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Tamarisk Beetles Move into Arizona

August 5th, 2020
After years of their anticipated arrival, tamarisk beetles have arrived on the Gila River in Arizona. Huge populations of beetles were seen earlier this year on both Imperial and Cibola NWRs, swarming in tremendous numbers in the evenings and landing on everything (including river recreationists) in search of green tamarisk. Beetles have since moved east from the Colorado River up the Gila from the city of Yuma. 
In the meantime, beetles have also descended on the Gila in the city of Buckeye, moving down the Hassayampa River. Several years ago beetle populations had moved from the Colorado, up the Bill Williams River and east as far as Wickenberg. There were very small numbers of beetles found the last few years along the Hassayampa, but this is the first year they have made a definitive push to the south and reached the Gila.
Both the populations in Yuma and Buckeye are extensions of the Northern Tamarisk Beetle (Diorhabda carinulata) that has moved down the Colorado River from both releases in Grand County, Utah (near the city of Moab) and releases in St. George, Utah, where they moved down the Virgin River.
Recent data from Dr. Dan Bean with the Colorado Department of Agriculture’s Palisade Insectary has shown that these lower latitude populations of the Northern beetle have evolved beyond physiological restraints that determined when populations of beetles entered diapause (akin to hibernation for insects). The release of this restraint has allowed these populations to persist much longer into the season (e.g. we now see beetles out and about in November when they used to disappear in September), having more generations of beetles and allowing populations to spread farther in a single year.
Beetles have also been found in the Gila Box section of the river, near Safford, Arizona. These beetles represent a different species, the subtropical tamarisk beetle (Diorhabda sublineata), and likely originated from populations in New Mexico. They were first discovered on the San Francisco near Clifton in early June by Gila Watershed Partnership (GWP) staff. This section of the river is just upstream of very large monotypic stands of tamarisk, through which the beetles are expected to move quickly. These large stands of tamarisk are utilized by the endangered southwestern willow flycatcher (SWFL). In expectation of the beetles’ arrival, efforts by GWP have been underway for several years to replace tamarisk with native willow and provide alternate nesting habitat for the birds.
It is yet to be seen how the beetles will affect SWFL populations on the Gila River, but it is possible that their arrival in key nesting areas occurred after the nesting season this year. Thus current-year eggs/fledglings would not be adversely affected and when the birds return in 2021 tamarisk will already be defoliated and they will utilize available native species for nesting. As beetle population movements are unpredictable in where and when they occupy a certain area, we cannot know for sure what will happen, but it may be a “best case” scenario for SWFL and beetle interactions in the Safford area.
Though tamarisk beetles are now in several new areas of Arizona, it is pertinent that these populations are allowed to spread on their own and are not helped by human hand. The original permit for tamarisk beetle release, issued by the USDA - Animal and Plant Health Inspection Service, requires that any intentional releases by people cannot be within 200 miles of a known southwestern willow flycatcher nest in tamarisk. This permit requirement virtually eliminates release within the state of Arizona, and definitely excludes all areas of southern Arizona. As supported by postulations from an expert panel, the beetle will eventually occupy all riparian areas of Arizona. We encourage landowners and organizations to utilize the time they have before beetles arrive in their area to accomplish native restoration projects to mitigate vegetation/habitat loss when the beetles do arrive and defoliate existing tamarisk stands.  
To learn more about RiversEdge West's work to monitor the movement of the tamarisk beetle, visit our website or watch our recent webinars on the tamarisk beetle and its effects on SWFL. 
More information about restoration work conducted by GWP to support SWFL populations can be found here.   

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