Establishment and monitoring of a sentinel garden of Asian tree species in Florida to assess potential insect pest risks

Sentinel gardens, also referred to as “ex-patria” plantings, sentinel plants, sentinel plantings, sentinel plantations, and sentinel trees, are a proactive approach for assessing pre-invasion risks associated with exotic pests and pathogens in their regions of origin (Eschen et al. 2019; Lovell-Read et al. 2023; Roques et al. 2015; Vettraino et al. 2015), a concept initially suggested by the United States National Research Council in 2002 (US NRC 2002). Establishing and monitoring these gardens certainly benefits both native and non-native regions by providing opportunities for early detection and risk assessment of insects and pathogens. They not only inform management of plants in non-native regions but also provide insights on the prospective effects of insect pests and pathogens prior to their arrival in the native regions of plants, thereby facilitating proactive phytosanitary risk assessment (Eschen et al. 2019; Mansfield et al. 2019).

In 2022, a sentinel garden of east Asian trees was established in Gainesville, Florida, USA. The garden is a part of a network of analogous sentinel gardens around the world (China, France, USA) established in 2018 (Dong et al. 2024; Ernstsons et al. 2022; Eschen et al. 2019). The Florida garden included six important east Asian (primarily Chinese) tree species – Pinus armandii Franch. (Chinese white pine; Pinaceae), Cryptomeria japonica (L.f.) D. Don (Japanese cedar; Cupressaceae), Quercus acutissima Carruth. (sawtooth oak; Fagaceae), Pinus hwangshanensis W.Y. Hsia (Huangshan pine; Pinaceae), Cunninghamia lanceolata (Lamb.) Hook (Chinese fir; Cupressaceae), and Ulmus parvifolia Jacq. (Chinese elm; Ulmaceae). The goal was to observe how American insects impact these trees in a new environment and provide evidence for managing potential forest pests in China, should any of these insects become invasive in China. This garden is a reciprocal complement to gardens of American trees planted in China, used to detect potential Asian pests of American trees (Dong et al. 2024).

The garden was established on the Millhopper campus of the University of Florida Institute of Food and Agricultural Sciences, School of Forest, Fisheries, and Geomatics Sciences (geocoordinates: 29.719 °N, 82.409 °W) (Figure 1A). To maximize exposure of the trees to ambient fauna, the garden plot was located adjacent to a large, protected state park and surrounded by natural forest composed of many native hardwood and conifer species. The plot was enclosed with an electric fence to deter deer browsing. All trees were purchased as bare-root seedlings from various horticultural providers within the United States; no tree has been imported from overseas.

Herbivores and wood borers were identified using two complementary approaches: 1) visual identification of adults and morphologically distinct larvae directly or through high-resolution photographs, and 2) molecular identification of morphologically non-descript specimens, particularly caterpillars, using DNA barcoding. For the latter approach, the universal primers LCO1490 and HCO2198 were used to amplify fragments of the mitochondrial gene COI. The resulting sequences were aligned and queried against the National Center for Biotechnology Information (NCBI) GenBank database for species identification. All sequences generated in this study have been deposited in NCBI GenBank under accession numbers PQ669713–PQ669724.

To facilitate the collection of herbivory observations, the Forest Entomology Lab team developed a custom mobile application that enables researchers to record data directly onto mobile devices, thereby reducing human error and streamlining the data collection process (Marais 2023). The application features integrated GPS tracking and photo documentation capabilities, enhancing the precision and efficiency in monitoring pest damage symptoms.

Currently, a total of 22 tree individuals have survived in the garden, representing five species with 3–5 individuals per species: Cr. japonica (5 individuals), Q. acutissima (3 individuals), Cu. lanceolata (5 individuals), P. hwangshanensis (4 individuals), and U. parvifolia (5 individuals). After three growing seasons, all tree species in the garden are thriving, with the exception of Chinese white pine (P. armandii). Despite repeated replanting efforts, this species failed to establish, and all seedlings ultimately died. This may be related to Gainesville’s higher summer temperatures compared to the species’ native range (Ning et al. 2021).

Detailed herbivory records are provided in Table 1. The observations can be summarized as follows: 1) frequent but minor herbivory was observed on leaves and needles, but it did not result in significant impacts on overall tree health; 2) no wood-boring pests were detected on living trees, and no primary pests capable of causing tree mortality were observed; and 3) Asian tree species are thriving and growing well under Florida’s environmental conditions. The project will continue to monitor the impact of herbivores and wood borers on these trees.

Figure 1:

Sentinel garden overview and representative interactions between Asian tree species and native insects in Florida. (A) Asian tree species planted in Florida sentinel garden. (B) A cocoon of Actias luna (L.) (Lepidoptera: Saturniidae) on a sawtooth oak. (C) Hypothenemus eruditus Westwood (Coleoptera: Curculionidae) colonizing on dead twig of Chinese white pine. (D) Huangshan pine fed on by the pine webworm, Pococera robustella (Zeller) (Lepidoptera: Pyralidae). (E) Neodiprion sp. (Hymenoptera: Diprionidae) larva feeding on Huangshan pine needle. Image credits: Yiyi Dong, University of Florida.

While this project aimed to assess the presence of potentially tree-killing woodborers, only three wood-boring beetle species were detected, all of which were found colonizing dead twigs or stems of Chinese white pine after the trees had died (Table 1, Figure 1). The most frequently recorded species was Hypothenemus eruditus (Westwood) (Coleoptera: Curculionidae); a generalist bark beetle known to colonize a variety of plant species, especially dead twigs, which does not cause host plant mortality. As the trees in this study were relatively young (2–3 years old), we were unable to assess patterns related to tree age or size. Tree health status, rather than age or species identity, appears to be the primary factor influencing susceptibility to woodborer colonization (Dong et al. 2024).

The establishment and monitoring of sentinel gardens, such as the one at the University of Florida, provide valuable insights into the interactions between native pests and non-native tree species, contributing to proactive biosecurity measures by offering early detection of potential pest threats. Continued surveillance of the trees during their growth and maturation will be crucial for identifying any new pests over time. Findings from this project will not only enhance our understanding of potential pest threats to Asian and other exotic tree species but also inform future phytosanitary strategies and forest management practices.

As the global network of sentinel gardens continues to expand, it strengthens international collaborative efforts to mitigate invasive species risks and safeguard forest ecosystems around the world (Eschen et al. 2019). Fostering global partnerships and facilitating information exchange among researchers, policymakers, and practitioners is essential for the growth and effectiveness of global biosecurity. Coordinated efforts across borders allow for the pooling of knowledge and resources, enabling more proactive and integrated strategies. Such collaboration is vital to addressing the complex, transboundary challenges posed by invasive species driven by global trade and climate change, ensuring the long-term resilience and health of forest ecosystems.