Some of you might be wondering why the white-lined sphinx moth caterpillars (Hyles lineata) are traveling in such numbers (see previous post). No one knows for sure what is going on. Two suggestions are that the caterpillars are looking for places where they can dig into the soil to pupate, and/or that they have run out of food and are looking for more. The caterpillars feed on common desert weeds, several wild relatives of four o'clocks. These plants are drying out since we haven't had rain in a little while, so it is possible at least some of them are looking for food.
I was able to do a small experiment to test whether the pupation idea holds water. If you place one of these wandering larvae in a terrarium filled with soft, moist potting soil, you will be amazed at how fast they dig in. I expected to see a bit of wandering, then dig into the soil. Nope, almost as soon as their legs touched the soil they were digging. The hard clay desert soil (yes, our desert soil is clay rather than sand) is almost like cement where these larvae were found.
Update: The white-lined sphinx moth emerged in September.