Does the Palo Verde Rootborer really kill Palo Verde Trees?

Check your porch lights: It's Palo Verde Rootborer season in Tucson. The huge, up to 4 in long beetles emerge from the ground, mate at night, and lay eggs into or close to dead Palo Verde roots (and probably other trees, too).  
Like every year, friends tell me that 'tree people' claim that the beetles are responsible for the demise of many trees. I don't think these 'tree experts' have done much conclusive research to base this opinion on. What they see is a downed Palo Verde tree with beetle grubs around its roots. 

Palo Verde Root Borer Pupa

 Some larvae are big enough or already pupated to make an identification possible, and they are indeed Palo Verde Rootborers. But did the beetle larvae even really damage the tree roots? Palo Verde Rootborers are prionins. Most (not all) of this subfamily of Longhorns feed on dead wood, mostly even on decaying substrate like old stumps, not on living tissue. 

When water is very scarce, branches die and break off, decreasing the evaporative surface area. Note that for this tree leaves are a luxury, only present right after a very good rain fall. Photosynthesis is performed in the green bark of trunk and branches.

 For Palo verde trees, leaves are a luxury that they only enjoy right after productive rains. Most of the year, they rely for their photosynthesis on the green bark of trunk and branches. That means that the bark also has pores that cause evaporation. During droughts, there are no leaves to drop. So Palo verdes famously react to adverse conditions by dropping whole branches. The branches are dead before they break off, but the main trunk survives with the potential to regrow when the drought finally ends. Maybe under ground, roots that aren't reaching any water source are also cut off  and left to die. Carl A. Olson and several other knowledgeable folks assume that the Palo Verde larvae are primarily feeding on those dead or dying roots.
Consider that desert trees and desert beetles evolved together and the trees survived the onslaught of the beetles just fine for eons. Of course,  climate change could  make the trees more susceptible and tilt that equilibrium. It's possible.

Even the competition of a saguaro that used it as a nurse tree might eventually kill a Palo Verde if there is not enough water for both

Still, most of the trees probably primarily succumb to drought damage, competition for resources, attacks by CA root rot, wrong irrigation, overheated surroundings of asphalt and concrete, or other human induced factors. Besides, Palo Verde Trees do not live as long as Ironwoods, for example, anyway.

Notopleurus lobigenis, a cousin of Derobrachus hovorei, shares its appetites

 But the activity of the ominously big beetles happens under ground, out of sight. So, when a tree finally falls over and we can see pulled-up roots, it's usually a dead or dying tree. Which naturally has plenty of rotting roots, and  Palo Verde Root Borer larvae feeding on those. But what's the hen and what's the egg here? Nobody is any wiser.

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