Edith -- Leaf Miners Attacking Cottonwoods

by Edith Isidoro-Mills -- A couple of weeks ago I noticed a tiny white worm dangling out of my poplar trees. I looked up and saw a few leaves with the characteristic brown blotches these worms create in my poplars. I expect eventually my trees will turn prematurely brown and not have that lovely golden yellow we look forward to in the fall. Driving around the valley, I’ve noticed other trees with these brown blotches on the leaves. My observation a couple weeks ago is probably the first generation of these leaf miners to descend from the trees this year. These small white worms are approximately ¼ inch in length dangling from long silky strands.  They will pupate in the debris on the ground to emerge as very minute, nondescript gray moths swarming around the base of the poplars.  These moths mate and produce more generations before the season is over.  In fact, this cycle is likely to repeat itself several times until the first killing frost when the eggs and pupae will overwinter in the dead leaves. The leaf miners attacking the leaves of the cottonwoods and poplars in Churchill County are members of the genus Bucculatrix.  This genus has many species and found around the world.  All species in the Bucculatrix are tiny, usually white, worms that descend from trees on silk like strands they make.  Once on the ground, they pupate and emerge as very tiny nondescript moths.  There are usually numerous generations of these moths every year.  The last generation of the growing season over winters in the dead leaves from the previous season. Not all species in the Bucculatrix attack cottonwoods and poplars.  Each species is specific as to their host plant.  This means that the leaf miners attacking the cottonwoods and poplars in your yard are not likely to attack any other species of tree, shrub or herbaceous plant in your yard.  They may form a cocoon on leaves of other plants but they are not going damage that plant unless it is a poplar or cottonwood. One reason we are seeing these leaf miners in our area more consistently every year may be the milder, shorter winters we have been having in recent years. Milder winters allow more of the overwintering leaf miners to survive and produce the next year. Nothing can be done to get rid of the blotches on the leaves and very little to get rid of the leaf miners since they are inside the leaves protected from any type of sprayed insecticide.  The only type of insecticide that might be effective on leaf miners are systemic forms either taken up by the roots of the tree or absorbed in the foliage.  Timing of applications of systemic insecticides is critical and care must be taken to avoid other trees that produce edible fruits do not take up these insecticides. Systemic insecticides should be applied either as a spray that is absorbed by the leaf tissue or by placing granules around the base of the tree and watering them into the ground.  These insecticides should be applied in spring before signs of the leaf miners appear. At this time the best you can do is rake up the leaves, and debris where the larvae land and start to pupate.  After raking up the debris, immediately dispose of it.  The use of bright blue sticky traps around the base of the trees may help reduce breeding moths but it won’t eliminate them entirely so there is still likely to be more leaf miners in subsequent generations this season. Any form of control, chemical or otherwise, will prove fruitless unless every tree in the county is treated and this may be impossible.  Properly timed applications of either Dimethoate or Imidacloprid may help control Bucculatrix if properly applied in the spring.  However, treatment must be a community  wide effort. If the cottonwoods and poplars turn prematurely brown this year don’t worry they are not dying.  Evidence of this is that the trees that turned brown last year have put on healthy leaves this year even if they are starting to get blotches again.     Never miss a meeting or community event – keep an eye on the community calendar at https://www.thefallonpost.org/events/ If you like what we’re doing, please support our effort to provide local, independent news and contribute to The Fallon Post, your online news source for all things Fallon.