YAMPA RIVER LEAFY SPURGE PROJECT
Inundative Biological Control Backstory
The term "inundative biological control" in association with leafy spurge has its origins in a 2010 scientific report entitled Inundative Release of Aphthona spp. Flea Beetles (Coleoptera: Chrysomelidae) as a Biological "Herbicide" on Leafy Spurge in Riparian Areas (click on title to read the article, or find it available on our Leafy Spurge Documents page).
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The authors of the experiment described in the 2010 report were looking for novel ways of treating small, isolated populations of leafy spurge growing in riparian environments subject to periodic flooding. While the biological control of leafy spurge with Aphthona in drier, upland environments has been quite successful, there have been fewer successes reported in riparian areas. Aphthona are particularly effective at leafy spurge control because the larval stage of the beetle’s lifecycle is spent below the soil surface, where they feed on the leafy spurge roots. The damage they do to the root system can seriously compromise the spurge's ability to grow, flower, and set seed. However, larval survival can be compromised if the soils are flooded.
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The authors of the report began with the assumption that the presence of periodic flooding in riparian area means that Aphthona are unlikely to establish long-term, sustainable populations—at least, not at high enough levels to significantly damage the leafy spurge. However, they hypothesized that a single massive, "inundative" release of adults during the early summer could result in a huge late-summer and fall crop of larvae, which then might cause substantial devastation to the leafy spurge in the short term. It was essentially a rethinking of the potential use of biocontrol agents, and as the report's title suggests, applying them to leafy spurge as if they were a single-use herbicide.
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As it turned out, the investigators' primary hypothesis was borne out—and the damage to the leafy spurge canopies in their plots the following year was reported to be an impressive 60 to 80 percent reduction in the biomass of established plants, as well as a corresponding decline in new seedling density.
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This was an intriguing experiment, with some interesting results, but not entirely applicable to the treatment of leafy spurge populations in the Yampa River basin’s riparian habitats. At the very least, those impressive leafy spurge damage statistics were only found in the plots where adult beetles had been released at the highest concentrations—a rate of 50 per leafy spurge flowering stem. Perhaps this is a feasible rate to apply to a small, isolated patch, but blanketing large areas of the Yampa River infestations at that rate is simply not practical. Using Aphthona as a one-shot biocontrol "herbicide" will probably never catch on in the Yampa River basin.
Nevertheless, the basic premise that more adults released means more larvae later munching on the leafy spurge roots has made good sense to the YRLSP. Our own version of an inundative biological control strategy is looking like a good fit with our goal of rapidly establishing large, persistent, and effective populations of Aphthona (and Oberea) in the Yampa River's floodplain.