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Although conventional herbicide treatment of upland leafy spurge infestations has been the norm in the Yampa River Basin, the use of biological control agents also dates back to at least 1989. Unfortunately, in the early years recordkeeping was sometimes incomplete, and (with the exception of Tepee Draw) virtually no follow-up monitoring was ever conducted. In the absence of such data, the relative success or failure of these “legacy” biological control releases was never substantiated.
Leafy spurge biological control involves the introduction of one or more different insect species that have co-evolved with leafy spurge in its original old-world habitats. Unlike most herbicides, biological control is safe to use in riparian habitats, does not negatively impact desirable species, and has the potential to be self-sustaining. The most commonly released leafy spurge biological control agents in the Yampa River Basin have been multiple species of flea beetles in the genus Aphthona, as well as a stem-boring beetle (Oberea erythrocephala). Recent investigations have also revealed the presence of another introduced species for which no record of local release has been found, the leafy spurge hawk moth (Hyles euphorbiae).
The goal of biological control is control of the target species, rather than eradication, and the impact of biological control insects on leafy spurge can be quite different from conventional herbicide use. While herbicide applications may provide obvious plant mortality in a short time, biological control acts more subtly to reduce the leafy spurge's viability. Tall, dense, vigorously flowering stands will typically be reduced to patchier, sparsely flowering stands after the introduction of biological control. The result is reduced seed production—an important step towards reducing the waterborne seed load in the Yampa River and its associated irrigation delivery systems.
More information on leafy spurge biological control can be found on the YRLSP Resources page.
Leafy spurge biocontrol agents photographed locally
Left top and bottom: Leafy Spurge Flea Beetles (black and brown species)
Right top: Leafy Spurge Hawk Moth Caterpillar
Right bottom: Leafy Spurge Stem-Boring Beetle
Biological Control Monitoring
The effectiveness of leafy spurge biological control in the Yampa River Basin cannot be quantified without a long-term monitoring program. The YRLSP’s efforts towards establishing a biological control monitoring program began in 2018, with the research and compilation of all available records for legacy leafy spurge biological control releases in Moffat and Routt counties. The location and year of release for over 40 sites have been identified. For most of these release sites we also have additional information regarding the type and quantity of insects that were released.
Legacy Leafy Spurge Biological Control Releases Date Back Three Decades
Beginning in the summer of 2019, volunteer Tamara Naumann, with help from individuals associated with the local Colorado Master Gardner program, and local, county, state and federal land agencies, began revisiting these legacy biocontrol release sites. Using a sweep-net protocol, the current presence or absence of biocontrol insects at each legacy site was established, and the current condition of the leafy spurge infestation was assessed. Other habitat variables (e.g., geomorphic location, aspect, soil type and dominant vegetation) were also inventoried. After these inventories are completed in 2020, each site will be evaluated for its appropriateness for continued biological control monitoring in the future.
Preliminary results from Tamara's monitoring suggest that past releases of biological control species in the Yampa Basin have been far more persistent than was conventionally thought. At least one biocontrol species has been found at virtually every visited legacy release site where leafy spurge is still present. Aphthona, Oberea and Hyles have also now been observed in many additional locations, often many miles from any documented release site. For example, leafy spurge biological control species have now been observed in the Axial Basin, the Little Yampa Canyon, remote patches of leafy spurge in upland habitats northeast of Maybell, and along the Yampa River in Dinosaur National Monument.
The widespread presence of leafy spurge biological control insects throughout the Yampa River Basin, even in relatively low numbers, confirms their viability in these habitats, and suggests the possibility that over the last three decades biological control insects have already been slowing the growth of leafy spurge infestations.
Sweeping for leafy spurge biocontrol insects
YRLSP Biological Control Releases
YRLSP's monitoring data is already contributing to a better understanding of the resiliency of the various biological control species in association with differing environmental conditions. This information will be invaluable for establishing enhanced biocontrol populations through additional releases in the future. Ultimately, YRLSP hopes to establish a number of viable nursery sites for local-sourced beetle releases.
To this end, in cooperation with our partners at the Colorado Department of Agriculture (CDA) and the Yampa River State Wildlife Area (YRSWA), in 2019 the YRLSP began an accelerated leafy spurge biological control release program. In the first year 7,300 flea beetles collected by the CDA on the Front Range were released on or in the vicinity of the YRSWA.
In June 2020, YRLSP volunteers upped the ante by traveling to the Front Range to aid the CDA in their annual collection of leafy spurge biological control beetles. Under the tutelage of the CDA's John Kaltenbach, on the first day approximately 27,000 flea beetles were collected from the former Lowry Bombing and Gunnery Range east of Denver. Day two was then spent at the CDA facility in Broomfield, sorting the flea beetles from stray plant parts and the rest of the insect "bi-catch," before packaging them into 1000-insect lots for distribution by the CDA across Colorado. In return for our contributions, the YRLSP's share of the take was thirteen 1000-insect lots of Aphthona, and one lot of Oberea (Oberea is typically released in lots of only 100 insects).
Upon our return to the Yampa Yalley, two additional days were spent releasing the biological control insects at thirteen separate locations in Routt and Moffat counties.
Finally, in July 2020, YRLSP volunteers also helped release an additional 10,000 flea beetles on the Yampa River State Wildlife Area, purchased by the YRSWA management from a vendor in Montana. This brought the total for 2020 biological control releases in the Yampa Valley to approximately 23,000 insects. The combined release total for 2019 and 2020 in Routt and Moffat counties represents roughly a thirty-eight percent increase to the total number of biological control insects estimated to have been released over the entire previous three decades.