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Title: Lack of establishment of the Mediterranean tamarisk beetle Diorhabda elongata (Coleoptera: Chrysomelidae) on athel (Tamarix aphylla) (Tamaricaceae) in south Texas

item Moran, Patrick

Submitted to: Southwestern Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2010
Publication Date: 7/12/2010
Citation: Moran, P.J. 2010. Lack of establishment of the Mediterranean tamarisk beetle Diorhabda elongata (Coleoptera: Chrysomelidae) on athel (Tamarix aphylla) (Tamaricaceae) in south Texas. Southwestern Entomologist. 35:129-146.

Interpretive Summary: Saltcedars (also known as tamarisks, and in Mexico as cedros salados) are widespread and destructive non-native shrubs or small trees in the western U.S., occupying millions of acres especially along wild rivers, removing precious water from desert farmlands, cities, and river systems, and making survival difficult for some native wildlife while perhaps helping some bird species. Since 2000, the U.S. Department of Agriculture-Agricultural Research Service has spearheaded a national effort to control saltcedars using insects introduced from Asia and the Mediterranean that eat the leaves and stems. These insects, known as tamarisk leaf beetles, have been released in 10 western U.S. states and have removed all of the leaves from over 100,000 acres of saltcedar so far. In South Texas and all along the U.S.-Mexico border, large trees known as athel, a close, non-native relative of saltcedars have been planted to provide shade and break the wind and dust on homesteads and ranches in Mexico and, to a lesser extent, in the desert southwestern U.S. Previous studies had shown that the tamarisk beetles prefer saltcedar over athel for laying eggs, and attack saltcedar first, going on to nearby athel saplings only when saltcedar leaves have been depleted. However, no previous study had asked the question: ‘What happens when beetles encounter big athel trees and there is no preferred saltcedar available?’. This study showed that tamarisk beetles damaged small 4-year-old athel trees when confined in field cages, but the trees recovered within one month. In the open field, the tamarisk beetles rapidly disappeared and did no lasting damage to large (30-40 feet tall) athel trees growing along two highways, even though each tree received about 400 beetles over a two-year period. This study concluded that ongoing releases of tamarisk beetles along the U.S.-Mexico border will not negatively affect large beneficial athel trees.

Technical Abstract: Adult Mediterranean tamarisk beetles, Diorhabda elongata (Brullé), a defoliator of exotic saltcedar (Tamarix spp.), were released into four field cages containing small saltcedar trees or closely-related exotic athel trees (Tamarix aphylla (L.). Karsten) and onto uncaged beneficial mature athel trees at sites in south Texas. In field cages, egg, larval and adult densities per m green foliage were equal on saltcedar and athel in the first 8 wk after release, but egg, larval, and adult numbers counted were at least two-fold higher on saltcedar. During this time, the beetles caused a 50% decline in green foliage on athel and a 20% decline on saltcedar. However, athel green foliage recovered after 10 weeks, while saltcedar foliage continued to decline, leading to starvation. At two roadside field sites containing mature athel and no saltcedar, D. elongata adults were released in bagged athel branches in July 2006 and May 2007 and produced larvae that defoliated these branches by the third wk, when bags were removed. A small number of larvae and adults were found on unbagged athel branches during the first 3 months after release, but beetles failed to establish populations on athel trees and removed no more than 3% of green foliage from athel branches, which were over 85% green through 2 years. The Mediterranean tamarisk beetle thus did not colonize athel or cause disfiguring damage to athel trees.