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Crabapple nursery stock with spongy moth larvae in Dodge County | DATCP L. Meils
The Nursery Program licenses nursery growers and dealers and inspects nursery stock for regulatory pests. Nursery inspections promote the production and sale of healthy plants, help prevent the introduction of invasive pests arriving on imported stock and facilitate interstate commerce through the issuance of Plant Health Certificates for exported stock annually. DATCP inspectors also partner with DNR staff to ensure invasive plants regulated under the DNR Invasive Species Rule (NR 40) are not sold at nurseries.
Nursery Licensing, Inspections, Regulatory Actions, and Compliance Agreements
DATCP provides an inspection every one to three years to licensed businesses. The program licensed 664 nursery growers and 1,265 nursery dealers in 2024, with staff inspecting 406 (47%) growing field locations and 431 (21%) dealer locations statewide. Annual inspections prioritized the 123 licensed nurseries who purchased Plant Health Certificates (PHCs), indicating intent to ship plant stock interstate. Since 2021, all nursery inspections are completed using the Survey123 app on mobile devices.
Inspectors continued to check for violations of the NR-40 Invasive Species Rule at licensed nurseries in 2024. Although detections continued to decline (see table below), inspectors documented nine instances of restricted or prohibited invasive plants offered for sale, including black alder, burning bush, woodland forget-me-not, Japanese honeysuckle, Japanese and Chinese wisteria, ‘Rose Glow’ barberry, and sawtooth oak. Three instances were at a chain store where the stock had been distributed to multiple locations throughout Wisconsin. Inspectors also recorded 13 cases of invasive plants growing in nursery production fields, including buckthorn, cut-leaved teasel, Japanese knotweed, and wild parsnip. Entities with multiple violations are referred to DNR for enforcement.
Inspectors issued 36 compliance documents in 2024 (i.e., activity reports, orders prohibiting sale, pest abatement orders, and warning notices), including 18 licensing violations or other pest problems, nine invasive plant sales, eight labeling violations, and one quarantine violation. As a measure to increase awareness of Wisconsin’s plant pest regulations, six rejection notices were sent to four states (Illinois, Michigan, Minnesota, and Oklahoma) following interceptions of invasive plants, virus-infected plants, and scale insects that were shipped to Wisconsin.
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Nursery Compliance Agreements
Inspectors also assist nurseries in determining if a compliance agreement is needed to import or export stock. Compliance agreements are annual contracts that provide limited pest quarantine exemptions to nurseries or businesses that implement prescribed risk reduction measures. Agreements associated with exterior quarantines are issued to nurseries importing plants from areas infested with regulated pests not found in Wisconsin, such as hemlock woolly adelgid or elongate hemlock scale. Nurseries exporting stock out of a spongy moth quarantine county or an area of the state infested with Japanese beetle may also enter into an agreement with DATCP.
Nursery Inspection Pests, Diseases, and Abiotic Stressors in 2024
A list of the top 10 pests, pathogens, and abiotic stressors observed most frequently during nursery inspections in 2024 is provided in the table below. Most of the issues found are common from year to year. Noteworthy inspection highlights from 2024 were the general decrease in pest counts (i.e., insects, mites, and nematodes) and an increase in pathogen detections, likely influenced by heavy rain during the first half of the growing season. In addition, spongy moth life stages were also less prevalent on nursery stock as the outbreak phase continued to subside in Wisconsin. Finds decreased from 37 in 2023 to 24 in 2024, which aligns with the larger statewide trend toward declining populations in 2024.
 Inspection Highlights
Elm Zigzag Sawfly: A New State Record - An invasive, introduced sawfly species was detected in Wisconsin for the first time in 2024, representing a new state record. The elm zigzag sawfly (Aproceros leucopoda) was observed on elm foliage in Portage County by a DATCP forest entomologist. Official USDA-APHIS confirmation was made on July 30, 2024. News of the July elm zigzag sawfly discovery led to subsequent records in 20 additional counties this season, including 24 reports of the pest on nursery stock in four counties. In each case, the nursery was required to treat the infested stock before it could be moved or sold.
