Tamarisks are shrubby trees that include ten North American species in the genus Tamarix and their hybrids. They are invasive exotics in the arid and semi-arid regions of North America, altering riparian and aquatic ecosystems and are considered serious pests in most Western states. Control efforts using mechanical removal, herbicides, and fire have been extensive. Research into biological control of tamarisk began in the 1960’s; open releases of the tamarisk beetle Diorhabda elongata finally occurred in 2001. Grand County Weed Department first released adult tamarisk beetles into the county in August 2004; additional releases took place in 2005, 2006, and 2007. Limited observations near release sites were made from 2004 to 2006; Grand County Weed Department began systematically monitoring beetle abundance and tamarisk response in 2007. We selected sentinel trees throughout the county, each with a target branch identified. We visited the trees (52 sites) weekly from 1 June to 5 October 2007; in 2008, we added 59 sites to cover expansion of the beetle’s range. Each tree was visited once every 7 to 14 days from 5 May to 6 October 2008. We counted numbers of beetles (by developmental stage) on the target branch in 15 seconds, and assessed tree canopy condition (percent green, yellow, and brown).
Results/Conclusions
Areal extent of browning was estimated to be less than 1.6 ha in 2005, roughly 405 ha in 2006, rising to 4050 ha in 2007, and 646,279 ha by October 2008. Beetle numbers varied in each year across sites and over the course of the year; patterns differed in 2007 and 2008 relative to when average numbers peaked for adults and larvae as well as where maxima occurred. Defoliation of tree canopies (> 90% brown) in 2007 was first observed on 7 June; in 2008, 90% defoliation did not occur at target trees until 23 June. A second defoliation occurred on some trees in late August 2007, but this did not occur in 2008.
Studies in the lab and in field cages do not provide data on dynamics of beetle-tamarisk interactions at the landscape level. Beetle populations “slosh” back and forth along riparian corridors, defoliating tamarisk, dispersing to new territories; subsequent generations then re-colonize re-foliated stands. Timing of defoliation, re-foliation, and re-colonization over a season or over years determine how quickly D. elongata kill tamarisks. Understanding this dynamic process is essential for managers trying to balance tamarisk control with other resources.
