In Mike wrote:
Both say pests locate your roses by smell and the
strong smells of these alliums prevent them from finding your
treasures.

That seems a valid theory to me. Then again, I underplant lavender
between my roses. It's supposed to have the same masking effect, but my
bushes still get infested.

This year the pests been far fewer than in previous years, but I
attribute that to a better spraying program this season. I say this
because my 'picking' bed at the back of my house is not underplanted
with anything - it suffers no more or less from pests than my display
bed which have lavender.