Laboratory rearing, establishment, subterranean survivorship, and thermal requirements of Laricobius spp. (Coleoptera: Derodontidae), biological control agents released for Adelges tsugae (Hemiptera: Adelgidae)

Open Access

Abstract

The hemlock woolly adelgid (HWA) (Adelges tsugae [Annand]) (Hemiptera: Adelgidae) is a non-native lethal pest to eastern hemlock (Tsuga canadensis [L.] (Pinales: Pinaceae), and Carolina hemlock (Tsuga caroliniana [Engelmann]) (Pinales: Pinaceae). In the early 2000's, a robust biological control initiative using Laricobius spp. (Insecta: Coleoptera) was launched with the goal of decreasing HWA's impact to hemlock and the associated ecosystems on landscape level. Since then, two Laricobius agents, Laricobius nigrinus Fender (Coleoptera: Derodontidae) and Laricobius osakensis Montgomery and Shiyake (Coleoptera: Derodontidae) have been evaluated, approved, and released. The production and subsequent release of these agents has largely occurred through mass rearing efforts by Virginia Tech. A descriptive analysis of these rearing and release data over the past 16 years at Virginia Tech revealed that the mass production of these agents has been consistently limited by: 1) excessively high rates of subterranean mortality and 2) early subterranean emergence. Very little is known about the subterranean life cycle of these species in the field in terms of survivorship and timing of emergence. Additionally, the thermal limits and rates of development are unknown for L. osakensis and were thus further investigated. In 2019, surveys within the urban environment in two counties in southwest Virginia were conducted to determine if these agents have dispersed from their original release location. Laricobius nigrinus was documented to have established outside of release sites in 100 and 75% of the surveyed grids containing HWA infested hemlocks. The establishments of these species in easily accessible locations allowed for the use of field-caught larvae, in addition to laboratory-produced larvae, for experiments. Experiments were conducted in 2019-2020 to determine the overall subterranean survivorship in relation to site condition, differences in survivorship between field-caught vs. laboratory-reared, and the seasonal timing of emergence. We found that the average field subterranean survivorship (17.1 0.4%) was significantly less than the historical production of these agents in the laboratory (37.5 13.6%). We found that as soil pH and organic matter depth increased, survivorship decreased. While the majority of emergence occurred as expected in the fall, a surprising amount of emergence also occurred in spring and summer. The rate of development and the minimum threshold temperature for each life stage were determined through constant temperature experiments, and were used in the construction of a phenological forecast model. Laricobius osakensis minimum threshold temperature (3.6) from egg to adult was similar to that of other Laricobius spp., but completed development at higher a constant temperature (22C). Our results suggest L. osakensis can handle the same colder climates as L. nigrinus, but may be better adapted for warmer locations than L. nigrinus. Laboratory rearing, establishment, subterranean survivorship, and thermal requirements of Laricobius spp. (Coleoptera: Derodontidae), biological control agents released for Adelges tsugae (Hemiptera: Adelgidae) ; Doctor of Philosophy ; Eastern and Carolina hemlocks are long lived, shade tolerant evergreen tree species that support a unique ecological assemblage of plants and animals. However, following the accidental introduction of a highly invasive aphid-like insect pest first found in the 1950s, known as the hemlock woolly adelgid (HWA), significant mortality of hemlocks in the eastern U.S. was eventually observed. In order to mitigate this pest's impact and to prevent the further collapse of this unique ecological system, a robust management program was initiated. Today, the primary management strategy to reduce HWA's deleterious effects is through the use of chemical and biological treatments. Biological treatments in the form of insect biological control agents, have focused on the use of two closely related beetle species in the genus Laricobius spp. These species were imported from their respective native ranges, biologically evaluated, and approved for release by federal and state agencies. Since then, the beetles have been mass-produced by multiple governmental and academic agencies and released on infested landscape on mostly public land. Key aspects of these species' biology in terms of their population patterns, development, distribution capabilities, and potential supplementary diet remains unknown. Therefore, investigations into these aforementioned aspects were conducted from 2018-2021 in both laboratory and field experiments. In order to supply beetle larvae for field experiments and to better understand how well these predatory beetles were establishing across the landscape, we investigated heavily urbanized areas for their presence. We found that L. nigrinus was established outside of release locations, in 100 and 75% of the surveyed grids containing HWA in southwest Virginia. Laricobius spp. spend a significant amount of their lives in a presumed dormant state beneath the soil during the summer months. Observations over the past 17 yrs. of rearing Laricobius spp. revealed relatively high rates of subterranean mortality and subterranean early emergence, both of which are substantial mortality factors. Through field experiments, the survivorship of these beneficial beetles was found to be 17.1 0.4% and their seasonal emergence was not limited to fall.Comparing the average survivorship from our field study (17.1 0.4% ) to that of the past 17 yrs. of laboratory production (37.5 13.6%), there was significantly less survivorship. The developmental and phenological biology of one of the two Laricobius spp., L. osakensis was determined and found to be similar to that of other beetles. However, L. osakensis was able to complete development at 22C, the highest developmental temperature recorded for any Laricobius spp. The similar phenological biology of these two predators will allow for precise larval sampling as a metric for establishment. Results and conclusions from these data have provided further knowledge to scientists, land managers, and public stakeholders regarding the benefits of Laricobius spp.