INTRAGUILD PREDATION AND CANNIBALISM IN ODONATE LARVAE - EFFECTS OF FORAGING BEHAVIOR AND ZOOPLANKTON AVAILABILITY

The foraging behaviours and vulnerabilities to predation of larvae of four odonate species were studied in the laboratory. Predation by large Aeshna juncea larvae was compared in the presence and absence of zooplankton, and with odonate species presented individually and together. The large larvae of A. juncea showed high foraging activity irrespective of odonate prey treatment. Predation by A. juncea was lowest on Coenagrion hastulatum larvae, which had a ''sit and wait'' foraging mode, and highest on actively foraging Leucorrhinia dubia larvae. However, both activity and predation on L. dubia were lower in the presence of zooplankton and large A. juncea, perhaps reflecting avoidance of predation risk when foraging. Small larvae of A. juncea showed about the same foraging activity and vulnerability to predation as did L. dubia. Cordulia aenea larvae had intermediate foraging activity levels which were lower in the presence of zooplankton, but suffered high predation, indicating a low odonate predator escape ability. When all four odonate prey larvae were presented together. C. hastulatum and C. aenea experienced the lowest and highest predation, respectively, whereas predation on A. juncea and L. dubia was intermediate. About 25% of the large A. juncea larvae from field populations had preyed on L. dubia and C. hastulatum. The low abundances of A. juncea and C. aenea in the study ponds may explain why these species were not found in the fecal pellets of large juncea larvae. The results show that large A. juncea larvae frequently prey on odonate larvae. The intensity of this predation depends on the foraging behaviour of prey odonate larvae. However, the presence of alternative prey (zooplankton) and escape ability of the odonate larvae may also be important.