Impact of predation by Laricobius nigrinus (Coleoptera: Derodontidae) and Leucotaraxis spp. (Diptera: Chamaemyiidae) on Adelges tsugae (Hemiptera: Adelgidae) and Tsuga canadensis (Pinales: Pinaceae) tree health

The hemlock woolly adelgid (HWA), Adelges tsugae Annand, (Hemiptera: Adelgidae) is an invasive species in the eastern United States and Canada. This scale-like insect feeds on the xylem ray parenchyma cells of eastern hemlock, Tsuga canadensis (L.) Carrière, (Pinales: Pinaceae) and Carolina hemlock, Tsuga caroliniana Englem., resulting in multiple physical and physiological symptoms, all of which commonly leads to tree mortality within 4 to 10 years. Currently, HWA has spread throughout most of the eastern hemlock range and all of the range of Carolina hemlock and threatens the forest ecosystems they serve. In its introduced range, there are no natural enemies specialized to feed on HWA and suppress HWA populations below damaging levels. In the urban environment and at some public locations, the use of chemicals, such as imidacloprid, have been used to temporarily protect hemlocks, however; this does not provide a permanent solution. In the 1990's a classical biological control program for HWA was created with the goal to suppress HWA populations below damaging levels to reduce tree mortality, and to reduce the negative effects associated with the loss of either hemlock species. Presently, four predators have become the main focus of this program: Laricobius nigrinus Fender, Laricobius osakensis Shiyake and Montgomery (Coleoptera: Derodontidae), Leucotaraxis argenticollis (Zetterstedt), and Leucotaraxis piniperda (Malloch) (Diptera: Chamaemyiidae). Throughout the eastern US, La. nigrinus has established populations and expanded its distribution beyond where it has been released. Although it has successfully established and continues to spread, its effect on eastern hemlock health has been unknown. A 1-year study, carried out in 2021, revealed that La. nigrinus predation on the sistens generation not only reduced the sistens generation, but also had a similar net photosynthetic rate to the negative control on 1-2 year old shoot growth. In addition, significantly more shoots were produced for treatment branches with the highest La. nigrinus density. This also led to a significantly reduced aestivating sistens density compared to the no predator treatment. However, predation was insufficient to reduce HWA populations below the suggested damaging threshold of eastern hemlock (< 4 HWA/cm), suggesting that additional predators would be needed. Recently, the western strain of Le. argenticollis had been released in the eastern US. However, its phenology in the eastern US was unknown. A 2-year study, from 2021-2023, revealed that Le. argenticollis eggs and larvae were present when both generations of HWA adults with eggs and HWA nymphs were present. In addition, Le. argenticollis was capable of completing development in NY and VA, showing promise that this species could establish populations in the eastern US. Timing the adult fly releases into cages at the time when sistens were producing eggs resulted in Le. argenticollis phenology to synchronize well with HWA phenology at both locations, thus providing additional support for its release in the eastern US. Another 2-year study conducted in 2021-2022, at five locations (MD, NC, PA, TN, and VA), sought to determine how the predation of La. nigrinus and Le. piniperda, alone and together, would impact the sistens and progrediens generations, as well as how their predation would affect the aestivating sistens density and new shoot growth production. From this study, La. osakensis was found as the dominant Laricobius species present at the TN site, and similarly to La. nigrinus, reduced intact ovisac density in. This provided a small preview in La. osakensis capabilities and how this predator may be impacting HWA populations at locations where it has established. In order to assess Le. piniperda's impact on the progrediens generation, mesh cages were applied to treatment branches at each site. The study found that Le. piniperda could reproduce at all field sites and that the percentage of disturbed progrediens was significantly higher for treatments with Le. piniperda present at all field sites, except at PA in 2022. However, the affect of both of these predators on the aestivating sistens density and eastern hemlock health was inconclusive. In addition, the percentage of new shoot growth was highly variable among treatments at all field sites, and there was no treatment effect on the mean length of new shoot growth produced at all field sites. The results of this study suggests that in order to fully understand the affects of these predators on HWA populations and on hemlock tree health, longer term studies should be conducted. However, this study does provide some insight on the capabilities of these predators in the short term and supports the continued release and redistribution of these predators.

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