Patterns of host utilization by two parasitoids (Hymenoptera: Braconidae) of the eucalyptus longhorned borer (Coleoptera: Cerambycidae)

We studied the behavior and biology of the Australian parasitoids Syngaster lepidus Brullé and Callibracon limbatus (Brullé) (Hymenoptera: Braconidae), which are being introduced into California as biological control agents of the eucalyptus longhorned borer, Phoracantha semipunctata F. (Coleoptera: Cerambycidae). Studies conducted in Australia revealed that (1) female S. lepidus and C. limbatus actively searched the bark surface of eucalypt logs and apparently located their wood-boring hosts by substrate vibration or sound; (2) size of hosts selected for parasitism varied across parasitoid species and sexes - the largest hosts were parasitized by C. limbatus females, and the smaller hosts were parasitized by C. limbatus males, S. lepidus females, and S. lepidus males; (3) C. limbatus tended to dominate in logs that contained larger larvae, whereas S. lepidus were more abundant in logs with smaller larvae, suggesting that the two parasitoid species partition available host larvae; and (4) hosts of S. lepidus were under bark that was slightly thicker than that overlying hosts of C. limbatus, and this relationship was consistent with the relatively longer ovipositor of S. lepidus. Furthermore, (1) very thick bark (>17 mm) provided a refuge for borer larvae from both parasitoid species, (2) rates of parasitism averaged 27% across study sites and were inversely correlated with densities of host larvae in individual logs, and (3) rates of predation averaged only 6% and were positively correlated with abundance of host larvae in logs.