By Shawn Bernick
We all know that applying the proper dosage rate for a pesticide is critical for ensuring success with any tree health care product. Application of not enough active ingredient can result in unacceptable or reduced performance against the target pest. Applying too much may result in higher product costs; increased risk to the applicator, environment and non-targets; and some products can cause adverse effects to the plant being treated if too high of a rate is applied.
Although this concept sounds simple, there are many steps that must be followed to guarantee the proper dosage is applied. To make it even more challenging, many labels for products that are used for tree care recommend a wide range of dosage and water dilution rates, provide vague application instructions, and do not include specific language for arborists who are making applications to trees. For example, many labels require a specific amount of product applied per acre or per 1,000 square feet; this works great for turf and agricultural applications but not necessarily for trees or shrubs. If that wasn’t challenging enough, arborists must also consider the size of the tree being treated, the tree species, target pest, level of infestation and desired length of residual. Furthermore, products must be applied correctly to ensure the desired concentration is achieved throughout the canopy of the mature tree that you are treating. Wow, this may not be as simple as first thought.
Some of the key distinctions to proper dosing of tree health care products are as follows:
Diagnose the problem correctly. It does not matter what dosage rate you apply if you are treating for the wrong problem. Some products have modes of action that are only effective on certain pests.
Identify the host species correctly. Products such as tree growth regulators have species-specific dosage rates that require the applicator to apply more or less active ingredient based on the species. Furthermore, certain products can also cause phytotoxicity on certain trees or at higher dosage rates. Read the label closely.
Don’t just use what you have sitting around in your pesticide storage. Pesticides can lose their effectiveness over time which can reduce the amount of active ingredient that actually makes it to the tree. As a general rule of thumb, use opened pesticides in two growing seasons. If stored properly, which is a big “if,” unopened pesticides have a three- to five-year shelf life. However, some products have much lower shelf life.
Read and follow the mixing and dosing instructions on the label. Some products break down in high pH, or do not readily go into solution at higher pH. For example, the active ingredient thiabendazole, a common fungicide used for treating elms for Dutch Elm Disease, will settle out of the solution at higher water pH. Using lower-quality water which has sediments or impurities, can also impact the effectiveness of sprays, and impede uptake of tree injection products.
When tank mixing read the label. If you are unsure about compatibility of two or more formulations, always conduct a jar test the first time by mixing a small volume of spray mixture in a glass jar, and observing the mixture for settling or separation. As a general rule, add formulations to the tank in the following order: Wettable powders, flowables, solubles, powders, surfactants, emulsifiable concentrates.
Know your units and use the appropriate measuring device (ounces, millimeters, liters, gallons, cups and pints are not the same). Also, read carefully as some label language refers to units of active ingredient versus units of formulated product. It is important to keep your measuring devices clean and readable.
Consider the following when selecting the appropriate dosage rate for the situation: First and foremost, you must use label dosage rates. Ideally, applications should be done at the lowest labeled dosage rate that will result in acceptable levels of control against the target pest. Because most labels recommend a range in dosage rates, it is important to seek advice from your university extension service on what specific dosage rates are backed by science and have been effective on a specific target pest. Be specific, and request what data has been generated on large trees in the situation that you are dealing with. In many cases, tree care products are screened by scientists for efficacy on small nursery trees and not on trees representative of what arborists are treating in the landscape. Tree size can certainly impact the dosage rate that you select (see below). It is important to network with your colleagues in the industry to determine the dosage rates with which they have experienced success.
Tree Size — Just as humans require larger dosage rates as they grow, so do trees. As trees grow there is simply more leaf, branch and trunk tissue to protect from pests, requiring higher amounts of active ingredient. This is especially true for systemic products that are applied to the soil or trunk injected.
Level of infestation — Treating trees prior to a pest infestation may require lower dosage rates than treating trees after a tree becomes infested with an insect, mite or pathogen.
Length of desired residual — Certain product labels allow for higher rates to be applied to achieve a longer residual length of control. For operational considerations you may select products that allow for higher amounts of active ingredient to be sprayed onto trees.
Measure the diameter of the tree correctly. Diameter at breast height (DBH) is the standard way in which applicators determine the dosage rate for systemic products. Ensure DBH is being measured at 4.5 feet above the soil. If someone else measured the DBH it is a good idea to re-measure prior to doing the application and don’t try to save time by using the DBH measurement from a prior year’s treatment — trees grow!
Calibrate your equipment regularly. Check for leaks, clogged nozzles and excessive wear.
Measure the pesticide concentrate carefully and double and triple check your measurement; mix to the precise concentrations with the desired amount of water.
Follow application instructions to ensure uniform uptake and distribution of product. For systemic products, the objective is to apply the desired dosage rate evenly to the entire tree to ensure that the dosage is evenly distributed throughout the canopy. This means that soil injection holes should be spaced evenly around the tree, drenches should be poured evenly, and tree injection sites should be spaced evenly based on the manufacturer’s recommendations.
Utilize surfactants to improve coverage of sprays and apply the appropriate pressure with the correct nozzle aperture. Use the finest mist possible that will deliver the dosage rate to the desired height of the tree while minimizing drift.
Does the label recommend that you spray to leaf wetness or leaf run-off? Spraying to run-off with products that call to spray to leaf wetness will only waste product, time, and in most cases will not result in better performance.
Following these tips and distinctions will help to ensure successful pest management and client satisfaction.
Shawn Bernick is director of research and technical support at Rainbow Treecare Scientific Advancements, Minnetonka, Minn. For more information, visit www.treecarescience.com