Survival, starvation, and activity in Heterorhabditis megidis (Nematoda: Heterorhabditidae)

Abstract

Infective juveniles (IJs) of entomopathogenic nematodes do not feed but have ample stored energy reserves and can survive for several months in soil or in water. Intraspecific variation in survival of Heterorhabditis megidis has been observed for eight isolates of H. megidis stored in water at 20 °C for up to 14 weeks with the 50% survival time (ST 50 ) ranging from 5.6 to 12.5 weeks. How important physical and behavioral attributes were in explaining this variation in survival was explored using stepwise regression. Included in the analysis were: initial energy reserves (measured by image analysis densitometry), size (area and length) the time at which 50% of energy reserves were depleted (ET 50 ), and motility (head movements/min in week 0). Energy depletion rate, ET 50 , is an important predictor of survival, explaining 93.8% of the variation in ST 50 among isolates. Energy depletion rate was highest in the first week, a time at which H. megidis IJs were spontaneously active during storage. By week 5, when IJs had begun to die, some individuals had completely depleted their energy reserves, supporting the hypothesis that death was a result of starvation. In a second stepwise regression, performed to explore what factors contribute to variation in rate of energy depletion, motility accounted for 60.1% of the variation in ET 50 . With the inclusion of initial energy reserves, length, and area, 88.5% of the variation was accounted for. We conclude that intraspecific variation in survival of H. megidis in water is due largely to variation in rates of depletion of energy reserves, which in turn is strongly associated with levels of locomotory activity.