Answer: Scale is very difficult to control on euonymus, partly due to the impervious form of the scale and partly due to the densely twiggy, leafy form of the plant. Control requires diligence over time. You would need to remove the worst affected branches and/or plants, rake up all plant debris and mulch, and spray with horticultural oil during the winter (correct spray timing is very important) followed by sprays when the insect is at the crawler stage. (Your local county extension should be able to tell you more precisely when to spray for this stage, it varies by geography and by the weather each year. They may also suggest other controls.) And then repeat as control is not going to be immediate and may in fact never be quite complete. Here is more information about it. You may need to cut and paste the complete url into your browser to make it work correctly.
Ivy can also be affected by pests. (Hedera helix)
To prevent erosion, you might want to work in stages with the replanting, or you can use a series of timbers laid crosswise along the hill to create a terracing effect, and lay natural fiber burlap (pin it down to hold it in place) over the soil and plant through it, then cover with mulch. Planting a steep slope is never easy.
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