Hemlocks have woolly adelgids

[Klebba]

I've got 12 hemlock trees infested with woolly adelgids. The local garden store recommended Bayer Tree & Shrub insecticide and recommended that I drench them. After measuring the circumference of the trees, also as they suggested, it looks like mixing one ounce of the insecticide per inch of circumference for all those trees in a 2-gallon watering can means that I'll need 6 gallons of this stuff @ $90 per gallon!

My question: is drenching really necessary or can I just use the alternate method of spraying them at a much lower concentration and expect decent results? I thought I might drench the ones that are too tall to reach with the sprayer and just spray the ones I can reach.

John, Maryland

[Chas Hurst]

I have used Safer Soap with good results, and much cheaper.

[Timothy]

Good day Jonh. Sorry to hear that you got the woolly adelgid. These are rather difficult to get rid of. Before you go and purchase a bunch of chemicals and the like, contact you local agriculture extension office. There is a process that needs to be followed to kill them off and the kill window has passed for you for now. Once the adelgid has the waxy fluff, they are next to impossible to kill effectively.

maryland agriculture extension

information on hemlock woolly adelgids.

[Klebba]

I guess you're referring to the use of horticultural oil sprayed on in the Spring (Mar-April) and fall (October). We've had this problem for a year or so, and many of my trees are too tall to spray. This article:

to indicate that drenching with Bayer or Merit is OK as long as it's not done near ground water sources, streams or other runoff areas, and is done in the spring or fall when there's enough ground moisture for the tree to absorb the insecticide. I found today that 2 gal. of Bayer drenched 8 trees with circumferences ranging from 22-54 in. I believe another 2 gallons will take care of the 12 smaller trees I have left. I think I'm going to go that route, despite the high cost ($90/gal) of the insecticide. In the past spraying them has not been sufficient to keep the critters away for more than a couple of months, and I've watched the trees go steadily downhill. Hopefully a couple of years adelgid-free will get them to spring back.

[newsreader]

Most of the reasonably knowledgeable folks I have discussed this with here (also Maryland) tell me they have had best luck with systemic treatment. You need to get the Merit into the soil so the roots suck it up. I had decent success using granular Merit & scratching into the soil / mulch area under my hemlocks, then watering it in thoroughly. It may be too late in the season by now to control them this way.

There are tons of articles on the subject by various regional Universities. One short concise one is here:

also used granular Merit in the soil at the base of my Purple Leaf Plum trees to help with controlling Japanese beetles, both the grubs in the soil as well as the adults munching on the leaves. The numbers of dead adults under these treated trees was amazing.

David in Olney

[JP]

I went with Bayer Tree & Shrub (1 oz. per 1" circumference in a 2gal. Watering can). 2 gal. covered 8 trees of between 22" and 54" and I've got to get another 2gal. for the rest of the hemlocks. As you say, it is late, and since Maryland tends toward dry summers I'm figuring on watering the trees heavily to get the systemic treatment absorbed by the roots. According to my local garden center (River Hill) it should take effect in about a month and the results are dramatic.

JP (West Friendship)

[Newsreader]

I'm in a new house now, about 1 year. It's an old somewhat overgrown property in Highland, right down the road from River Hill. Small world!

There are a couple Hemlocks on my property line, and many on the neighbors property. He hired an arborist to spray all his about a month ago, & I kicked in bit to have 'mine'?? done.

I also recently bought 2 quarts of the Bayer Tree & shrub to try on some flowering cherries I put in late last spring. Hopefully the Jap Beetles damage will be minimized. Plan to put it on tomorrow. Hope it works. Pretty hefty price tag on that stuff.

David