MOVEMENT PATTERNS AND FORAGING TACTICS OF A CATERPILLAR HUNTER INHABITING ALFALFA FIELDS

Hunger level influences a predator's motivation to search for food. Whilst foraging a predator may be faced with situations involving a trade-off between energetic gain during feeding or the pursuit of other activities that are necessary for survival and reproduction. The movements of the caterpillar hunter Calosoma affine Chaudoir were studied in short (recently cut) and full-grown (tall) alfalfa by tracking beetles in the field at night, using a portable harmonic radar system. Full-grown alfalfa contained high densities of prey for Calosoma affine (during an outbreak of alfalfa caterpillars/armyworms), while stubble fields, i.e. short alfalfa, represented low prey densities. Hungry Calosoma affine displayed directed movements in short alfalfa (low prey density), whereas the movement pattern of hungry beetles in full-grown alfalfa (high prey density) could be described as a correlated random walk related to near-search activities; foraging inside the base of crown stems. Satiated beetles moved randomly, i.e. displayed a correlated random walk within both short and full-grown alfalfa. Satiated beetles, only, climbed and foraged in the plant canopy. The longest distance covered and the velocity were highest for hungry beetles tracked in short alfalfa. A significant reduction in speed was obtained for hungry beetles in full-grown alfalfa, while satiated beetles exhibited a similar speed within the different growth stages of alfalfa. Time to ingest caterpillars/armyworms, biomass gained per minute, and the amount of prey traces left after feeding, increased with increasing size of the prey. Mean biomass ingested increased with increasing temperature. However, the overall rate of ingestion decreased with time due to satiation. The influence of motivation on predatory movements displayed by adult Calosoma affine within short and full-grown alfalfa is compared with differences in prey density and prey availability. Moreover, the foraging behaviour exhibited by Calosoma affine within extrinsic (temperature) constraints, and the importance of energy requirements for various parameters of the time budget are discussed.