park

 Last Monday we must have been on Mt Lemmon just in time for another species, Monochamus clamator. This one isn't rare in other western states, but last year Patrick Gorring, a Cerambycid researcher, contacted every collector and entomologist in Arizona for help to find local specimens in our skyislands. I met him on Mt Graham by the end of his trip, and I don't think he got any Monochamus at all.  I had only once seen a specimen that a birding friend, Gary Waayers, spotted and netted in flight at Rustler's park in the Chiricahuas.

How many?

So one week after an unseasonably early rain Randy and the dogs were resting in the lush grass along Meadow Trail on Mt Lemmon and I was beating some pine branches without much enthusiasm because we hadn't found much until then. I was quite surprised when the first big male Monochamus  plopped into the beating sheet and calmly walked around on it with his long antennae extended in front of him. When a second one landed right afterwards I had a search image and began seeing more of them on pine branches and clinging to grasses. They all looked fresh and perfect, as if newly emerged. Their elegant checkered pattern camouflaged them efficiently. They were mostly in the branches of a live, upright pine, but a freshly dead tree was lying near by. Their larvae are known to bore in sick and dying pine trees, and there are still many fire -damaged trees from the big Mt Lemmon Fire (9 years ago? Time flies) that are succumbing only now.

Monochamus clamator from Mt Lemmon