Field Notes 10-31-24

Pest Survey Program field and lab specialists carried out several early detection surveys for high-risk plant pests during the 2024 season. Our survey targets included USDA APHIS national priority pests such as the box tree moth (Cydalima perspectalis), which has never been found or reported in the state. Detection efforts also focused on emerging threats like the spotted lanternfly and velvet longhorned beetle, two insects that have been periodically intercepted and require ongoing survey and inspection work to prevent their introduction into Wisconsin.

In addition to these exotic pest detection projects, DATCP’s Spongy Moth Program completed its annual trapping survey to identify emerging populations and determine future treatment areas. This issue of Field Notes highlights the results of our 2024 forest and nursery pest surveys and provides an update on the recent discovery of elm zigzag sawfly in the state.

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^{Spongy moth caterpillars | Melissa Hart}

After four consecutive years of increasing moth counts from 2020 to 2023 and destructive caterpillar outbreaks, spongy moth catches plummeted in 2024. The sharp decline in numbers indicates that populations may be collapsing following a multiyear outbreak phase that peaked in 2023.

The 2024 spongy moth trapping program recorded an annual catch of 213,702 male moths in 8,921 traps, or 24 moths per trap average. This count is a 46% decrease from the 397,415 moths collected in 9,733 traps in 2023 (40.8 moths per trap average) and is comparable to the 201,549 moths collected in 2022. Factors influencing the population collapse were the biological controls, nucleopolyhedrosis virus and the fungus Entomophaga maimaiga. These natural enemies are particularly effective against spongy moth caterpillars during wet, rainy seasons and once larval populations have become dense enough for the virus to spread rapidly. DATCP and the DNR received many reports of caterpillar mortality this summer from landowners, residents, and foresters.

While the spongy moth outbreak phase could be subsiding in some parts of the state, unusually high trap counts in the west-central area are a reminder that this pest continues to spread westward. Follow-up surveys for egg masses now underway will provide an indicator of reproducing populations and should point out potential areas of population expansion in 2025.

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^{Spotted lanternflies and egg mass | Elizabeth Barnes Purdue University}

DATCP plant pest specialists continued detection work for spotted lanternfly (SLF) this season. A total of 72 visual surveys were conducted in 19 counties along the perimeter of high-risk introduction pathways (i.e. railyards, warehouses), and as follow-up responses to sightings reported by the public. Locations of tree-of-heaven, the primary host of spotted lanternfly, were also documented. As of October 31, no live SLF adults, nymphs, or egg masses have been found in Wisconsin.

Although SLF has not yet been detected in the state, it is established in the Chicago area of Illinois, in several Indiana counties, and in eastern Michigan (see the Cornell University SLF reported distribution map). At this time of year, DATCP encourages the public to be on the lookout for spotted lanternfly egg masses. The gray, mud-like masses are laid on a variety of outdoor surfaces, including tree bark, firewood, nursery stock, patio furniture, stones, trailers, and vehicles, making it easy for this pest to spread to new areas. Each egg mass is about one inch long with a waxy coating that hardens, cracks, and deteriorates in spring, exposing 30 to 60 eggs laid in vertical rows. For more information on this pest or to report a sighting, visit slf.wi.gov.

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^{Emerald ash borer larval galleries | M. Lannan DATCP}

In the 16 years since the first discovery of this invasive insect in Wisconsin, emerald ash borer (EAB) has spread to all 72 counties. Burnett was officially the last county in the state to confirm EAB, following the detection of infested ash trees in July 2024. New records were also established in April 2024 in Taylor and Washburn counties. Another 76 municipal detections (all in previously confirmed counties) were added to the EAB distribution map in 2024, bringing the total to 1,287.

The rapid range expansion of EAB across the state and in North America has led to the destruction of millions of ash trees in its path. The DNR Forest Health Program reports that efforts are underway to identify and propagate surviving ash with potential resistance to EAB. Referred to as “lingering” ash, these resistant trees in combination with introduced biological control agents may eventually offer opportunity for ash recovery in U.S. forests.

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^{Velvet longhorned beetle | Boris Laboda UkrBIN 28516}

This exotic wood-boring beetle (Trichoferus campestris) was initially detected in Milwaukee County survey traps in 2017 and DATCP has conducted surveys for the last seven years to understand its distribution in the state. Although velvet longhorned beetle (VLB) infests a broad range of forest, orchard, and urban trees and has been intercepted several times in Wisconsin in rustic log furniture imported from China, its pest potential in Wisconsin is unresolved.

In 2024, the survey included 10 trap locations in five counties where VLB has not been documented. One of the sites was a residence in Price County where live beetles were found after emerging from log furniture. To date, no additional VLB specimens have been found in the trap samples screened, but sample processing is still underway. The known Wisconsin distribution of VLB currently includes Columbia, Dane, Fond du Lac, Kenosha, Milwaukee, Ozaukee, Racine, Rock, Sauk, Walworth, Washington, and Waukesha counties.

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^{Elm zigzag sawfly larva | Nathan Siegert US Forest Service}

A new invasive insect was detected in central Wisconsin this season by DATCP’s Forest Entomologist. The elm zigzag sawfly (Aproceros leucopoda) was observed on elm foliage in Portage County in July. This unexpected discovery marked the first Wisconsin observation of the elm pest and a new state record. Official confirmation was made by USDA-APHIS on July 30, 2024.

Its capacity to produce multiple generations each season, in combination with hitchhiking on vehicles, plants, and other outdoor items, has allowed the elm zigzag sawfly (EZS) to spread rapidly. Since the first North American (in Canada) report of EZS in 2020, this native East Asian insect has been recorded in several eastern and Midwestern states in the U.S. In Wisconsin, the July EZS discovery quickly led to reports in 20 more counties, as shown in the map below. The growing list of positive counties suggests it is established and widely distributed in the state.

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^{Box tree moth trap | S. Jentz DATCP}

DATCP conducted a new survey for the invasive box tree moth (Cydalima perspectalis) in 2024. A total of nine pheromone traps were set from mid-June through mid-September in five southeastern Wisconsin nurseries that grow or import boxwoods for public or wholesale purchase. No box tree moths (BTM) were collected in the survey traps.

This USDA national priority pest poses a significant economic threat to the ornamental boxwood trade and is quickly infesting new areas through the movement of infested boxwood plants and via natural spread (up to 3-6 miles per year). With recent detections in Michigan and Ohio, surveillance for BTM in Wisconsin has become especially important for protecting the market for boxwood and contributing data on its current status in North America. The Pest Survey Program is planning an expanded BTM trapping effort for 2025.

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