Observations on the Fall Movements of Isabella Moth Larvae (Lepidoptera: Erebidae: Arctiinae) in Northwest Missouri

Abstract. Despite their visibility, abundance, and widespread distribution, the movements of Isabella moth larvae (a.k.a “woolly worms:” Pyrrharctia isabella (J.E. Smith); Lepidoptera: Erebidae: Arctiinae) have been poorly studied. We observed these larvae to assess the relationship between their movement and local environmental conditions. Twice each day for 83 d, nearly 52 km of highway were observed in northwest Missouri. A total of 264 larvae were encountered during 664 total visits to four road sections, peaking in the first week of October. Analyses revealed increased larva abundance (n ≥ 159 road-section visits) associated with temperature, wind speed, pale pavement tint, and afternoon time-of-day. The best overall model to explain larval movement patterns, based on 163 visits and all measured conditions, incorporated temperature, north wind speed, west wind speed, and humidity (P = 0.015), but no other factors. In addition, larvae showed a significant tendency to travel in the same direction as the wind, and to travel eastward in the afternoons. These observations provide insights into the fall movements of a common larva, and provide directions for future research.