Thursday, June 7, 2012

Tent Caterpillar Perspective

A few summers ago I was working in the forest on the northwest Olympic Peninsula and thought it odd that even though it was late June, the red alders were leafless. On closer inspection I realized that they had been stripped of leaves by caterpillars.

Marian Edain of Frosty Hollow Ecological Services had a write up at part of the Whidbey Environmental Action Network that provides a bigger picture perspective.

Tent Dwellers of the Northwest Woods

Once upon a time there were ancient forests of enormous Douglas firs and Western red cedars in the Puget Trough. There were Red alder present near streams and wetlands, and in the openings where the occasional giant tree came crashing down, but they were not particularly common. One of the species native to these forests was the tent caterpillar. It ate mostly Red alder leaves and converted the vegetation into compact packets of fairly intense fertilizer. Those packets (the polite term is “frass”) provided a shot of nutrients to the surrounding conifers. Good system.

           Then white folks settled the area and cut down the conifer forests. Nature abhors a vacuum and filled a lot of that bare ground with a band-aid: the quick growing Red alder. We ended up with large areas of alder where formerly there had been conifer forest. Given enough time (say 50+ years), Douglas firs would grow up among the alders, top out over them, and slowly shade out the shorter lived alders. Tent caterpillars, which had formerly been a pretty small component of the forest system, took advantage of those large areas of alder to explode their population. The exploding population ate the leaves off the alders, which allowed sunlight down to the forest floor where the little Doug firs were struggling along, at the same time as it delivered a great big jolt of fertilizer in the form of all those frass pellets. Even with the system out of kilter, the tent caterpillars were performing a very useful re-balancing service. And because they only produce one generation per year, they rarely actually killed any trees. They came, they went, and the trees produced another set of leaves.

           The only fly in this ointment is that the tent caterpillars also like to munch on almost everything in the rose family - like your apple, cherry, plum, peach, apricot, and pear trees. A lot of people want things all neat and clean, and they reach for the pesticide spray to kill those nasty tent caterpillars. Not good. Counterproductive. Does more harm than good.

           The caterpillars go through 5 stages called ‘instars.’ When they first hatch out they form those lovely tents in the trees. As they go through the stages they wander farther and farther from the tent in search of leaves. Eventually they migrate down the trunk of the tree where they were hatched and search for another high place where they will pupate. That will produce a brown moth who will lay eggs which look like a blob of grey styrofoam wrapped around a stem, and the whole process will begin again next year.

           There are several things which nature does to control the caterpillars without any help from us. The most useful is a virus (Polyhedrosis). One day the world is crawling with caterpillars. The next day all you see are dead bodies draped all over the trees. And that’s the end of the invasion. The other thing you see is bright white spots, usually on the forehead of the caterpillar. This is the egg of a wasp (kids love this part). The egg hatches and the larva burrows into the caterpillar where it eats it from the inside out. By the time it is ready to  emerge there’s just the shell of a caterpillar. When you pick up a tent caterpillar it will whip its head back and forth. This is its way of trying to avoid the wasp laying an egg on its head.

           In most cases the caterpillars cannot kill your trees so the thing to do is nothing. If you really can’t stand them, the best thing to do is to use plain old water. Get one of those nozzles that makes your hose act like a fire hose and just hit those tents. The spray should knock them out of the tree. If you feel vindictive, you can then pick up the tents and burn them, but you really don’t need to because you’ve destroyed the caterpillars’ home and they don’t know what to do next. If you have a particularly vulnerable young tree, you might want to wrap the trunk with cloth on which you then smear sticky stuff (you can buy this at the garden store). Just remember to remove the cloth after the caterpillars are gone.

           Bottom line: the tent caterpillars are working to re-balance our out of balance ecosystem. They don’t know when to stop, so just hose them out of your fruit trees.
No need for any toxics.


Tree Service Bronx said...

Seems like our ecosystems are not quite balanced, as seen with the abundance of these caterpillars. The last post made me happy, use a hose not chemicals.

Happy Gardening!

-Oscar Valencia

Elizabeth said...

Eight years old.
Running through the woods with my brothers, collecting caterpillars (army worms, we called them) in our hair.
Into the house (also in the woods). Very upset mother.