Host location of gall parasitoids—A case study using the <i>Quadrastichus mendeli</i>–<i>Leptocybe invasa</i> system

Abstract Galls are thought to be induced by insects for protection, yet many gall‐forming insects experience heavy parasitization. Little is known about how parasitoids locate gall‐forming insects. Quadrastichus mendeli Kim &amp; La Salle (Hymenoptera: Eulophidae) is an obligate ectoparasitoid of the larvae of Leptocybe invasa Fisher et La Salle (Hymenoptera: Eulophidae), which is an invasive gall‐forming pest in eucalyptus ( Eucalyptus spp., Myrtaceae) plantations. In this study, Q. mendeli and L. invasa were used to explore parasitoid behavior when locating gall‐forming insects. Our results revealed that host location by Q. mendeli involved five discrete steps: walking, resting or grooming, antennating, probing and stinging. Antennation was the most frequent behavior, accounting for about half of the total time. Host location behavior of Q. mendeli occurred in the photophase. Female Q. mendeli exhibited a clear circadian rhythm: Almost all behaviors occurred during the photophase, and behavior peaked at the sixth hour of the light phase. The olfactory response rates of Q. mendeli to volatiles from galls decreased significantly after their antennae were detached. However, there was no significant difference in the host location behavior of Q. mendeli after their eyes were obscured. Our results provide essential information to explore the host location mechanisms in gall parasitoids.