Effect of tenthredinid leaf miner invasions on growth of Alaska white birch in Anchorage, Alaska, USA, and the interaction with biological control of amber-marked birch leaf miner

The amber-marked birch leaf miner, Profenusa thomsoni (Konow) (Hymenoptera: Tenthredinidae), invaded the Anchorage region in southeastern Alaska, USA, about 1991 and by 1996 caused high levels of mining, resulting in browned foliage annually by Aug during the outbreak years of 1996 to 2007. A biological control program began in 2004 based on the importation of the larval parasitoid Lathrolestes thomsoni Reshchikov (Hymenoptera: Ichneumonidae), which previously had suppressed an outbreak of amber-marked birch leaf miner in Alberta, Canada. By 2008, this introduced parasitoid and 2 resident species (native or self-introduced) reduced damage to Alaska white birch by > 50%. In 2020, as a follow up, 100 Alaska white birch in forested parks in Anchorage were cored to see if leaf mining had reduced tree growth during the outbreak and whether biocontrol of amber-marked birch leaf miner allowed tree growth to recover. Compared to 12 yr (1984-1995) before the start of the outbreak, radial growth was suppressed by approximately 16% during the outbreak (1996-2007). During the 11 yr after suppression of damage through biocontrol (2008-2018), tree radial growth of Alaska white birch did not recover, but rather declined further. This pattern may be due to the invasion around 2008 of another birch leaf miner, Heterarthrus nemoratus (Fallen) (Hymenoptera: Tenthredinidae). These findings show that outbreaks of birch leaf miners caused an important drop in growth of Alaska white birch, which has implications for forest productivity and carbon sequestration by birch-dominated forest stands in southeast Alaska.