2021 gypsy moth prediction

Forests in northeastern Lower Michigan are challenged in many ways. Some challenges are immediate (e.g., oak wilt fungus, invasive plants), while other challenges are more abstract (e.g., climate change, white-tailed deer browse, invasive earthworms). To provide options for an ecologically sustainable future, most scientists argue for forest management that maintains or promotes native species diversity and forest complexity. Progressive forest management based on natural models can produce forests that are more resistant and resilient to current and future stressors.

The gypsy moth is a non-native (exotic), invasive insect that has impacted many area forests in the last couple of years. Gypsy moths undergo “complete metamorphosis.” Within one season, eggs (found in 1-inch, beige, furry masses on trees) develop into larvae (caterpillars), then into pupae, then into reproducing adults. Eggs begin to hatch mid-April and the ¼-inch black caterpillars eat and grow and change over the next 40 days.

Gypsy moth caterpillars are voracious eaters. In high numbers, such as the last two years, they can cause widespread tree defoliations. In rare instances, gypsy moths can cause mortality of unhealthy trees.

Gypsy moths do not come and go. Instead, populations increase and decrease over time. In a 2006 paper in the journal Ecography, data illustrated how gypsy moth outbreaks occur over different time frames among different forest types. Dry forests, dominated by oak and pine, were more susceptible to more frequent (and severe) defoliations compared to forests on more moist sites dominated by trees less preferred by gypsy moth, such as sugar maple, American beech, and yellow birch.

How about gypsy moth numbers in northeastern lower Michigan in 2021?

During the fall of 2020 and the spring of 2021, an inventory of gypsy moth egg masses in 1/40th-acre plots was conducted in Montmorency County. A total of 108 plots (83 in fall 2020 and 25 in spring 2021) were sampled.

Data indicated that, similar to the findings in the Ecography paper mentioned above, variability existed in the average number of egg masses found across different parts of the county. No two areas were alike. Areas dominated by oak (e.g., Avalon Lake and Avery Lake areas) had more gypsy moth egg masses compared to areas with more pines (e.g., north-central Montmorency County).

Fortunately, gypsy moths have many natural predators and pathogens that reduce their numbers. A comparison of plot data from the fall of 2020 and spring of 2021 shows an average reduction in the number of egg masses per plot. This finding suggests that the natural enemies of gypsy moth have increased and are starting to have an impact. If this is true, defoliation this summer Is likely to be much less than in the past two years.

So, what can landowners do on their property?

Techniques suitable for backyard trees are dissimilar from what a landowner with 40 acres or more should do. For backyard situations, keep trees well-watered, but do not fertilize. Reduce the number of egg masses by scraping them off trees or use a pressure washer at low settings. Place the egg masses in a soap water solution for a couple of hours or burn them. Emergent caterpillars can be impeded and collected by a combination of tree barrier and collection bands (see link below).

Forest landowners should take a step back and consider their property from the perspective of its capabilities based on soils, climate, seed sources, and past management. As demonstrated by a number of studies, past forest management actions have generally increased the dominance of oak and aspen relative to historic conditions in northeastern lower Michigan. Landowners are therefore well-served by promoting forest tree diversity within the context of a given soil type. Irregular planting of bare-root, native conifers, such as white spruce or eastern white pine on better sites with partial sun or red pine or jack pine on drier sites with full sun, may be a simple and cost-effective way of promoting diversity.

Gypsy moths are here to stay for the foreseeable future. Just because you don’t see their impact does not mean they have disappeared. Making our forests less susceptible when gypsy moth populations are high may be the most time-and cost-effective way of impact mitigation. Managing for resistance and resilience in our forests are key.

More about natural enemies of gypsy moths that we should appreciate can be found here: https://www.canr.msu.edu/uploads/files/e2700.pdf.

For more about gypsy moth management techniques, see: http://www.alpenamontcd.org/gypsy-moth.html.

Greg Corace is the forester for the Alpena-Montmorency Conservation District. For more information, including sources used in this article, Greg can be contacted via email at greg.corace@macd.org, or by phone at 989-356-3596, ext. 102.


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