Resource utilization in the sibling species Trupanea nigricornis (Coquillett), a polyphage, and the narrowly oligopragous T-bisetosa (Coquillett) (Diptera : Tephritidae) in southern California

The flower heads of the hosts of the sympatric, morphologically similar, pr obable sister species, Trupanea nigricornis (Coquillett) and T. bisetosa (C oquillett) offer an annual, stable food source in southern California. In t his study, larger flower heads supported the development of more larvae per head than smaller flower heads. The size of the puparia of T. nigricornis was found to vary among the different hosts and with the density of larvae per flower head. The larvae of T. nigricornis and T. bisetosa exploited the flower heads in a similar way, eating their way downward from the florets first, then into the achenes. The only difference detected was in the secon d instars of T. bisetosa which continued to feed on the florets compared to the second instars of T. nigricornis which fed on the achenes. This variat ion reflected the larger achene size of the hosts of T. bisetosa. The perce ntage of damaged achenes per flower head was found to be proportional to th e size of the flower head. In general, a single T. bisetosa larva damaged f ewer achenes per head than a T. nigricornis larva because of the relatively larger achene size and flower head diameter of T. bisetosa hosts. Larvae o f both tephritids did not exhaust all their food resources in a particular flower head regardless of the size of the flower head and the density of th e infesting larvae. In general, the percentage of flower heads infested by T. bisetosa was lower than the percentage of flower heads infested by T. ni gricornis. Plant hosts with larger flower heads did not support more insect species than hosts with smaller heads. There was little evidence of inters pecific competition between the various insect species exploiting the flowe r heads of T. nigricornis and T. bisetosa because of the many evasion strat egies adopted by those insects to minimize interspecific contact.