Summary -Infective juveniles (IJ) of three Heterorhabditis megidis isolates, HF85, EU17 and UK211, were stored in water at 20 • C for up to 10 weeks. At 2-week intervals, activity, infectivity, energy reserves and survival were measured. There was no difference between the three isolates in infectivity, which increased significantly over the first 2 weeks and declined gradually thereafter. IJ became inactive during storage. Out of storage, the highest activity was recorded in week 0: nearly all IJ were active within the first minute of observation and remained active for the 20-min observation period. With increasing storage time, an increasing proportion of IJ were inactive in the first minute, reaching 83-96% by week 6. The time taken by 50% of the IJ to become active (AT 50 ) initially increased with nematode age, reaching a maximum of 3-7 min in week 4 or 6 (depending on isolate) but subsequently declined to 2-4 min in week 10. By the time the IJ were becoming more readily activated in weeks 6-8, 75% of the lipid reserves had been depleted, and IJ had started to die. This greater propensity to become active with age may represent a switch to risk-taking behaviour in the face of impending starvation.

The eVect of storage temperature (9 and 20ë C) on North West European Heterorhabditis megidis isolate UK211 for control of Otiorhynchu s sulcatus larvae at 9ë C is assessed. O. sulcatus mortality increased from 2 5.3% (corrected mortality) using freshly produced nematodes, to 27.1% using nematodes that had been cold-stored for 12 weeks. The number of nematodes invading the insect larvae increased almost 27-fold. Nematode storage at 9ë C for 11 to 12 weeks weeks resulted in signiWcantly higher O. sulcatus mortality (41%) than storage at 20ë C for 2 to 3 weeks (12%). Thus, cold storage does enhance nematode infectivity for O. sulcatus larvae.

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. IntraspeciWc 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 Wrst 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 intraspeciWc 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.

The fatty acid composition of three North West European isolates of Heterorhabditis sp. from different geographical origins, UK211 (England), HF85 (The Netherlands) and EU17 (Estonia) was assessed directly after harvest and, for UK211 and HF85, after 5 weeks storage in water at 20°C. Lipid represented 34 -43% of the dry weight of fresh nematodes. Of this, neutral lipid (NL) comprised from 70% (HF85) to over 90% (UK211, EU17). The fatty acid patterns were similar between the three isolates. Oleic (C18:1n-9), palmitic (C16:0), and linoleic (C18:2n-6) acid predominated with 51, 13 and 12%, respectively in the total lipid (TL) of fresh nematodes (average for the three isolates). Levels of unsaturation (U.I.) of fresh nematodes were on average 110, 112, 113 and 152 for the TL, NL, phospholipid and free fatty acid fractions, respectively. EU17 had a slightly lower U.I than the other two strains, despite its more northern origin. Changes in fatty acid composition due to storage were most significant in the NL fraction. The U.I. for the NL fraction increased during storage, suggesting a preferential use of saturated fatty acids.