By: Paul Oester, Extension Forester and David Shaw, Forest Health Specialist, Oregon State University Extension Service
David Shaw and Paul Oester have been leading research into whether the parasitoid wasps that were released to control the larch casebearer in the Blue Mts. of Oregon have persisted since their release in the late 1970’s. We report on our findings for the Blue Mts. here. Mailea Miller-Pierce and Ari DeMarco have been critical to this work.
The larch casebearer (Coleophora laricella) is a nonnative insect that has impacted western larch (Larix occidentalis) in its native range. Impacts were mitigated through a biological control program that released 7 species of parasitoid wasps, including Agathis pumila (Order Hymenoptera: Family Braconidae) (Figure 1) and Chrysocharis laricinellae (Order Hymenoptera: Family Eulophidae) (Figure 2) during the 1970’s. Roger Ryan, an entomologist with the US Forest Service PNW Research Station studied population dynamics of larch casebearer before and after establishment of these two wasps on 13 sites in the Blue Mts. of Oregon. He found they reduced populations of larch casebearer from an average of over 50 moths/per 100 larch buds (fascicles) to an average of 1.6. The biological control program is considered a major success.
Over the past decade, the larch casebearer has increased defoliation of western larch in the Blue Mts. of Oregon: total defoliation detected during USFS/ODF cooperative aerial detection survey was over 80,000 acres in 2007, and over 40,000 acres in 2008 (Forest Health Highlights in Oregon 2007-08).
This may also be confounded by foliage diseases, but regardless, during these years it is evident that larch casebearer was defoliating more western larch than expected. In 2009, aerially detected defoliation declined to 14,000 acres and in 2010 there were only 2,000 acres detected. This pattern is consistent with a parasite taking a little while to build up in the casebearer population and then really having an effect. However, since the mid-1990’s, no sampling has occurred to determine whether the introduced parasitoid wasps of larch casebearer are still present in this region.
Therefore we sought to reassess casebearer densities and parasitoid presence and abundance. In June 2010 we re-sampled Ryan’s sites and 16 additional ones at varying distances from his to determine landscape persistence. Agathis pumila was recovered at all 29 sites; Chrysocharis laricinellae at 14 (Table 1, Figure 3, 4). Twelve native species of larch casebearer parasitoid wasps were also found. At Ryan’s 13 sites, the mean density of the larch casebearer was 7.26 moths/100 buds compared to the 10-year average of 1.63 ending in 1995. Parasitism by Agathis pumila ranged from 1.8% to 53.4% , rates comparable to those in Ryan’s “after” period although a bit lower. Agathis pumila was the most abundant and widespread parasitoid suggesting that it may continue to be an effective control agent.
In summary, two of the introduced parasitoid wasp species are now well established in the Blue Mts,
while five of the released species were not recovered.