Effects of Site Characteristics and Release History on Establishment of Agapeta zoegana (Lepidoptera: Cochylidae) and Cyphocleonus achates (Coleoptera: Curculionidae), Root-Feeding Herbivores of Spotted Knapweed, Centaurea maculosa

We evaluated several factors to identify features or practices that might increase the probability of establishment following the release of two root-feeding insects (the cochylid moth Agapeta zoegana L. and the weevil Cyphocleonus achates Fahraeus) that attack the invasive plant, spotted knapweed (Centaurea maculosa Lamarck). At each of 99 sites where releases had been made in previous years, we assessed (1) the number of insects of each species released, (2) the number of years in which releases were made, (3) the number of years since the first release, (4) the size of the knapweed infestation, (5) its shape (linear, large single patch, many small patches), (6) the knapweed plant density, and (7–18) 12 physical site characteristics: habitat type, elevation, percentage slope, aspect, topographic type, forest structure at or by the site, disturbance factors, land use category, percentage forest canopy at or near site, percentage bare soil, annual precipitation, and soil type. We found that continuous, nonlinear patches of spotted knapweed on loamy soil that were surrounded by even-age forest stands had the highest rates of establishment for A. zoegana, but likelihood of establishment was not greater for larger releases (>200 adults). This suggests that roadside strips of knapweed should not be selected as release sites and that many, smaller releases (100–200 adults) are better than fewer, larger releases (300–500 adults or greater). For the weevil C. achates, we found that the probability of establishment was also greatest in continuous knapweed patches, rather than in strips, and that larger infestations (>2 ha) were better than smaller infestations. Establishment was also highest at mid-elevations (750–1500 m), compared to higher or lower locations.