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Soil injection application

Many of our remaining mature eastern hemlock trees are only alive today because they have received chemical pesticide treatments that have acted as an immediate life-support system. Chemical treatment is still the only completely reliable way to save an individual tree from mortality due to HWA (trees can still die due to other factors).

The good news is that chemical treatment is significantly cheaper than it was 4-5 years ago and we now have a better understanding of how to apply it, so you can get up to 5-7 years or longer protection from one application. One fairly large tree can be treated for less than $10. This is still by far the best option for most homeowners with a few dozen trees or less.

Imidacloprid 75WSP soil drench

The neonicotinoids, imidacloprid (Merit and others) and dinotefuran (Safari), provide excellent control of hemlock woolly adelgid (HWA) when applied as a soil drench or soil injection. Both products are loosely bound to soil particles but are sufficiently water-soluble to be absorbed by tree roots following soil application.

Always make sure to follow state law and the chemical label when applying pesticides. 

Safari soil drench is typically used on heavily infested trees and/or super stressed trees to give a quick knock down to HWA. It will only last 1-4 years, so it is good to go back the next year and do a follow up treatment with the longer lasting Imidacloprid. Treatments work best during the spring or fall, and after a soaking rain has just occurred.  Soil should be moist, but not saturated or frozen.

Dinotefuran has a faster uptake and is therefore a good choice for very large or stressed trees that may be in need of immediate attention.  Dinotefuran is much more water-soluble than imidicloprid and less tightly bound to soil particles. This characteristic is beneficial by increasing accessibility of the active ingredient for root uptake but can be perceived as detrimental regarding potential water contamination through run-off and soil leaching. Note: Safari bark spray is an alternative application method that does not put chemical into the soil and is therefore a good choice for treating trees adjacent to water, in a seep or drainage, in sandy soils, or where roots of a flowering tree may be exposed to chemical in the soil drench method.

Because the Safari product is still under patent, it is much more expensive than imidacloprid and protection may only last a year or two. Imidacloprid, on the other hand, is no longer under patent. Therefore, the generic product can be acquired at a much lower cost. It also has a longer residual protection of 5-7 years or more. Imidacloprid is preferred and most practical for most situations; however, a combination of the two chemicals may be appropriate in certain situations.

Imidacloprid can safely be used on streamside hemlocks as long as you avoid injecting into a scarped bank or into deep sand on the stream side of the tree.  No buffer zone is required when treating hemlocks in ornamental landscapes, but direct contact with water is prohibited.

Detailed instructions for chemical HWA-control are available from the Hemlock Restoration Initiative “Info for Landowners” page.

Other resources:

NC Cooperative Extension Service

US Forest Service Northeastern Area – Forest Health Protection

US Forest Service Southern Region – Forest Health Protection

Save Georgia’s Hemlocks (Note: the information on SGH’s page is directed towards GA residents; NC residents must follow NC laws regarding pesticide application–check the label!)

Although chemical treatments provide essential short-term HWA-control, they require repeated applications and are impractical in many situations. Therefore, other long-term options are being tested, evaluated and cautiously pursued. Learn about some of them here.

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