Tuesday, July 3, 2018

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.

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