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Hemlock Woolly Adelgid Treatment



Pesticides can be used for hemlock woolly adelgid treatment to save hemlock trees. Proactive management is necessary to combat the HWA to avoid infestation and death of healthy trees.  HWA can be managed with a variety of strategies that prevent new infections and can actually save trees that are lightly infested.  Hemlock woolly adelgid is a very small non-native aphid like insect that infests hemlock trees, eventually killing the trees within about six years.


This insect was first found in North America, in 1951 in Virginia and has since expanded to include numerous states in the eastern US.  It has recently been found in southwest Michigan.




The woolly adelgids are so small that the individual bugs are difficult to see.  They have a white cotton-like furry covering on their bodies and egg masses that is visible when numerous adelgids infest the areas along the branch tips. The HWA feed on sap at the base of the hemlock needles which kills the needles and they fall off, defoliating the tree.   It takes about 4 to 6 years for the infestation to stress the hemlock tree enough for numerous large branches to die back, and eventually the tree is killed.   But the good news is that the white furry infestation along the needle twigs are noticeable and, with adequate treatment, the HWA can be removed.  If a tree is not too far gone (too many dead branches) then it can be saved.



HWA is spread primarily by the wind, and by birds and mammals.  They stick to the fur and feathers and are transported as hitchhikers. Work is being done to identify and use native parasitic predators that feed on HWA, but this solution is in the early study stages.  The conventional preventative and corrective responses for an invasive species outbreak are being pursued as detailed below.


To slow the spread of HWA there are state quarantine programs which restricts the movement of materials which might harbor the insect. It is difficult to control the spread of the insects that blow in with the wind.  But much of the spread of infestation has occurred because of human transport of infected hemlock wood, branches and chips which carries HWA to new areas, and this behavior can be managed.  Also, if trees are showing signs of infestation on or near your property there are some things you can do to limit or slow the infestation, including removal of the host infested trees, and chemical treatment. The goal of HWA control is to reduce the expansion of the HWA population, to limit or slow the attack of healthy trees and the spread to new geographical locations.

Remove Infected Trees

If you do not want to attempt chemical treatments (discussed below), then trees that are infected with or have died from HWA should be removed and properly handled to prevent continued propagation of the pest. These treatments include debarking, chipping, burning, burying or splitting, and drying the wood.

Do Not Transport Infested Wood

Cutting up trees infected by HWA for lumber, firewood or tree trimming and transporting the infected wood to new areas will transport the HWA to infest and kill trees in these areas.  Do not transport any wood, bark or brush from HWA killed trees.


HWA kills hemlock trees but it takes years and there is time identify the infestation and take action to save trees. Chemical insecticide treatments can be conducted to protect and save valued trees.  Only infested trees need to be treated.   These applications include drenching the soil around the tree with insecticide, spraying the tree trunk, and trunk injection treatments. If done properly, the insecticide will kill the resident HWA and keep new infestation away from the trees.  If a tree is lightly infested, the insecticide treatment kills the HWA and the tree can start to heal and ultimately thrive again.

Soil Drenching

This method involves mixing the appropriate concentration and volume the insecticide imidicloprid, diluted with water, and pouring onto soil or injecting into the soil around the base of the tree.  The tree roots take up some of the insecticide as it moves downward through the soil, then the treatment is circulated throughout the tree, leaves to roots.    Soil application of insecticides can take 6 to 8 weeks to reach the effective levels throughout the tree.

This method is relatively simple and easy to perform by landowners and the comparatively inexpensive insecticide imidicloprid is the weapon of choice.  There are good instructions on the bottle labels for how to mix and apply as a soil drench around the trees.

Trunk Spray

With this method of HWA tree treatment, the lower portion of the tree trunk is sprayed with insecticide which penetrates through the bark and into the vascular system of a tree.   The insecticide dinotefuran is used for this method, and is sold under the brands Safari 20 SG, Transtect and Zylam .  Trunk spray treatments are done annually with dinotefuran.

Trunks spray is implemented by spraying all around the tree wetting the bark from the base up to about five feet high, taking care to avoid windy or rain conditions during application.   The insecticide is quickly absorbed into the vascular system, and is circulated throughout the entire tree within two or three weeks. Trunk spray can be conducted on the hemlocks during mid to late spring after other trees have leafed out and throughout the summer months.

