In the past few days I have received multiple phone calls and questions regarding some very large, horned caterpillars. This appears to be the annual arrival of the white lined sphinx moth. I alluded to these hornworms being thus far strangely absent from Hale & Swisher in the June 27 edition of the Plains Pest Management Newsletter. Since that time they have made their presence known from Hale Center to Vigo and beyond.
Dr. Ed Bynum reports finding this caterpillar north of Amarillo and Dr. Pat Porter has reports of it near Muleshoe.
As an adult moth the white lined sphinx is also commonly referred to as the humming bird moth or the hawk moth. The moths are large, somewhat motley colored interesting Lepidoptera that lean toward mimicking humming birds in behavior and body shape when in flight.
There are actually several species of sphinx moths, but these tend to be the most predominate in this area. We noted several in the area through the months of April and May. The larva can typically grow to be surprisingly large, up to 3 ½ inches in length, are very plump, colored with a light green base with yellow, black, and sometimes white markings, and a large usually yellowish spine or horn near the tip of the abdomen.
These hornworms are known to feed on a variety of plants which include portulaca, primrose, wild grapes, and a few types of succulent trees. Despite a list of host plants, they do appear to be very picky eaters. Over the past few years, I have noted them mostly feeding on purslane weeds, almost completely ignoring other plants and healthy foliage nearby.
The massive group population and potential movement of the white lined sphinx caterpillars can be very impressive, with many describing it as downright intimidating. These worms only seem to appear in mass populations of at least thousands (if not by the million) with just one local generation annually.
The larva will feast on their preferred vegetation, devouring it quickly and fully before moving to find another pocket of that preferred vegetation with a mass migration that many describe with a statement like, “the ground seemed to be moving and wiggling.” Meanwhile the pasture, sorghum, and cotton plants they were near or fields they were in are left almost completely untouched with any damage being coincidental and minor, usually to field margins.
Just about all of my encounters with this hornworm over the past 15 years have ended pretty well. The sheer number of veracious larva of this size moving into production fields or healthy pasture is enough to make anyone nervous and is most certainly worth keeping daily tabs on.
To date, I have never been forced into recommending treatment for these hornworms. The fields they have infested are eventually left almost completely unscathed with a serious hit to the weed population following either their migration away from the field, or eventual pupation of the larva. I can state with some confidence this pattern of nerve racking benefit to production fields should continue this season.
I cannot be nearly so certain of a respectable out-come of this hornworm’s impact to a grape vineyard or garden plot, horticultural or vegetable. There is likelihood that something within those situations could be just what the white lined sphinx moths picky diet would find delicious.
If treatment for these hornworms becomes necessary it is believed a good general pyre-throid would do a good job of achieving control. Thus far, there has not been a need for an efficacy trial in this region so this assumption has not been verified. By incident I have noted that bollgaurd cotton offers full protection against these hornworms.