Defoliation by introduced Diorhabda elongata leaf beetles (Coleoptera: Chrysomelidae) reduces carbohydrate reserves and regrowth of Tamarix (Tamaricaceae)

Diorhabda elongata (Brullé) sensu lato leaf beetles have been released in the United States for the classical biological control of invasive Tamarix L. species, which are exotic trees that are causing deterioration of riparian ecosystems in western North America. The impact of D. elongata defoliation on Tamarix nonstructural carbohydrates (NCHOs) was measured in both manipulative field cage, and non-manipulative natural experiments. Additionally, spring above-ground growth was measured following beetle defoliation in manipulative field cage experiments in Texas. There was no significant difference in the proportional change in NCHOs between beetle-damaged and undamaged control-treatment trees in the manipulative field cage experiment. However, spring above-ground regrowth was reduced by 35% on trees which experienced beetle defoliation the previous fall. In the natural experiment, root crown tissue was sampled in 2005 and 2006 from stands near Lovelock, Nevada in which trees had experienced 0–4 years of beetle defoliation. In 2005, mean NCHO concentrations were statistically different between tree stands and ranged from 9.0 ± 0.8% (mean ± SE) in trees that had not been defoliated to 3.2 ± 0.4%, 2.1 ± 0.4% and 2.3 ± 0.4% in trees in stands that had been defoliated for 1, 2 and 3 successive years, respectively. In 2006, NCHO concentrations were again statistically different between stands and ranged from 13.6 ± 0.9% in trees that had not been defoliated to 7.6 ± 0.8%, 2.3 ± 0.4%, 1.5 ± 0.3% and 1.7 ± 0.4% in trees in stands that had been defoliated for 1, 2, 3 and 4 years, respectively. These results indicate that D. elongata herbivory reduces nonstructural carbohydrates and inhibits regrowth which may lead to reduced survival and reproduction of Tamarix.