Physiological ageing and behavioural plasticity of Heterorhabditis bacteriophora infective juveniles

Heterorhabditis bacteriophora is a cruiser strategist that exhibits long distance ranging searches for insect hosts in soil. We characterised age-based variations in constituent behaviours and total lipid reserves of laboratory-aged H. bacteriophora infective juveniles. Individuals demonstrated extensive behavioural variability within the first 7 days, followed by accentuated ranging behaviour during the next 14 days and, finally, a decline in activity during the last 15 days. Frequency, bout duration and total time investment decreased almost 50% for forward crawling but almost doubled for backward crawling and curling. Both pausing and head waving frequency, a theoretical correlate of searching intensity, varied considerably without relation to age. Lipid content declined steadily, decreasing by 56% after 28 days. Behavioural changes became apparent after a 20% reduction in total lipid, and performance continued to decline with further lipid depletion. These findings emphasise the combined influences of environmental and physiological factors on behavioural plasticity in entomopathogenic nematodes.