The Elm Spanworm (Ennomos subsignarious) and Fluctuating Bird Populations

An infestation of pine bark beetles in Stevens Creek Park has killed a grove of pine trees, resulting in a natural opening.  I want to visit the park and write about this interesting ecological occurrence, but it’s just too hot this time of year to do any kind of nature rambling.  Instead, I’m staying in the air conditioned environment of my house, and I’m writing about another type of insect infestatation reported in the scientific literature.

I think the average person underestimates the influence insects have over plant species composition.  During the warmer months of the year the biomass of insects far exceeds the weight of vertebrates, and they consume more organic material.  Just 1 species, the Elm Spanworm, can defoliate as much as 500 square miles of forest.

Larval stage of the elm span worm, aka an inchworm.

Adult stage of the elm spanworm.

Forest defoliated by elm spanworm.  The only green left is from pines and tulip trees, species the spanworm won’t touch.

The elm spanworm is a misnomer–it feeds on every hardwood species with the exception of tulip trees.  They highly favor ash, hickory, and walnut, but they also like basswood, beech, elm, oak, dogwood, locust, maple, and willow.  During cyclical outbreaks they drop from the tree tops and feed on understory plants as well.  Trees survive defoliation, but it slows their growth and weakens them, increasing their mortality rate.  The trees are forced to expend more energy refoliating and are more likely to succomb to disease and other insect infestations that they normally could endure.  The chestnut borer is notorious for finishing off trees that earlier suffered defoliation from the elm spanworm.

The elm spanworm has many color variations, but during cyclical outbreaks when the population explodes exponentially, most are black with red heads.  The adults are white moths with a tan spot.  The female moth lays eggs on twigs or in bark crevices.  The eggs hatch and the larva begin feeding immediately.  They shoot silk strands that serve as parachutes, allowing the wind to spread them from tree to tree.  The pupal stage lasts for 10 days.  Then they emerge as adult moths.  Twenty cyclical outbreaks occurred during the 20th century, so defoliation events occur about every 5 years, though, of course, the damage varies, depending on each individual geographical location.  The species ranges from Canada south to Florida and Texas.

The cyclical outbreaks of the elm spanworm are a feast for birds.  One study determined bird density was 15% higher in areas of forest infested with the elm spanworm, and bird species abundance was as much as 33% higher.  Elm spanworm infestations attract many species of warblers, as well as deep forest species such as woodpeckers, nuthatches, and ovenbirds.  Their numbers all increase following spikes in spanworm populations.

Ovenbird, a deep forest species of thrush.  Ovenbirds are known to increase the number of clutches they lay during insect infestations.

Black poll warbler.  They are attracted to insect infestations in forested areas.  I think I saw some of these while staying at a vacation home in the Nantahala National Forest 3 years ago.

However, birds aren’t the elm spanworm’s most devastating enemy.  An army of invertebrate predators including spiders, beetles, bugs, and wasps take their toll, but a tiny parasitic fly (Telenomus alsophilae) is the #1 agent of spanworm control.  This fly parasitizes overwintering eggs.  Larval diseases may doom even more elm spanworm spawn than all predators combined, and unusually late freezes prevent outbreaks by killing newly hatched larva.

A parasitic fly layed 50 eggs on this moth larva.  Parasitic flies kill more spanworm larva than any other predator including birds.

Insects surpassed the megafauna in shaping the forests of the Pleistocene.  Insect-killed trees became dry deadwood and tinder for fires that transformed forest into prairie.  Temporary defoliation of trees allowed more sunlight to reach the forest floor, drying the soil, and changing the compostion of the undergrowth.  A dead tree here and there served as hollow snags and shelter for some species.  Several dead trees opened up the canopy for shade intolerant saplings and bushes.  Insect infestations are just another example of the constant flux found in the natural environment.

Reference:

Haney, Christopher J.

“Numerical Response of Birds to an Irruption of Elm Spanworm (Ennomos subsignarious; Geometridae: Lepidoptera) in Old Growth Forest of the Appalachian Plateau USA”

Forest Ecology and Management July 1999 Volume 20

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