Trunk Injection

Direct injection of pesticides through the bark and into the trunk of hemlock trees is an environmentally sound and economical way of treating HWA.  With direct injection into the circulatory system of a tree, a much lower volume of chemical is needed because it all goes into the tree, leaving none on the ground or on the exterior tree bark for exposure to unintended receptors.  Trunk injection is the most efficient and works the fastest of HWA control methods, with the pesticide circulation throughout the tree within a few days. Trunk microinjections should be conducted during spring after other trees have leafed out and throughout the summer.

Healthy or lightly infested hemlock trees can be treated using an insecticide to prevent infestation by HWA.  This can be easily conducted using Chemjet® Tree Injectors and a special injectable form of imidacloprid into small drilled holes at several locations around the circumference of the tree. The chemical is then carried throughout the tree, from up to the needles down to all of the roots, effectively killing all HWA currently in the tree, and preventing future infestation.

With implementation of microinjection using the inexpensive Chemjet® Tree Injector, it is economical for protection of many more hemlock trees. This brings to reality an economical option of DIY tree injection for all of the affected hemlocks on your woodlot to save your trees and control continued expansion of the infestation.

Injection Procedure for Hemlock Woolly Adelgid

This example injection procedure for HWA is adapted from the Chemjet® manufacturer suggested procedure, and from the Ima-Jet tree injection insecticide label.  Ima-Jet is 5.0% solution of imidacloprid,  containing about 0.055 gram of imidacloprid per milliliter.

  1. Confirm with a tree expert that this procedure is appropriate for your tree.   You may not have an HWA risk or there may be other circumstances that would cause this procedure to be inappropriate. Also there is always the risk that this procedure may harm your tree.
  2. Only inject hemlock trees during the seasonal time period necessary to be effective for treatment of HWA (during warm months when transpiration is occurring, indicated by green leaves on other trees).
  3. Only inject trees after a good rain or after substantial watering. Do not inject trees during drought conditions.
  4. Plan for use of one Chemjet® Tree Injector every eight inches around the circumference of the tree.
  5. Put on rubber gloves and safety goggles for use during all mixing, injection and cleanup work.
  6. Disinfect Chemjet® injector and drill bit by washing and scrubbing in diluted bleach solution or Lysol disinfectant solution. Pull disinfectant solution through the nozzle filling the injector several times.  Rinse with clean water.
  7. Prepare the imidacloprid injections per the labeled instructions.  From the Ima-Jet label the maximum dose is 8 milliliters of Ima-Jet for every inch diameter of the tree. Calculations using this dose and typical tree diameters yield just under 8-inch spacing between injection points around the circumference of the tree using Chemjet Tree Injectors filled with 20 ml of Ima-Jet.
  8. Drill the first three holes at eight inch intervals around tree at 45 degree angle downward no more than 1.25 inch deep. Only use a 11/64 drill bit.   Drilling more than three holes may heat up the bit and scorch the wood which will inhibit injection.
  9. Pull the chemical mix from step 7 into the injector until full at 20 ml and lock the handle back while twisting.
  10. Insert into recently drilled hole. Push into place and seal (don’t twist because the nozzle may break off).
  11. Using both hands, hold the Chemjet® in place and then twist and release the red plunger so that injection begins. If there is leakage push the nozzle in harder to seal.   Repeat for remaining two holes.
  12. After all chemical is in tree, pull injector straight out of hole. If chemical starts oozing back out of open hole, replace Chemjet® and leave there for another half hour or use a plug.
  13. Repeat with three more holes, going around tree.
  14. If an injector is taking chemical slowly, leave it in place and move on. It could take up to 24 hours.
  15. IMPORTANT: Disinfect drill bit and all Chemjets (inside and out) before using on any other tree per step 6.  Disinfect your hands and put on new rubber gloves to avoid potential for transferring diseases to next tree.


Additional information

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Hemlock Woolly Adelgid Treatment

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Chemjet Tips and Tricks

Video: Injecting for Oak Wilt

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