Winsome fly - Introduced into New Jersey in 1923 as a biological control agent of Japanese beetle, the winsome fly (Istocheta aldrichi) is a parasitoid that lays conspicuous white eggs on the pronotum and wings of the Japanese beetle. The fly is found in many of the same areas as the Japanese beetle and is established in the northeastern US, as well as in Quebec and Ontario, Canada. In neighboring Minnesota, releases of fly pupae were made by the Minnesota Department of Agriculture 20 years ago (1998-2004), and the winsome fly is now beginning to have a measurable impact. Monitoring in 2023 documented parasitism rates of 15-55% at the University of Minnesota (UMN) Arboretum, and 15-39% at commercial vineyards, according to UMN-Extension.
A citizen science survey was recently started by UMN-Extension to better understand the distribution and biocontrol impact of winsome fly. Using the iNaturalist app, winsome fly was recorded in 14 new Minnesota counties in 2024, and its known distribution currently includes 22 counties in the state. In Wisconsin, DATCP assisted with the effort this season and has so far documented the fly in eight Wisconsin counties.
This tachinid fly has several characteristics that make it a promising biological control of Japanese beetle in the Midwest, including its synchronous emergence period with its host (late June to early August), high parasitism rates resulting in direct mortality of current-year beetles and major reductions in egg laying, and its ability to adapt to a variety of climatic conditions.
Viburnum Leaf Beetle - Inspectors found viburnum leaf beetle (VLB) at 18 locations, primarily in southeastern Wisconsin. This established invasive pest feeds exclusively on the leaves of viburnums, and both the adults and larvae cause severe defoliation and eventual shrub mortality. Native viburnums are an important understory component of many Wisconsin woodlands and are at risk as this insect becomes more widely established. Viburnum leaf beetle has been found in 14 Wisconsin counties since 2009. All VLB detections in 2024 were in previously confirmed counties (Dane, Kenosha, Milwaukee, Ozaukee, Racine, Sheboygan, Washington, and Waukesha). No new county-level detections were recorded in 2023 or 2024.
Lily Leaf Beetle - First detected in Marathon County in 2014, the invasive lily leaf beetle was found at two nursery locations this season and has now been confirmed in 23 Wisconsin counties. Chippewa and Kenosha are the latest additions to the Wisconsin lily leaf beetle distribution map. The adult beetles are remarkable for their bright red color, while the larvae can be found by inspecting Asiatic lily leaves for defoliation.
Bagworm - A severe infestation of bagworm (Thyridopteryx ephemeraeformis) was observed on arborvitae nursery stock in Kenosha County in 2024. Although this insect pest sporadically appears in southeastern Wisconsin, it is more common in the southern US and reaches Wisconsin by migrating on weather systems or by hitching a ride on nursery stock. The infested arborvitae plants were destroyed.
Ramorum Blight - Seven symptomatic plant samples collected by DATCP inspectors or forwarded by the UW-Madison Plant Disease Diagnostics Clinic were tested for Phytophthora ramorum at the Plant Industry Bureau Lab this year. All tested negative for this invasive pathogen. Although P. ramorum has been intercepted at Wisconsin plant nurseries and nursery dealers several times in the past, it has never been detected on plants in the Wisconsin landscape. State and federal quarantines for this water mold, which causes the disease sudden oak death, restrict and require prenotification for host material shipped from known infected areas in the western US.
Boxwood Blight - This disease of ornamental boxwood plants was first confirmed in Wisconsin in 2018. During 2024 nursery inspections, 29 symptomatic boxwood samples were collected for testing at the Plant Industry Bureau Lab. One sample from a retail nursery in Ozaukee County was positive for boxwood blight, which led to a trace-forward effort at four more nursery locations that received potentially infected boxwood plants from the same Oklahoma nursery. Infected boxwood stock was found and destroyed at two of the four locations. Twenty-eight of the other boxwood samples were diagnosed with different diseases, including Volutella blight, another serious fungal pathogen affecting this popular ornamental.
Other Pathogen Detections of Note - While none of these were found in significant numbers, they were unusual or noteworthy detections. Leafy gall caused by Rhodococcus fascians was detected on one shasta daisy. Phytoplasmas (Candidatus Phytoplasma sp.) were detected on three samples: coneflower (1) and speedwell (2). Fireblight (Erwinia amylovora) was detected seven times: on apple (3) and crabapple (4). Verticillium wilt was detected on a redbud and a sugar maple.
Nematodes - Foliar nematodes (Aphelenchoides sp.) were detected on anemone, hosta, and masterwort. Root Knot Nematodes (Meloidogyne sp.) were detected on astilbe and hosta. Root Lesion Nematodes (Pratylenchus sp.) were detected on astilbe, bleeding heart, false sunflower, and peony.
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