The news broke on July 14th. DuPont's Imprelis®* herbicide is the prime suspect in a series of tree deaths, the main victims being eastern white pines and Norway spruces. Owners of these conifers are pointing the finger at DuPont, but is Imprelis® to blame?
Washtenaw Acquisition LLC , Polo Fields East LLC and Polo Fields Golf & Country Club LLC certainly think so. On the same day the tree-icide story hit the news, the golf and polo group filed a federal class action lawsuit.
The chemical at the center of this legal drama is 6-amino-5-chloro-2-cyclopropyl-4-pyrimidinecarboxylic acid, known by its chemistry nickname 'aminocyclopyrachlor'. Perhaps thinking aminocyclopyrachlor was too long and not sexy enough, DuPont dubbed it Aptexor™.
DuPont's Imprelis® herbicide contains aminocyclopyrachlor and its potassium salt (Figure 2). Like other herbicides, aminocyclopyrachlor severely inhibits or kills undesirable plants while leaving the desired ones (mostly) alone. The undesirables targeted by aminocyclopyrachlor are various broadleaf weeds and bushes, such as those listed here (page 1).
Aminocyclopyrachlor is part of a group of compounds that mimic the behavior of plant hormones called auxins. These hormones play various roles in plant growth and development. Auxin mimics like aminocyclopyrachlor can play these same roles. As with many chemicals, it's all about the dosage when it comes to helping or hurting. At high concentrations, auxins and their mimics behave as herbicides.
Imagine if the invitation to your intimate dinner party went viral. A flash mob turns up at your front door, rushes in and soon your home is a rave with hundreds of club kids. These ravers soon spill into the street and it's an impromptu block party. You, your house and your yard were overrun - and now look run-over.
Now imagine a flash mob of auxin mimics showing up in a patch of weeds, getting inside the weeds through the leaves and roots. The many roles auxins play in plant growth and development gives a auxin mimic flash mob several modes of attack. The weeds' cells rapidly proliferate clogging up the plant's vascular transport system, cell membranes and their resident proteins don't work as they should, RNA production is interfered with... An overdose of aminocyclopyrachlor doesn't just mess up one thing, it messes up several things - too many things for a weed to handle in short order. What's left is a stunted, malformed weed which will die in days to weeks.
If a high concentration of aminocyclopyrachlor doesn't sound like the best thing for turfgrass, don't worry. Turfgrasses can handle these concentrations of auxin mimics. But can trees like Eastern white pines and Norway spruces? DuPont says conifers were included in their trials with no negative effects observed. Given the reports of tree deaths, both DuPont and the EPA are said to be investigating aminocyclopyrachlor's possible role. In scientific literature, little on-point research has been published.
Imprelis injury seems to be related to the soaking spring rains of April and May (I am not aware of any tree injury following fall applications), and to some particular characteristics of the herbicide. Even though applicators I have spoken with did not apply the herbicide within the “drip line” of affected trees (as directed on the Imprelis label), injury still occurred. Research has shown that root spread of trees far exceed the branch spread; thus, root uptake from leached herbicide residue can occur outside of the drip line (Freucht, 1988). Although leaching of herbicides is more of a risk in sandy soils with low organic matter content, Imprelis-related damage occurred in several locations on heavy, clay soils.
It's possible that, like the targeted weeds, the conifer victims absorbed aminocyclopyrachlor through their roots from soil. Perhaps those heavy rains spread the auxin mimic farther than intended and within range of tree roots. But "possible" and "perhaps" are a far way from proof of Imprelis®'s culpability. To quote Dr. Landschoot, "Right now, there is much speculation about the details surrounding tree damage due to Imprelis applications, but the exact reasons still need to be sorted out."
______________________Notes ^DuPont's Imprelis® is a herbicide sold to lawn care professionals. Image Attribution Figure 1: Office 1010 clip-art Figure 3: Image was captured from http://bit.ly/nRqwWx