Q We're passionate about keeping our birch tree for a long time, so we've sprayed it for birch leafminers for several years. Now that the tree is too big to spray easily, we've been painting an insecticide called Cygon on the trunk. What else can we do?
A Birch leafminers aren't a major problem.
According to Jeff Hahn, an extension entomologist at the University of Minnesota, leafminers tunnel between the layers of leaf tissue, where they feed for a couple of weeks starting in mid-May in the Twin Cities. Their feeding creates large brown blotches on some of the leaves, but healthy birch trees usually can withstand even a heavy infestation, although it might make the tree look bad.
If you see a sickly birch tree, the damage is likely to have been caused by the bronze birch borer. Attracted to already weakened trees, birch borers feed under the bark, creating channels that block moisture and nutrients from flowing from the roots up to the leaves. The first telltale symptoms are yellowing and dying leaves and twigs, usually at the top of the tree. As the infestation worsens, the dead area expands. If the problem is caught in time, a professional arborist can treat the tree with chemicals that are available only to registered pesticide applicators.
The best way to take care of your birches is to keep them healthy. Birches are shallow-rooted trees that grow best under cool, moist conditions. To keep the roots cool and the soil moist, apply 2 to 4 inches of mulch (preferably wood chips or shredded bark) around the base of the tree. (Don't mulch right up to the trunk. Leave a couple of inches for air to circulate.)
Water birch trees regularly -- even larger, well established trees -- when the weather is dry. A good soaking once a week is usually enough. And don't trim your birches in May or June. (Female borers appear to be attracted to fresh pruning wounds.) In fact, it's best to prune birch trees minimally, if at all.
If you'd like to add a tougher birch tree to your landscape, check out the river birch (Betula nigra), which is less likely to suffer from bronze birch borers than paperbark birch (Betula papyrifera).Name that beetle
Q I've grown hybrid tea roses successfully for 40 years, but last summer they were attacked by insects I've never seen before. They were about 1/2 inch long, greenish-black and beetle-like. I tried several insecticides on them, but nothing worked. What's been eating my roses? How can I protect them this summer?
A I'm afraid you've got Japanese beetles. They've been in the Twin Cities for several decades, but during the past five to six years their numbers have been increasing and their range has been expanding. Roses are among their favorite targets, although they'll attack grape vines as well as many flowering annuals and perennials.
Japanese beetles spend much of the year underground as grubs, feeding on the roots of grass. They emerge as hungry adults in summer. There are insecticides labeled for use on Japanese beetles, but the beetles are extremely mobile. So, even if you kill some by spraying them, more are likely to fly in. Also, spraying the flowers can kill or discourage bees and other beneficial insects in the garden.
Mary Meyer, an extension horticulture professor with the University of Minnesota, recommends getting rid of them by hand-picking them and dropping them in soapy water. Watch for them in late June or early July. If you hunt early in the morning when it's cooler, the beetles will be more sluggish and easy to catch.
Deb Brown is a garden writer and former extension horticulturist with the University of Minnesota. To ask her a gardening question, call 612-673-7793 and leave a message. She will answer questions in this column only.