The raspberry cane borer, Oberea bimaculata, is a particularly nasty garden pest and for some reason there seems to be an unusually large infestation of them this year. Our property contains all manner of cane berries including a large patch of local wild black raspberries (my favorite), wild blackberries, wild raspberries (usually too small to bother with) and various cultivars of red raspberry, yellow raspberry, and blackberry. Cane borer damage occurs pretty much every year but it tends to be isolated and infrequent. Several days ago I noticed that one or two of the red cultivars had wilting tips. As we had been a little shy on rain, I assumed that was the cause. Stupid me. Then I saw a few more and upon closer inspection of a black raspberry saw this:
That’s what I call “the “purple death”. If left unchecked, the cane will be dead the following year.
The raspberry cane borer adult is of modest size, maybe the size of a small housefly, with body-length antennae. The female lays an egg in the new tip of a cane, perhaps 10 to 15 cm down and girdles the cane a few millimeters above and below the insertion point. You can see it pretty well in this photo (also of a wild black raspberry):
The larva then begins to burrow toward the root, four to five cm in the first season. It overwinters and continues its journey to the root where it overwinters for the second time and emerges the following spring. The cane, of course, is now hollowed out and dead.
At this point virtually every new cane has been attacked by these beasties. I’ve never seen anything quite like it. I consider them to be particularly nasty because they do not attack the fruit bearing canes, just the new leaders (and occasionally, new lateral shoots). Thus, to the uninitiated, the plant yields fruit and seems otherwise fine. By next year though, you’re screwed.
The most effective treatment is via pruning. Once you locate the double ring girdling, simply prune the cane several centimeters below. Ideally, these pruned bits should be burned to kill the larvae (picture here at bottom). Unfortunately, we have a “no outside burning” law in our town so these bits are chopped up, submerged in water, and will be placed in a plastic bag come garbage day. Fortunately, they don’t take up much room.
In any event, the black raspberries are just coming into production. Today was the first real harvest and I managed to pull in four pints. I am interested as to whether or not this infestation is an isolated event or if others have seen an increase this year.