Toadstool Tuesday
Entomophaga maimaiga - The Spongy Moth Caterpillar Killer
Good evening, friends,
This week we look at a fungus that we can’t visually see, but one that is actively helping our native trees. Not through a mycorrhizal symbiosis with the roots, however, but by parasitizing one of North America’s most noxious invasive insects. I mentioned a few weeks ago that spongy moth caterpillars (formerly known as the gypsy moth, Lymantria dispar) were chewing up our oak trees. In the time since many have pupated into moths. Many caterpillars never made it to moths, however, and the fungus Entomophaga maimaiga instead turned them into fungus food.
I found out about this fungus just last week on the New York Mycological Society zoom ID session and have been thinking about it since. Buckle up, folks, because this is going to be a thorough examination of the life cycle, ecological impacts, and biological controls of these prickly little caterpillars.
Where did these caterpillars come from?
In 1869, spongy moths were introduced to the US in Medford, MA (they’re native to Eurasia) by one Leopold Trouvelot. He wanted to breed them and produce a hardy silkworm that would allow the US to produce its own silk. This did not work, but the caterpillars did escape cultivation and have since established themselves in the northeastern part of the continent. Brutal, you think you have a get rich quick idea and end up causing a centuries-long ecological disaster. Tough break, Leopold.
Interestingly, the Hudson River was originally a “Hold the Line” barrier where, just after WWII, DDT was aggressively sprayed east of the river to prevent the moths from crossing and spreading west. That also did not work. They can now be found throughout New England across to Minnesota, and down to North Carolina. They’re also on the other side of the Rockies from California up to British Columbia.
Life cycle
The caterpillars hatch in the spring from a “spongy”, tan-colored egg mass located around the base of trees. The juveniles are a quarter-inch long when they begin their climb up the tree to feed on leaves in the canopy. They love oak trees, but will be happy munching on birch, beech, apple, linden, the blueberries I planted outside my door, and in bad outbreaks they’ll even hop over to conifers like pine and spruce. They can feed and develop on more than 300 different species of trees/shrubs. They don’t seem to eat ash nor red maple (Acer rubrum), and anecdotally have left most of the sugar maples at the preserve untouched.
The young caterpillars possess an interesting adaptation known as ballooning. If there aren’t enough leaves for them to consume, or their tree is too crowded, they repel themselves from the canopy on a thread. Like some fungal spores, they let the wind decide their fate Of course, they don’t have any control of where they land so they end up inside cars, houses, and other places that are completely unsuitable.
As the caterpillars continue to feed, they will molt (shed their skin) 3-6 times on a near weekly basis. The periods between molts are known as instars. Females need more energy to lay eggs and typically molt once more than their male counterparts. They’ll usually grow an inch longer as well. A mature caterpillar will have five pairs of blue dots followed by six pairs of red dots running down their back. They then descend the tree and find a nice dark crevice to spin their cocoon. This can be in the tree’s bark, wrapped between leaves, or on outdoor furniture and car tires.
The moths emerge from the cocoons 10-14 days later and will live for around two weeks. The moths do not feed and are solely focused on reproduction - in fact the females are flightless. White with brown markings, females emit a strong pheromone to attract the tan-colored males. Females will lay a single egg mass that can contain 200-1000 eggs which will overwinter until next spring.
Ecological Impact
These hungry caterpillars wreak havoc on the predominantly oak canopy we have at the preserve and throughout the Hudson Highlands region. While an oak typically won’t die from the defoliation, they do become more susceptible to death from drought or disease. The loss of their leaves prevents the tree from photosynthesizing and also deprives them of other crucial nutrients like Nitrogen. A study from the Cary Institute of Ecological Studies in Millbrook, NY claims that an oak will reabsorb “about 70% of the nitrogen in their leaves before dropping them in fall. In spongy moth defoliation years, only about 23% of the nitrogen in leaves gets reabsorbed back into the plant” (Reference 2).
