Praying for an Epidemic

I sat out in the backyard for an hour a few days ago and stood up covered with tiny gypsy moth caterpillars.  I would guess there were over fifty--mostly on my pants and around my shirt collar.  Thankfully, I didn't find any in my hair.  After the bumper crop of eggs laid last year I expected this would be a bad year, but thought I'd be okay under my little shelter.  I'm guessing they parachuted down at an angle on caterpillar silk. 

Gypsy moth caterpillars start out tiny, spindly and blackish, but quickly grow to become colorful and (gritted teeth) quite handsome with red and blue spots.  In a bad year, their frass (poop) falling on dry leaves can sound like rain.  Besides soiling the wash hanging on the line, the frass causes an allergic reaction in some people. 

Of course, the caterpillars are merely an annoyance to us; to trees it can be deadly.  I've seen trees (even whole forests) denuded by gypsy moths come back later in the season with a full suit of new leaves--but growing that new foliage can deplete a tree's resources so that other stresses tip it over the edge. 

Recent rain is very welcome: wet encourages the spread of the caterpillar's nemesis, the parasitic fungus Entomophaga maimaiga.  This fungus--which affects only the gypsy moth caterpillar--spreads through the air from an infected, dead caterpillar to others.  It cannot wipe them out, but keeps the population in check most years.

(The story of this imported, species-specific fungus is interesting in its own right: a century-old attempt at biological control that failed--then later succeeded when the fungus was accidentally reintroduced thirty years ago.) 

Technically, I'm praying not for an epidemic (which is "upon the people"), but an epizootic.

Almost-full-grown caterpillar in June; male with female moths laying eggs in July last year.

In other news,

the rains these last few months have been ending the drought.  Our local reservoir, Silver Lake, went from one hundred inches below normal at the beginning of the year, to just a foot below normal recently.  This weekend's rain may be a double blessing.

A good year for caterpillars--

 

--means a bad year for trees.  And a bad year for sitting in the shade, with frass* raining on your head.

I first noticed the caterpillars in early spring.   I'm in the habit of plucking twigs of trees that interest me on my walks, and putting them in water at home to see how they turn out.  (Kind of a waste of time actually: even in the best of circumstances, these developing twigs need resources from the rest of the tree to do much growing.)  But not only did my twigs not develop, the leaves would rapidly be eaten by caterpillars that, unnoticed, had ridden home with me.  In the beginning, these caterpillars were more varied, but as spring progressed the dreaded gypsy moth caterpillar dominated overwhelmingly.  

Gypsy moths would be kind of pretty if not so frightening.  They prefer oaks, and will even eat young pine needs--both well-equipped with chemical defenses against most herbivores.  If the infestation is bad enough, the trees will spend so much in resources to continue producing leaves that they starve to death, or or so weakened that they succumb to secondary diseases.  I remember doing field work  in an oak forest as a college student doing field work in the early 80's.  It was a bizarre experience: the rain-like patter of frass falling on the dry leaf litter contrasted weirdly with the sunlight streaming in through the leafless crowns.  This year is bad, though not nearly as bad as that.  

Gypsy moths were deliberately imported into Massachusetts from Europe in 1868 by a French scientist who wanted to breed them to produce silk.  Like many such introductions before and since, this did not go according to plan, and an outbreak occurred  only a decade later.  Lawns and smaller plants of the forest floor will likely see a fertilizer effect from the caterpillar droppings.  The caterpillars are a food source for many birds and small mammals such as mice and squirrels, which should see their own population booms.  At high population densities the insects suffer from epidemics of fungi and a virus.  (I have seen many dead caterpillars hanging from twigs.)  But the fungus, in particular, needs moist conditions to spread, and this has been a dry spring.  

It's too late to do anything about this year's infestation, although trees could be sprayed early next spring to avoid a repeat performance.  Then we could get back to business as usual: battling periodic outbreaks of winter moth caterpillars--another alien invasive.

Red oak, alder (tiny caterpillar), ash-leaved maple, and witchhazel with caterpillars in early spring.

Maples like this red maple have seen a lot of damage.

 Black oak seems to be a favorite--and is the most common tree in the neighborhood.

Swamp white oak is tasty, too, but a pretty rare tree hereabouts: first sign of trouble, then later on.

 

 Both paper birch (above) and gray birch (below) are nice snacks.

Witchhazel is good, but rare.

 I'm not sure about white oaks; many seem to go almost unscathed, but not all.

Sand-grain-sized frass is good fertilizer, but makes a mess of cars, lawn furniture, etc.

*Insect poop.