Post hoc assessment of an operational biocontrol program: efficacy of the flea beetle Aphthona lacertosa Rosenhauer (Chrysomelidae: Coleoptera), an introduced biocontrol agent for leafy spurge

Mixed populations of Aphthona lacertosa and Aphthona czwalinae were released at more than 50 locations in Alberta in 1997. Two and 3 years post-release, beetle populations were primarily A. lacertosa, with A. czwalinae forming less than 0.5% of the sampled populations. Beetle densities were moderate (10–70 beetles per m²) or high (>70 beetles per m²) at 14% and more than 60% of the sampled sites in 1999 and 2000, respectively. Larger beetles had greater instantaneous egg loads (r²=0.424,P=0.003). In 2000, the largest beetles were found at moderate density sites and there was a significant negative relationship between beetle size and the time taken to accumulate a degree day threshold of 1230 (for females: r²=0.678,P=0.001). Sites with the most rapid accumulation of degree days have the greatest potential for beetle population growth based on potential fecundity. Changes in leafy spurge percent cover, stem density, and canopy height from 1997 to 2000 were assessed across sites with low (<10 beetles per m²), moderate, and high beetle densities in 2000. Sites with high beetle densities had significantly greater reductions of leafy spurge within 5 m of the release point than sites with low beetle densities (P<0.017). Damage caused by the beetles at high-density sites was often visible as a halo-shaped patch of dead leafy spurge stems. The significant overall reduction of leafy spurge within release patches makes A. lacertosa a promising biocontrol agent for leafy spurge in Alberta.