Have a thorny problem with roses? Solution depends upon type of insect

I need some help on my roses. Something is eating the leaves. The missing pieces form almost a perfect circle. I’ve never seen anything chewing on the plant, but every day it seems like more holes appear. What can I do to get rid of them?

— Phyllis

What you have is a little insect called a leaf cutter bee. This bee takes pieces of leaf from your rose (or redbud, privet, lilac, among others) to make a nest. Unlike honeybees, these bees don’t form a big hive; they’re solitary. Instead, they make a nest in any place they can squeeze into — usually rotten or other soft wood (like the pithy center of your rose canes), but they can take advantage of small tubes, holes in a board or in concrete, holes in the soil, or the dry hollow stems of plants.

The bees cut these pieces of leaf to line their nest for an egg chamber with several individual cells in each. The finished chamber looks almost like a cigar with overlapping pieces of leaf. Inside each cell the female will lay one egg and place a ball of pollen for the larva to feed on after hatching.

As alarming as it might look, they’re not hurting the plant. They don’t eat the leaves; they’re just making a nursery for the little ones. You don’t want to be spraying for them; they’re extremely beneficial pollinators for a lot of different plants (in fact, they are usually several times more efficient at pollination than honeybees). Besides, it takes the bee no time at all (usually 10 to 20 seconds) to cut the piece of leaf out and be gone, so insecticides don’t work well on them anyway. This is something I tell everyone to just ignore.

 

I brought in some rose stems last week. The lady I spoke with told me to check for cane borers and I have them in a major way and apparently have had them for many years. I think last year’s borer problem never went away over the winter and they just continued to eat. My roses are old and were neglected before I moved and there’s a lot of dieback on them. I noticed that most of the current damage goes down to the plant where I can’t cut it out. I moved here wanting a huge rose bed but that isn’t going to happen if I can’t handle 11 of them.

Do systemics ever take care of such a chronic problem? Or is it better to take out the worst of the old plants and plant new ones? If I decide to take out a few, when is the best time to replant here?

— Kris

 

This little borer enters the soft, pithy end of the cane and usually kills the cane down as far as it tunnels down. The sad part about it is that the damage is unintentional. These little wasps or bees are not eating the rose; they’re just digging out an egg chamber. In fact these guys are actually beneficial as they parasitize plant pests like aphids and perform a lot of pollination.

The best way to avoid their damage is to prevent them from tunneling into the cane in the first place. Placing a physical barrier on the cut end will keep them out of your rose. They can only get in through the soft pithy center; they can’t bore in through the side of the cane.

The way I like to prevent cane borer in roses is to paint the freshly cut ends with a standard black pruning paint. It’s waterproof and one good coating should last you the entire season. Elmer’s glue works fine as a barrier, but it doesn’t last. It breaks down in the sun and moisture and will have to be reapplied, which is a painful job since the rose has grown out when you need to do this. I try to avoid reaching down into that dense, thorny plant whenever I can.

Some people like using the old-fashioned flat thumbtacks just stuck into the end of the cane. That will work; I guess it’s a bit fussier than I want. I’ve also heard of people using nail polish to paint the ends of the canes. It will last outdoors but the solvents in it will kill plant tissue back a bit and delay healing of the wound. However, if you want your roses to match your toenails ...

The one complaint I’ve heard about using the pruning paint is that it just seems to magically be attracted to fingers and clothes. I know I’ve had my run-ins with it. It’s helpful to wear a pair of those disposable latex gloves and some old clothes when you’re doing this.

As for using systemics, I’m afraid they don’t work on cane borer. If they’ve tunneled down to the graft union, it doesn’t necessarily mean that the whole bush will die, but the cane they’ve tunneled down will. Cut the cane off entirely and then you just have to decide whether it’s worth hanging in with the plant or not. Sometimes you can rebuild the plant into a nice specimen and sometimes it’s just too far gone to bother with. Ultimately, it boils down to what you’re willing to put up with — rebuilding a bush over the next two, three, or four years or just starting over.

Dennis Hill is the nursery manager at Bookcliff Gardens, bookcliffgardens.com. Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or email .(JavaScript must be enabled to view this email address).


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