Backyard Gardening
Nightmare Before – and During – Summer!
You may have heard that the spotted lanternflies (Lycorma delicatula) – SLF for short – have now surrounded Clinton County. They are getting closer! The devastation from the SLF is yet to come. The current tree damage is mostly due to an outbreak of the spongy moth (Lymantria dispar), formerly known as the gypsy moth. Who needs to worry about the future effects of the SLF, when spongy moth larvae have munched their way through Pennsylvania’s trees this year? Well, we might actually be able to learn something from dealing with L. dispar that can help us prepare for L. delicatula.
According to the Entomological Society of America, as of 3/2/2022, ‘spongy’ moth has now been accepted as the common name for the moth species Lymantria dispar. This name is based on the appearance of the egg masses.
It has always been used in L. dispar’s native European range (https://www.entsoc.org/news/press-releases/spongy-moth-approved-new-common-name-lymantria-dispar). For the remainder of this article, I will use the term ‘spongy moth’ as the common name for L. dispar.
Spongy moths belong to the order Lepidoptera (moths and butterflies). Egg masses are yellowish-tan, and have a furry or spongy appearance. The flightless females deposit fertilized eggs in mid-summer. The egg masses don’t hatch until mid-spring of the following year. Young larvae, or caterpillars, are dark brown or black, and only a quarter-inch long. According to Penn State Extension (PSU), “mature larvae can be two to three inches long, with a yellow and black head. There are five pairs of blue spots followed by six pairs of red spots (https://extension.psu.edu/gypsy-moth).” See Photo 1.
According to the Smithsonian Institution, Leopold Trouvelot, an amateur lepidopterist, imported L. dispar specimens to his home in Massachusetts. He hoped to breed a new type of silk-producing moth that could compete with the silkworm moth (Bombyx mori). In 1868 or 1869, some spongy moths escaped from Trouvelot’s enclosure, and L. dispar has been spreading across North America ever since (https://www.si.edu/spotlight/buginfo/gypsy-moths).
Have you noticed any trees that leafed out in the spring, but are now bare? Spongy moth caterpillars favor oak leaves most of all, but will also eat the leaves of more than 300 different tree species. As the caterpillars mature, competition for food increases, and L. dispar will chew on many leaves that they can reach. There are some tree species that are resistant or even immune to spongy moth attacks; they should still have their leaves.
We have three magnificent oak trees in our yard. All three were completely defoliated. Our pussy willow tree also got chewed on (see Photo 2). For the most part, our blueberry bushes are leafless. Our native red maples also took a hit. My young witch hazel now has very holey leaves.
The spongy moth caterpillars have caused most of this leaf loss. In May and early June, they were all over our deck, on our garage doors, and crawling on the trees. They were an assault on my senses: I saw and felt them. I could even hear their feces dropping to the ground, like falling raindrops. There are brown patches on many mountains in Clinton County, where the spongy moths have wreaked havoc. I don’t remember the trees ever being defoliated to this extent, but my husband remembers having to cut down a lot of dead oaks – killed by spongy moths – in the early 80’s.
Spongy moth caterpillars are very hairy. Much like poison ivy (Toxicodendron radicans), any contact may cause an allergic reaction – typically an itchy rash. According to the Minnesota Department of Agriculture (MDA), as the caterpillar outgrows its bristly skin and sheds it (four to six times), the empty skins pile up on the ground. Detached bristles may irritate the skin and eyes. The hairs can even be inhaled, which can affect respiratory systems (https://www.mda.state.mn.us/consequences-not-treating-gypsy-moth).”
Fortunately, there is an end in sight to this current torture by spongy moth! Most caterpillars should already have pupated, forming cocoons. Adults have already started emerging. According to Michigan State University (MSU), adult spongy moths do not eat, and only live for a few days. Be aware that pheromone traps are available that will attract the male spongy moths, but will not kill them (https://www.canr.msu.edu/resources/btk-one-management-option-for-gypsy-moth).
Spongy moth outbreaks usually only last two to four years, and then the population collapses, often due to viral or fungal diseases. “Parasitoids, predators, starvation, and unfavorable weather [very cold winters and/or wet springs] may also help control spongy moth numbers (MSU).”
Another promising tool for battling the spongy moth is Bacillus thuriengensis var. kurstaki HD-1 (Btk). Btk is a variety of native bacteria commonly found in soil. “Btk is widely used to control spongy moth and certain other caterpillar pests because of its effectiveness, ease of use, and its low toxicity to other animals, including other insects.” When Btk spores and toxin crystals are eaten by caterpillars, they stop eating, and die. Since it works best with younger larvae, Btk should be sprayed on leaves in April (MSU). It’s too late to use Btk this year. Next spring, if you start to see tiny, dark, fuzzy worms hanging or floating from silky threads, then you might successfully use Btk. If you feel the need to use a chemical pesticide for spongy moths, please follow the instructions on the label very carefully.
The first line of defense used by the Pennsylvania Department of Conservation and Natural Resources (PA-DCNR) in their spongy moth spraying program for state forestlands was Btk. PA-DCNR has already completed its spraying program for this year. They did NOT spray private property. For more information, see www.dcnr.pa.gov. Search for “spongy moth.”
Now that the spongy moth caterpillars have stripped the leaves from many trees this year, it will be important to find and eliminate egg masses that will be deposited after the spongy moths mate this summer. Keep looking for egg masses until next spring. This activity can also be applied to the SLF. Both pests hitchhike in egg masses to travel long distances. The females of both L. dispar and L. delicatula lay their egg masses on any outdoor surface that sits still long enough for egg deposition. This includes tree trunks, firewood, lawn furniture, and inside vehicle wheel wells or bumpers. Even a single egg mass can start a new infestation. Scrape the egg masses off whatever surface on which they have been deposited, and dispose of them properly. Place them in a jar filled with soapy water, rubbing alcohol, or vegetable oil. They can then be burned or put in a sealed plastic bag, and placed in the landfill trash. This can also be done with the caterpillars next spring.
Unfortunately, conifers (hemlocks, pines, spruce, etc.) cannot grow a new set of needles, and may die after a single season of defoliation (PA-DCNR). Healthy deciduous trees may be able to recover from first-time leaf loss, often growing a new set of leaves before the end of summer. However, once defoliated, the tree’s entire system is weakened and stressed. The tree is then susceptible to other disease-causing organisms. Deciduous trees rarely survive a second year of defoliation.
Next spring, inspect your property for dead trees. If there are any near your house or garage, you might consider being proactive, and cutting them down before they fall on something or someone. I am hoping that my oak trees will survive this spongy moth outbreak!
Kathryn Dorman has been a Penn State Extension certified Master Gardener of Clinton County since 2017. She can be reached at clintonmg@psu.edu, or through the Clinton County Master Gardener Hotline at (570) 858-0198.
