Laboratory and field evidence of post-release changes to the ecological host range of Diorhabda elongata: Has this improved biological control efficacy?

A large-scale biological control program against saltcedars (Caryophyllales: Tamaricaceae) has been implemented in the western United States. Repeated releases of the biological control agent, Diorhabda elongata (Coleoptera: Chrysomelidae), have resulted in delayed establishment success in northern California. Evidence that the target weed species at northern California release sites is a lower ranked host led us to test for shifts in host preference of D. elongata, by comparing the field-established population to its source laboratory colony. In greenhouse experiments, laboratory-colony D. elongata exhibited ovipositional preference for Tamarix ramosissima over Tamarix parviflora, whereas D. elongata reared from the field population had no significant preference between the two host plants. In an open-field host choice experiment conducted prior to widespread field-establishment (2006), higher oviposition and number of adult beetles over time was found in T. ramosissima treatments when compared to T. parviflora-only and mixed-host treatments. We repeated the open-field host choice experiment in 2008 for the laboratory colony and beetles reared from the field-established population. Field-established beetles remained longer than laboratory-colony beetles, irrespective of host. Significantly more laboratory-colony D. elongata remained on T. ramosissima treatments, but there was no treatment effect on field-established beetles. Our findings suggest the relaxation of a host acceptance threshold by field-established D. elongata, which may have improved performance on the target weed, T. parviflora. If such trait shifts are common, then strategies implementing repeated releases may not improve establishment, and studies comparing pre- and post-release realized host use by biological control agents may be useful to detect novel host use patterns.