Potted Seedlings


Large Trees & Shrubs

Ecological Services


Invasive Plant Control

Ordering By Mail

Email White Oak Nursery

Contacts & Directions

Natural Landscapes

Growing Plants From Seed

Purchase Manual: Using Native Plants in Urban Landscapes

Essays & Articles

Nursery Hours, Plant Sales & Classes

Customer Comments

Related Sites

Treating Canadian Hemlock for Hemlock Wooly Adelgid

Hemlock wooly adelgid (HWA) is a recent invasive insect pest affecting Eastern hemlock trees throughout the Fingerlakes region. Eastern hemlock has little resistance to HWA. Hemlock trees throughout the eastern US, from Georgia to New England, have been deveastated by this pest.

HWA is native to Japan and was first observed in downstate NY in 1985. In recent years it has established in the Ithaca area and has spread northward along the fingerlakes and continues to spread. Spread and dispersal is unknown but thought to occur by wind, hitch hiking on birds and human movement of plant material.

HWA feeds at the base of needles, causing the needles to dessicate. The needles turn a grayish-green color and the tree to take on a gray cast. HWA feeding causes the needles to prematurely drop from the tree and prevents new buds from forming, so little new growth is produced. Dieback of entire branches will occur on infested trees as needles and buds are killed, progressing from the lower branches upwards to the crown. Trees mortality may occur in as few as 3 years or take up to 8 years. Mortality of Hemlock trees has occured and is currently taking place throughout the fingerlakes region.

For more information go to CCE suffolk county and search for Hemlock Wooly Adelgid Fact Sheet

or to the NYS DEC website Hemlock Woolly Adelgid Brochure (PDF)


LIFE CYCLE: Hemlock Wolly Adelgid has a two generation overlapping life cycle.
Adult nymphs produce a single white ovisac from March through May, which can contain up to 300 eggs.
The ovisacs are white and cottony looking and are an easy field identifying feature as seen from the pictures.
Young nymphs, called crawlers, hatch from the ovisacs beginning in April and search for a sutable feeding site at the base of a needle.
The crawlers quickly develop through 4 instars (molts) and mature into adults in June. The adult HWA produce ovisacs in June - July.
Crawlers that hatch from the June - July ovisacs settle at the base of hemlock needles where they feed for a few days and then go into the summer
aestivation (dormant) period, where they remain until October. In October the young nymphs resume development and continue feeding until February when the nymphs mature into HWA adults.
The adults then begin to oviposit in March through May completing the 2nd generation life cycle of HWA.


FIELD IDENTIFICATION: Look at the underside of small hemlock twigs at the base of the needles.
HWA ovisacs (egg cases) will appear white and cottony. This is the best idication of the presence of HWA and provides a good measure of the severity of the infestation.
HWA often infest lower branches first where detection is easier to obeserve. Trees that do not have lower branches can be scouted using binoculars to look into the upper canopy.
Look for the tell tale signs of white at the base of needles on the underside of twigs.

TREATMENT: Infested trees will succumb to HWA feeding in from 3 to 7 years. There is no reliable treatment for HWA other than applying a systemic insecticide to the tree.
I provide two different treatments that can be used to treat HWA. Each treatment has its benefits and limitations. Both materials are relatively expensive to apply but the expense needs to be balanced against the value of the hemlock trees you will lose if you do not treat. There is also the cost to have a tree removed professionally, which can run into the thousands of dollars

TIMING:Trees can be treated anytime the tree is actively growing and uptaking water: Fall through early winter (Sept, Oct, Nov.) and late winter to early spring (April, May, June) is the best time to treat trees.

Safari Basal Bark Spray:
Safari is applied as a basal bark spray. The insecticide is sprayed on the bark of the tree from ground level up to 5 feet above ground level. The insecticide is absorbed through the bark and into the tree. The active ingredient used is dinotefuran, Product name is Safari 20SG. Safari 20SG is a restricted use insecticide and can only be applied by a licensed certified pesticide appplicator. Dinotefuran is very mobile inside of the tree, it will be absorbed through the bark of the tree and be transported within the tree up into the canopy and distributed to all parts of the tree, where it protects the foliage from HWA. Research has demonstrated that control begins a couple weeks after application and can last for 8 to 12 months. Dinotefuran gives very quick and effective control of HWA. Elimination of the HWA population after treatment may delay recolonization for a longer period of time, but monitoring should be used to verify this.

Dinotefuran gives fast effective control of HWA and is quick and easy to apply to infested trees. This is important for heavily infested trees and old trees that need immediate protection. The limitation of dinotefuron is that it is expensive to use and control is limited to about 1 year from date of treatment so reapplication needs to be done every year or every other year.

Imidacloprid Basal Bark Spray: The active ingredient used is Imidacloprid. Imidacloprid is a restricted use insecticide and can only be applied by a licensed certified pesticide appplicator. The insecticide is sprayed on the bark of the tree from ground level up to 5 feet above ground level. The insecticide is absorbed through the bark and into the tree and then moves slowly through the phloem of the tree to the foliage. It may take from a couple of months to a year or more for the active ingredient to move up to the foliage. But once the imidaclopid is in the foliage it will provide long term control of HWA.   Control is estimated to last from five to seven years.

Where hemlock trees are already heavily infested and trees are in decline it is advised to do an emergency treatment using dinotefuran (Safari) as a basal bark spray to get fast effective knockdown of HWA. Applying Imidacloprid at the same time as Safari will give long term control and reduce the labor cost of making two separate applications.

After treatment for HWA contiue to monitor your trees for HWA egg sacs. There may be residual cotton on the tree for a period of time or it may take some time for the chemical to move within the tree and control HWA. Positive signs of HWA control and tree recovery is the development of new buds and new light green shoots in the spring. It may take a couple of years for infested to trees to produce new buds. Long term monitoring up to several years after treatment will indicate when the effectiveness of the treatment is declining and additonal treatment may be necessary. There are ongoing efforts to establish biological control agents, to control HWA but these will take many years to determine if or when they are effectiveness.

Estimate of Cost to treat hemlock trees: These estimates are provided to give you a basis of calculating the cost to treat your trees. Naturally there are other factors that will determine the actual costs, such as ease of access to the trees, total number of trees to treat, travel time to site etc. This estimate will also help you determine if you are getting a fair price for having your trees treated. Application rates are based on Diameter at Breast Height, DBH (the diameter of the tree measured 4 ft from the ground). To calculate the diameter of the tree use a tape measure or length of rope and measure the circumference of the tree, divide by Pi ( 3.1416") = the diameter in inches.

Cost to Treat 100" DBH of Hermlock Trees
Product (Active ingredient) 100" DBH Cost
Safari 20SG (Dinotefuran) 100" DBH $200.00
Imidacloprid 2F ( Imidacloprid) 100" DBH $80.00
Joint application, Safari & Imidacloprid 100" DBH $240.00

Demonstrating treating a hemlock tree using a basal bark spray. Trunk of tree is treated from ground level up to 5ft high and around the entire circumference. Both Safari and Imidacloprid can be applied at the same time.