Did you figure out the mystery insect from last week?
The patches of mud you see on the trunk of the saguaro are made by desert encrusting termites (Gnathamitermes sp., probably Gnathamitermes perplexus).
These desert termites build layers of mud on the bases of saguaros or palm trunks. They also plaster mud over dry grass or twigs on the ground, particularly after a rain. Over time they eat the grasses or twigs, leaving a hollow tube of dried mud.
On trees, they gently scrape the surface clean of dead material.
Desert encrusting termites are different from other termites because they do not enter or eat sound, strong wood. They could even be considered beneficial. Have you ever heard that termites have protozoa in their guts to help them digest the cellulose in wood? Desert encrusting termites have bacteria instead. Those bacteria are capable of fixing nitrogen, which means they actually fertilize the soil. Because the termites work on dry grass and twigs on the ground, they reduce fuel for wildfires. Finally, by tunneling in and moving soil, they aerate it, making it better for plants.
You could say that desert encrusting termites are part of nature's clean up crew.