Get Growing: Good news, good riddance to gypsy moth caterpillars this year

  • Gypsy moth egg masses are shown on a tree June 8, 2017 in Belchertown. FILE PHOTO

  • Gypsy moth caterpillars crawl on the leaves of a white oak tree June 8, 2017 in Belchertown. FILE PHOTO

For the Gazette
Published: 4/5/2020 4:01:13 PM

Good news! Good viruses!

Many of you undoubtedly recall that between 2015 and 2018, our region experienced a dramatic increase in the population of gypsy moth caterpillars (Lymantria dispar). These voracious critters munched their way through vast amounts of foliage in the summer, denuding our treescape and showering us with tiny black pellets of excrement called “frass.” I wrote about this unpleasant infestation in a “Get Growing” column dated June 29, 2017, the year the gypsy moth infestation was at its peak. 

We have plenty to complain about these days, but this week the UMass Extension Service issued a piece of good news: We can expect to see little or no evidence of gypsy moth caterpillars this year. According to Tawny Simisky, an Extension entomologist, the decline in the gypsy moth population is mostly due to the recent surge in Entomophaga maimaiga, an airborne fungus that kills gypsy moth caterpillars. This fungus overwinters in the soil and leaf litter and is activated by damp spring weather. Because we had spring drought spring 2015 and 2016, the fungus declined, allowing the caterpillar population to soar. In 2017, gypsy moth caterpillars defoliated 923,000 acres of forest in Massachusetts alone. Fortunately, damp spring conditions have returned, increasing the fungus population and putting a serious dent in the number of gypsy moth caterpillars. 

But Entomophaga maimaiga is not the only enemy of the gypsy moth caterpillar. Simisky said we can also thank the nucleopolyhedrosis virus (NPV) for curtailing the gypsy moth population. According to Wikipedia, the virus was first reported in 1891 as Wipfelkrankheit, which is German for “treetop disease.” This name comes from the fact that while unaffected larvae feed at night and hide during the day, larvae infected with the virus climb up towards the tree canopy and basically turn into goo. As Simisky explained, the liquid from the infected caterpillars leaks onto other caterpillars, spreading the disease. NPV is density dependent, meaning that it kills more caterpillars in areas where the caterpillar population is higher. 

In the early 20th century, NPV was aptly known as “caterpillar cholera.” I’ll spare you further gory details, except to say Wikipedia also notes that infected larvae have been referred to as “zombie caterpillars” because of the gruesome way in which they die. I admit that the phrase is new to me. I’m happy that I probably won’t need to use it this year. 

At a time when just the word “virus” makes us shudder, it’s good to be reminded that not all viruses are bad. NPV is spread through the droppings of birds and other animals that prey on gypsy moth larvae, but it is harmless to all but the gypsy moth larvae. 

Most gardeners I know are fascinated by insects: the good, the bad and the just plain weird. The UMass Extension Service recently launched an online video series produced by Simisky called “InsectXaminer.” Simisky said she hopes to “increase the visibility of the beautiful world of insects, even those we consider to be pests in our managed landscapes. We want to showcase the complexity of insect life cycles, cataloging as many life stages for each species as possible.”

The aim of the Extension Service is to provide professionals and land managers with footage that is helpful for learning the identification of insects throughout the season, rather than at any single point in their life cycle. 

If you’re already missing gypsy moths, or if you just want to know more about them, you’re in luck. Episode 1 of the series examines the fascinating life cycle of the gypsy moth. Simisky got the video’s lively soundtrack free from YouTube’s audio library. “It’s a little funny, a little goofy,” she said. “It matches the insect’s theme.” She added, “Maybe when InsectXaminer gets really big, we can get local artists to offer us free clips.” 

This is three minutes of video won’t want to miss. I promise. Here’s the link: 

Mickey Rathbun, an Amherst-based lawyer turned journalist, has written the Get Growing column since 2016.


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