I live in Niagara, Welland to be precise and have had a continuing problem through the summer with my yews. Leaves yellow somewhat and drop. They show signs of some sort of sucking insect but I have never actually seen an insect on them. I have sprayed them with soapy water, diazanon, malathion and pyrethrins without any apparent effect. Any help would be appreciated.
So sorry to hear that your Japanese Yew (Podocarpus macrophylla) is not doing well.
Based on the information you have provided it may be that your tree has scale which is a sucking insect common to Japanese Yew.
If the infestation is not too far along the plant can be saved but it will take some care on your part. First, cut off any heavily infested branches or where leaves are dropping. If your plant has just a few scale you might be able to pinch them off with your fingernail. Then spray with water to wash them off. Keep a close eye on the plant and pick off new scale as they appear.
If the plant is heavily infested one option is to clean the tree with rubbing alcohol applied gently with a cotton swab or Q-tip. Test a small part of the tree overnight first to ensure that the alcohol does not damage the plant. Clean carefully under each leaf and on the stems and trunk. This may require another treatment if the scale return.
Another option is to use an organic mineral oil to coat and kill the scale although this is best done when the plant is dormant. The simple process for applying oils is described in this excellent piece on bonsai scale. https://kitsunebonsai.blogspot.ca/2009/07/closer-look-bonsai-pests-scale.html
Once your tree is pest-free the trick will be prevention and early intervention if the scale comes back. Keep good air circulation around the plant, water well but keep the drainage good and use only slow-release, low-nitrogen fertilizer. Make sure to keep the interior of the tree open enough to allow light and air to penetrate.
This answer has been adapted from a previous question to Toronto Master Gardeners which you can read in full at: