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