Effect of soil moisture on overwintering pupae in Spodoptera exigua (Lepidoptera: Noctuidae)

Spodoptera exigua Hubner (Lepidoptera: Noctuidae), can overwinter as pupae at depths of 0- to 5-cm beneath the ground. In order to understand the effect of soil moisture on the overwintering success of this species, we tested the supercooling point and survival of pupae, and the growth rate of prepupae that were exposed to different temperature and soil moisture in the laboratory. Results showed that supercooling points, body water contents, and survivals after the different pre-treatments were not significantly affected by the soil moisture. The developmental progress of prepupae, survival of prepupae and pupae were negatively correlated with soil moisture as well as the delay of exposure time, which supports the hypothesis that soil moisture acts as a developmental modulator. We presumed that the delay of pupation and lower prepupae survival under higher soil moisture was due to lack of a complete pupal chamber for protection at low temperatures. Low pupal survival was likely attributed to lack of oxygen in the soil, especially under the condition of higher soil moisture. We suggest that using strategies of irrigation and soil tillage during winter may decrease the overwintering population of S. exigua from the perspective of integrated pest management.