Stress avoidance: vertical movement of entomopathogenic nematodes in response to soil moisture gradient

Abstract

Survival of entomopathogenic nematodes (EPNs) in soils is attributed to their entering a dormant state—anhydrobiosis—as soil moisture decreases, but EPNs with poor desiccation tolerance and low anhydrobiotic capabilities may practice desiccation avoidance. We compared the effect of soil moisture gradient on downward movement of the highly desiccation-tolerant Steinernema carpocapsae and the poorly desiccation-tolerant Heterorhabditis bacteriophora. Infective juveniles (IJs) were applied to the surface of moist (11–13% w/w moisture) sandy soil in buckets. Nematode distribution was monitored at different depths 3, 14 and 28 days after application. In uncovered buckets, soil moisture decreased to 1% in the upper 5-cm layer after 28 days. H. bacteriophora IJs abandoned the upper soil layers as dryness intensified with >80% found in the bottom (20–25 cm) layer. In contrast, >70% S. carpocapsae IJs remained in the upper layer. In covered buckets, with 10% moisture throughout the experiment, heterorhabditid IJs were equally distributed between the 10–15 cm and 20–25 cm layers; only 7% remained in the upper layer. Again, >70% S. carpocapsae IJs remained in the upper layer throughout. Soil type influenced H. bacteriophora IJs' downward migration. In sandy and sandy loam soils, with rapid evaporation, >80% IJs were in the bottom layer 14 and 28 days after application. In the loam soil, with higher moisture retention, >75% IJs remained in the 10–15 cm layer and <20% migrated to the bottom. Results provide initial evidence of a possible stress-avoidance strategy in H. bacteriophora under natural conditions.