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