TAWAS CITY – Gypsy moth caterpillar populations were extremely high throughout the oak forests in northern Michigan this year, according to officials with Michigan State University Extension. 

Officials said the presence of caterpillars has been a nuisance, and many oak trees have been severely defoliated. According to a press release from MSU Extension, at this point, caterpillars have pupated and turned into adult moths, or have died.  Those that have died were likely infected by a fungi called Entomophoga or a virus called NPV.  

Caterpillars will no longer feed on trees at this point, but homeowners can adopt a few practices that can help their trees both this year and next year:

• Water your defoliated trees during times of drought, but do not be overly concerned with fertilizer (See details below.)

• Resist cleaning up too many dead caterpillars right now because the fungi and virus that killed some of the gypsy moth caterpillars this year can continue to reproduce if left alone.  Those additional fungal spores and viruses will overwinter and re-emerge next year to kill caterpillars again in 2021, ideally in larger amounts.

• Lightly spray dead caterpillars with water to add humidity needed by the fungi that killed them to reproduce and make more spores for next year, but NPV does not need the same humidity.

“MSU staff have observed widespread gypsy moth damage in Northeast Lower Michigan, including areas in Iosco County,” said Sarah Rautio, MSU Extension consumer horticulture educator in Iosco County. “Many residents have called our office with concerns.  Gypsy moths have completed their larval stage, and observations have been made of gypsy moth fungal and viral predators kicking in, which can help bring them down in future years. 

“The focus now should be on ensuring affected trees stay healthy as they regrow leaves. Most healthy trees will survive even severe defoliation if they are able to regrow leaves. The most important way to support the leaf regrowth is by making sure trees get enough water, especially during periods of drought. Contrary to belief, tree fertilizers are not necessary, but certainly not harmful.  The focus should be on watering.”  

Julie Crick, MSU Extension natural resources educator in Roscommon County adds that “watering will relieve some stress on the trees as they work to grow leaves to replace what the gypsy moth caterpillars consumed. This is especially important while the temperatures remain in the 80- to 90-degree range. To properly water a tree, use a sprinkler to distribute water to the area under the branches of the tree. 

“Most of the fine roots grow further out from the trunk and dispersed watering allows the tree to take up more water than if a hose is left to run at the base of the tree. Apply one inch of water every five days to each affected tree. Use a baking pan placed in range of the sprinkler to learn how long it takes to deliver one inch of water via your sprinkler.” 

She said the bulletin on the MSU Gypsy Moth website titled, “Dealing with gypsy moth around your home or property” provides additional detail. 

Rautio gives a final tip: “Feel free to run that sprinkler across the infected tree trunks, too, as that moisture can help the Entomophaga fungi reproduce and prepare to infect gypsy moth larvae next year.  It is difficult to predict how high gypsy moth numbers will be next year since caterpillar populations and their predators are dependent on survival rates this year and unpredictable weather patterns next year, but the science does show caterpillar numbers will come down again once the predators kicks in.  

“Any efforts residents can do to help the predators multiply can certainly help” later in the fall, MSU Extension can provide some tips on how to reduce egg masses, too.

For more information, visit Michigan State University’s Gypsy Moth website at https://www.canr.msu.edu/ipm/invasive_species/gypsy-moth/.

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