It keeps appearing in the press and is spreading on the Internet: the rumor that our lilacs could be devastated by the emerald ash borer (Agrilus planipennis). But it simply isn’t true. I’ll explain why in a second, but first a short history.
Ashes (genus Fraxinus) are the main hosts of the emerald ash borer, a piercing insect accidentally imported from Asia and Eastern Russia and first discovered in Michigan in 2002. It will attack all true ashes, but rarely kills Eurasian ashes. It will not attack mountain ashes (Sorbus spp.) either, as they are not true ash trees and are not even related to ash trees.
North American ashes are the most susceptible to this insect and inevitably die from the infestation unless very expensive treatments are applied. The pest has eliminated tens of millions of ash trees so far and is spreading across North America, so tens of millions more are expected to die of the infestation. So owners of North American ash trees should indeed be concerned.
But where does the rumor the pest can also attack lilacs (Syringa spp.) come from? From a supposition that went a bit too far.
You see, lilacs and ashes are in the same plant family, the Oleaceae, as are, by the way, the olive tree (Olea europaea), privets (Ligustrum spp.), forsythias (Forsythia spp.) and jasmines (Jasiminum spp.), among others. And sometimes an insect specific to a certain plant can make the jump to a close relative.
This has happened in the case of the emerald ash borer. It has been found on the white fringe tree (Chionanthus virginicus) and a few specimens have died from the attack. However, it has shown no sign of a general move to fringe trees and indeed, in general larvae that do move to healthy fringe trees quickly die before doing any serious damage. They only seem to cause damage to weak or dying fringe trees.
The fringe tree is sometimes grown as an ornamental, but not nearly to the extent of lilacs. The latter are among the most popular garden shrubs. And the emerald ash borer has been tested on lilacs (as well as on other ash relatives, such as forsythia) and it simply cannot survive on them. Even when given no other choice of food species, borers quickly die when placed on lilacs.
And if they did attack lilacs, we certainly ought to know by now, as many lilac species are grown in Asia and Russia, where the emerald ash borer comes from, and in fact quite a few are native there. The fact that the emerald ash borer has never been a problem on lilacs growing inside its native range should already be a sign that this “problem” is in fact a tempest in a teapot.
So let’s quash that rumor once and for all: the emerald ash borer is a disaster for North American ashes, may sometimes attack weakened fringe trees, but it will not harm lilacs. So lilac lovers can rest in peace: their precious shrubs are in no danger whatsoever from this new pest.