Treatment of Canadian Hemlocks against Woolly Adelgid

Asked March 31, 2017, 8:43 PM EDT

For a number of years, my row of Canadian hemlocks has been attacked by woolly adelgid. To suppress these insects without undue harm to beneficial insects (which live in the plants below and next to the trees), I have been contracting with a tree service to "Perform [two annual] foliage treatment[s with] Horticultural Oil, Pyronyl. . . Estimated Treatment Dates: 6/15/2017 thru 7/14/2017, 11/16/2017 thru 12/15/2017. 2% OIL in June, 4% OIL in November." Now the tree service would also like to apply one treatment of phosphites "to promote vitality and improve resistance. Estimated Treatment Date: 3/1/2017 thru 5/29/2017. • Application of phosphites enhances the activity of the plant’s own defense systems. This includes the formation of ‘necrotic blocking zones’ (dead cells limiting spread of a resultant lesion), rapid changes within the cell, production of ethylene and hypersensitive cell death (death of infected cells), production of lytic enzymes, thickening of cell walls and phytoalexin (antibody) accumulation in infected plants. Interestingly, when concentrations of phosphite residues are no longer detected there is still protection within the crop (an “immunisaton” type effect)." My questions are about doing good, but no harm: 1. How much would the phosphites treatment help my hemlocks stay healthy and fight off the woolly adelgid? In other words, is it worth spending the money for the treatment? 2. Would the phosphites hurt beneficial insects or other aspects of the environment? 3. Does it make sense spraying the hemlocks with both the horticultural oil and the phosphites? Thanks for your help.

Baltimore County Maryland

1 Response

We suggest having another certified arborist come in and give your their recommendations.
The oil sprays can help with scale insects, but there are better treatments for wooly adelgids like trunk injections. Once may last years.

Phosphite treatments are for root rots, not wooly adelgid control.

Here is our page on the pest: https://extension.umd.edu/hgic/hemlock-woolly-adelgid-trees

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