Coexistence of three specialist predators of the hemlock woolly adelgid in the Pacific Northwest USA

Abstract

The hemlock woolly adelgid (Hemiptera: Adelgidae: Adelges tsugae Annand) is an invasive insect, introduced from Japan to eastern North America, where it causes decline and death of hemlock trees. There is a closely related lineage of A. tsugae native to western North America. To inform classical biological control of A. tsugae in the eastern USA, the density and phenology of three native western adelgid specialist predators, Leucopis argenticollis (Zetterstedt), Le. piniperda (Malloch) (Diptera: Chamaemyiidae), and Laricobius nigrinus Fender (Coleoptera: Derodontidae), were quantified in the Pacific Northwest. Infested branches were collected from western hemlock (Pinaceae: Tsuga heterophylla (Raf.) Sarg.) at four sites around the Puget Sound, Washington and three sites in Oregon. Immature Leucopis were identified to species using DNA barcodes. Leucopis argenticollis was roughly twice as abundant as Le. piniperda. Laricobius nigrinus larvae were more abundant than the two species of Leucopis during the egg stage of the first adelgid generation, but Leucopis were present as feeding larvae during the second adelgid generation when La. nigrinus was aestivating in the soil, resulting in Leucopis being more abundant than La. nigrinus across the entire sampling period. Adelges tsugae and La. nigrinus densities were not correlated, while A. tsugae and Leucopis spp. densities were positively correlated. Leucopis spp. and La. nigrinus densities were negatively correlated. These results support the complementary use of La. nigrinus and the two Leucopis species for biological control of A. tsugae in the eastern USA, and point to the need for further investigation of spatial and temporal niche partitioning among the three predator species.

Phenology of Leucotaraxis argenticollis, a specialist predator of the invasive hemlock woolly adelgid, in the eastern United States

“…In the Pacific Northwest, Leucotaraxis spp. larvae were also found during the time when A. tsugae adults of both generations were producing eggs (Kohler et al 2008, 2016, Grubin et al 2011, Rose et al 2020), and when aestivating sistens were present (Grubin et al 2011). It is possible that Le.…”

Sensitive environmental DNA (eDNA) methods to detect hemlock woolly adelgid and its biological control predators Leucotaraxis silver flies and a Laricobius beetle

“…Data from eDNA surveys in conjunction with traditional sampling techniques can also be used to advance the ecological knowledge of target species. Previous studies have found a temporal overlap of these two Leucotaraxis species during the larval stage when aggregating observations from multiple locations (Dietschler et al, 2021; Rose et al, 2020), but that adult emergence is temporally asynchronous at the site level (Dietschler et al, 2021; Neidermeier et al, 2020). Our study only found both species in the same tree sample in two out of the 21 samples where Leucotaraxis were detected, with overlap occurring in only the May 13 and October 10, 2020, sampling periods.…”

“…During the time of western collections for this study, we could expect to find three larval instars or puparia (enclosed or intact) of Leucotaraxis flies, and larvae of La. nigrinus on western hemlock foliage (Dietschler et al, 2021;Grubin et al, 2011;Kohler et al, 2016;Rose et al, 2020;Zilahi-Balogh et al, 2003). For the enumeration of A. tsugae on eastern hemlock, approximately half of the sampled Measuring the foliage while collecting and dividing can increase the probability of eDNA contamination and was thus avoided.…”