Last week we went for a drive and in one area the white-lined sphinx caterpillars were crawling across the road. They weren’t in large numbers, but noticeable. Why were they doing that?
Adult white-lined sphinx
The caterpillars could have run out of food plants and were looking for another snack, but the more likely explanation was that they were in the “wandering phase.” When caterpillars have finished eating and growing, they may wander around looking for an ideal place to dig into the soil and pupate. Sometimes they wander onto roads, but they really do want to see what’s on the other side.
Have you seen any white-lined sphinx caterpillars or moths lately?
I was asked a question last week about what white-lined sphinx moth caterpillars eat. I had read that they eat wild relatives of the four-o’clock, a garden plant. A few weeks ago we visited Boyce Thompson Arboretum State Park and I got some great first-hand information about what kind of plants the caterpillars feed on.
At the arboretum we found caterpillars on a native plant that is being used as a landscape perennial called pink guara (Guara lindeimeri). I noticed, however, the caterpillars were only eating the flowers. Often the flowers lack toxins or feeding deterrents found in the leaves or stems, although I don’t know for sure this is the case here.
Some of the caterpillars were working on a plant called white ratany (Krameria grayi).
I needed a friend’s help to identify that one. The plant has pretty purplish-pink flowers, but they are inconspicuous. She said the plant is a partial parasite that takes food from the roots of fellow desert plants like bursage or creosote bush. I also found out that the flowers produce an oily substance rather than nectar (weird!), but that some native bees will take it to mix with pollen.
Finally, we cheered the caterpillars when we found this batch eating the noxious weed, spotted spurge. Go white-lined caterpillars, go! (Sorry, the photo isn’t all that great).
For more information about white-lined sphinx moths and their caterpillars, check these previous posts:
Some of you might be wondering why the white-lined sphinx moth caterpillars (Hyleslineata) 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.