Pupation behaviour, depth, and site of Spodoptera exigua

The beet armyworm, Spodoptera exigua (Hubner) (Lepidoptera Noctuidae) is a persistent agricultural pest in many areas of the world. There is considerable controversy regarding the overwintering regions of S. exigua, although its pupae have been considered by many researchers as the most adaptable overwintering stage. Pupae are not consistently found in the field because of sheltered pupation sites. Direct evidence of S. exigua overwintering is only obtained by examining surviving pupae excavated during winter. In the present study, we observed the larval pupation behaviour of S. exigua in the laboratory, and investigated its pupation sites and depths in the field as parameters for field investigations during winter. Pupal survival in a natural pupal chamber at low temperature (4 degrees C) indicated that S. exigua has a protective mechanism against chilling injury. The pupal chambers were mainly found at depths of 1.1-2.0 cm in the field. Field investigation indicated that the pupation sites were located directly beneath plant canopies, and that soil structure influenced larval preference for pupation.