Wednesday, May 7, 2014

Western Tent Caterpillars Returning to Kitsap

If you've lived in Kitsap county since the early 2000's, you probably remember the tent caterpillar infestation.  Many deciduous trees, especially those in the Birch, Willow, and Rose families were more or less defoliated by June.  I remember seeing whole neighborhoods where every Red Alder was completely bare of foliage due to this voracious, leaf-feeding insect.  On heavily infested trees, their numbers were so great that you could hear the "rain" of their frass as it fell from the tree.  Frass is insect may want to mind where you stand outside.  Truly it was not a pretty sight.

And that sight is returning to our county.  I’ve seen them.  They are here. In the scientific literature they are called Malacosoma californicum.  The moth is an unassuming brown flyer.  The caterpillar is a much showier hairy orange and black with tiny bits of blue.

Western Tent Caterpillar
Jerald E. Dewey, USDA Forest Service,

Early in their years-long presence, the infestations do not start out as eye-catching spectacles, but rather as random tents here and there in the spring.   They may go largely unnoticed from year to year until their numbers progressively build until you can’t help but notice them.

They hatch out of their golden egg masses and begin feeding at night, retreating to their tent colonies for safety during the day.  As they get older they seem to leave their tents and feed during the day as well.  These writhing clumps of caterpillars are efficient defoliators.  Should they run out of foliage on a tree before pupation, they will crawl or drop off in search of fresh food, a.k.a. your garden plants particularly your deciduous (and sometimes evergreen) trees.  When they’ve had their fill, it’s time to pupate.  They leave their food trees and crawl around looking for a place to snuggle up and spin their cocoons.
Tent produced by feeding caterpillars
Brytten Steed, USDA Forest Service,

After a brief pupation, they metamorphose into moths and begin flying around in search of mates and then a suitable place to lay their egg masses.  By “suitable place”, I mean the trees in your yard and neighborhood. The females locate a decent tree for their offspring to feed and grow where they land and lay a golden mass of eggs in a single patch.  Usually they pick a spot at the end of a branch on the outside of a canopy.

Adult moth with golden egg mass.
Jerald E. Dewey, USDA Forest Service,

At the peak of a heavy infestation, the moths can reach biblical plague numbers.  I recall during the most recent infestation, I stopped by the Kingston Albertsons before sunrise on my way to work.  The trees nearby had been heavily infested that spring and the adults had emerged from their cocoons.  If you have a porch light, you know light’s attractant effect on moths.  Now imagine many giant parking lot “porch lights” in an area with hundreds of thousands of moths.  It was like a scene from a horror movie. Every light had a frenetic shroud of beating wings encircling it. The moths flutter around the lights until tiring and falling to the ground to die.  I literally could not walk through the parking lot past the lights without crushing dead moths under my boots. Well, mostly dead. There were that many. This spectacle will surely be repeated in this same parking lot.  Maybe not this year but one or two years very soon. The last infestation lasted about five years; each successive year birthing a few less moths as the previous year until the cycle ramps up the other way.   

You may ask what there is to do to fight back, but the truth is a healthy tree can withstand an early Spring defoliation and bounce back. The caterpillars only attack for a short amount of time, only reproduce once a year, and are typically gone by the end of June. Fortunately we have a couple tiny champions to fight the onslaught of the black and orange caterpillar hordes:  a virus and a parasitoid fly.  This is not to say that birds and yellow jackets don't help decrease numbers with predation, but the tiny ones pack the bigger punch and over the span of a few years, they beat the caterpillars into submission.  

If you’ve either had trees affected by this caterpillar or have had to deal with millions of the little grubbers, you know the sense of satisfaction you feel from finding them parasitized.  The first of our two common parasites is a virus.  Affected caterpillars are pretty obvious after they’ve died.  They tend to shrivel up and look wilted; hanging down from the branch or leaf where they died.  With each passing year, more and more succumb to this virus, reducing the following year’s brood.

Another less obvious but very important parasite of the western tent caterpillar is a Tachinid fly.  This fly takes down the caterpillars in a most insidious manner.  The adult female will locate an caterpillar and lay a tiny white egg on the caterpillar, usually around the head. The egg hatches and a tiny larva burrows into the caterpillar to feast on it’s insides, leading to the caterpillar’s eventual gruesome death.  Look for these little white dots (eggs) before you squash the caterpillar!  If you find a caterpillar with an egg on its head, leave it so the parasitic fly may survive to produce more flies next year.

