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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. |
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