The defoliation of entire canopies has a cascading effect as well - nothing in nature occurs in a vacuum. Lots of sunlight now hits the forest floor, warming up/drying out the soil, and allowing for new seeds to germinate. Less carbon is absorbed by the trees as well, as the leaves are the organ through which they respire. Most trees leaf back out a few weeks after they’re defoliated, resume photosynthesis, and hopefully continue on to a long, healthy, and prosperous life.
You might expect that the caterpillars would serve as a bountiful food source for whatever likes to eat caterpillars. Some species of birds like blue jays, downy woodpeckers, gray catbirds, cuckoos, and grackles will snack on them; as will grey squirrels and white-footed mice, but no one is solely subsisting off these lumpy larvae. The caterpillars are covered in filamentous hairs which likely dissuades their consumption - and can even irritate human skin. Since they’re not originally from North America, they have no natural predators.
Entomophaga maimaiga
Enter the fungus, E. maimaiga. Like the spongy moth, it was also introduced to the country - but with the sole intention of controlling spongy moth populations. Native to northeastern Asia, the entomopathogenic (insect-eating) fungus was introduced to the US in 1910 and 1911. Interestingly, the first recorded sighting of the fungus in the wild wasn’t until 1989. Scientists are unsure whether the fungus survived for decades in the wild after introduction, undetected, or became naturalized later through different means.
The fungus uses what are called resting spores to persist in the environment - in the soil, on the trees - for an extended period of time. The current thinking is at least 10-20 years. Resting spores, like many fungal spores, need moisture and the right amount of humidity to germinate on the caterpillar. A moist spring is necessary for the fungus to really impact the Lymantria dispar caterpillar population.
The fungus parasitizes and kills mature caterpillars within a few days of spore germination. Caterpillars killed by the fungus hang vertically from the side of a tree - their hindlegs gripping the tree and their forelegs dangling limp as the body desiccates. In addition to resting spores, the fungus will digest the body and produce asexual spores (conidiospores). These conidiospores are released in the same season and can spread to other caterpillars as quickly as fourteen hours after the host’s death (Reference 3). Wow.
One study showed a reduction in defoliation of up to 85% across 11 states in the years from 1995 to 1996 (Reference 6). The study occurred just as the fungus was becoming naturalized to those areas… and I’m not sure if it’s as inhibitive now (looking around the forests here we definitely suffered some serious defoliation). It should also be noted that a different study collected 1,500 dead insects spanning 53 species and only two of the 1,500 individuals were infected with E. maimaiga. This suggests that the fungus is leaving native insects alone and isn’t jumping hosts from spongy moths.
Another Caterpillar Killer
There is another invisible internal invader that curbs these caterpillar populations. Nuclear Polyhedrosis Virus (NPV) is a naturally-occurring virus that afflicts spongy moths. Like E. maimaiga, the virus only infects spongy moths. The USFS will actually spray GYPCHEK in some of their forests - an insecticide that just contains NPV.
For the virus to be most effective, however, there needs to be a high density of caterpillars (i.e. an outbreak like we have at the preserve). That’s not the case with E. maimaiga which consistently kills caterpillars - even in down years where there aren’t dozens dangling from each tree. Caterpillars killed by NPV usually hang in an upside down V (perhaps a lowercase n, it’s all about perspective) on the side of the tree.
Whew.that was a fun one. It’s always a nice achievement to hit the “near email length limit” banner. I understand that fun may not translate for the reader, however ;)
I’m going to the NYMS Gary Lincoff Backyard Walk this Sunday in Central Park. I’ll leave the caterpillars up here but hope to see some of you there,
Aubrey
References:
https://dec.ny.gov/nature/animals-fish-plants/insects-and-other-species/spongy-moth
https://www.caryinstitute.org/news-insights/feature/spongy-moth-chronicles
Hajek AE. Pathology and epizootiology of Entomophaga maimaiga infections in forest Lepidoptera. Microbiol Mol Biol Rev. 1999 Dec;63(4):814-35, table of contents. doi: 10.1128/MMBR.63.4.814-835.1999. PMID: 10585966; PMCID: PMC98977.
https://portal.ct.gov/deep/forestry/forest-protection/the-spongy-moth-a-brief-history
https://biocontrol.entomology.cornell.edu/pathogens/entomophagamaimaiga.php