What’s that?  You don’t want to wait a few years for the parasites to make the caterpillars go away? They’re eating your prized flowering cherry NOW?  Well there are some possible options for you.

Option number one:  live with it. I know you may not want to hear this, but the reality is most tree species can live with a few successive, annual defoliations provided they’re healthy enough to begin with.  Hundreds of thousands of trees were affected by the last cyclical infestation of western tent caterpillar and we weren’t left with forests and neighborhoods of tree corpses. Certainly we lost some; not all trees are perfectly healthy.  The majority of affected trees survive and after the caterpillars disappear in June, the trees will leaf out again and be looking normal by August if not sooner.

Option number two:  prune out the silken tents produced by the caterpillars.  The best time to do this is when the caterpillars are still young.  In Kitsap county, this is usually mid to late april.  The younger the caterpillars are when you remove their tents, the less damage will have been done to the trees.  With an extension pruner or pole saw, you can reach up and prune out branches with tents on them.  Ideally you should do this pruning in the early morning when most individuals are still in or around their tents.  You will leave behind fewer caterpillars wandering on other parts of your tree to continue feeding.  

Option number three:  It’s too late to do it this year, but NEXT winter when the branches are bare and easier to see, look over your tree’s branches looking for the golden egg masses of the western tent caterpillar (see the photos included with this article).  When you find some, scrape them off.  If left on the tree, the eggs will hatch and the tiny little caterpillars will proceed to eat and grow.  But if removed, they can’t do any damage.

Option number four:  I debated whether to include this option in this article.  I think for most residential homeowners this isn’t really necessary (and possibly ill-advised).  But in the interest of being thorough, I’ll mention it: spray the little buggers.  Whole blog posts (even a whole blogs!) could be spent on discussing the pros and cons, how-to’s, and whats of spraying pesticides, but there’s not room in this article to do it so let me try to keep it brief but relevant.  Here are some rules for spraying (maybe someday i’ll turn them into a blog post):

  1. Don’t do it!  It’s hard for me to justify the use of pesticides for purely ornamental purposes.  If you’re growing food that your family relies on, that’s another matter, but no one’s going to actually suffer if your prize roses get defoliated (options 1-3 will be better for that anyway).
  2. If you’re going to ignore rule #1 then PLEASE follow rules 3 -5.
  3. Read and follow the label instructions.  The pesticide label is a legal document.  Deviating from the label instructions is illegal and could land you in hot water.  Not to mention that the instructions explain proper use of the pesticide and ignoring them could harm you, your kids, your plants, or create greater harm to the environment.
  4. Don’t spray during bloom.  Most pesticides that will kill the western tent caterpillar will also kill bees attracted to your tree’s flowers.  Wait until your tree is completely finished blooming.  We already have enough problems with colony collapse disorder, a serious situation that’s already impacting agriculture.  Seriously.  Be proactive and get out there with extension pruners in April and early May or hire someone to do it for you.
  5. Use bt instead of synthetic or inorganic pesticides.  Bt, formulated as a pesticide, is spores of the bacterial species Bacillus thuringiensis. It is much safer for everyone.  There are various strains of this bacterial-based pesticide depending on what you want to attack.  Read the product label to make sure what you buy will actually work on caterpillars (look for B. thuringiensis kurstaki).  Perhaps the most important bit of advice here is to apply Bt while the caterpillars are young.  Older caterpillars are much more resistant to it.  Additionally, sunlight degrades Bt so apply it late in the day before the caterpillars leave their tents at night to feed. For Bt to work, it must first be ingested by the caterpillars so make sure to apply it directly to the plant’s foliage.

From a gardener’s perspective, this may all sound scary.  Voracious worm-like creatures devouring our precious garden plants. Truly it’s not fun to watch your hard work disappear down the gut of these little beasts.  I know from experience as do many who will read this.  But the biologist in me has this to say to the gardener in me:  The western tent caterpillar is just one of many of nature’s grand spectacles.  It shows up at your door, makes a big, frightful show of things, and then goes away.  Everything will return to normal.  Maybe the best thing to do is relax, enjoy the show, and marvel at the beautiful complexity of nature.

Updates to this article:


Special thanks to Tess Haddon for editing assistance.

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