Ask the Master Gardener: Tips for fending off a garden-destroying fiend

Japanese beetles are widespread and make life miserable for gardeners. They eat flowers and leaves on some of our favorite things to grow: roses, most fruit trees and vines, herbs, dahlias, lilies, birch and other trees, even ragweed and poison ivy.

A Japanese beetle feeding. Photo by Jeff Hahn / University of Minnesota Extension

Dear Master Gardener: Is it true there’s a parasite that kills Japanese beetles? They eat so many things in the Twin Cities I don’t want them up here in Brainerd.

Answer: Japanese beetles arrived in the U.S. in a shipment of bulbs from Japan in 1912. Unfortunately, they didn’t bring along their natural predators that help keep their population down in Japan. Japanese beetles are now widespread and make life miserable for gardeners. They eat flowers and leaves on some of our favorite things to grow: roses, most fruit trees and vines, herbs, dahlias, lilies, birch and other trees, even ragweed and poison ivy, and the list goes on.

Related: Ask the Master Gardener: Deterring deer from treating your garden like a smorgasbord Outside of tall fencing, there are no foolproof ways to keep deer from grazing on gardens. However, there are methods that work better than others when protecting flower and vegetable gardens from hungry deer.
The most effective way to deal with Japanese beetles has been to walk around your garden, pitching the bugs into a bucket of soapy water. Traps are available, but the pheromones that are used to attract the beetles will actually draw more of them to your yard. Spraying either isn’t effective or kills off all your insects, including the beneficial ones. Treating your yard usually doesn’t make much difference anyway because these critters will just fly in from next door.

But a tachinid fly called the winsome fly (Istocheta aldrichi) is offering some hope. This parasitic fly appears a few days before the Japanese beetles do, mates and fills up on nectar from a variety of shallow flowers, like daisies, coriander, dill, yarrow, and sweet alyssum. The female then lays her eggs on a Japanese beetle’s thorax, leaving an obvious white spot. When the egg hatches 24 hours later, the larva burrows into the beetle, eating the wing muscles first. The Japanese beetle falls to the ground before mating and digs in, where it dies. The winsome fly pupates and spends the winter inside its victim, only to emerge the following year. Each parasitized Japanese beetle failed to lay 40-60 eggs, so although this won’t be an instant cure, over time it should help keep the number of Japanese beetles in check.

So, if you have Japanese beetles, plant flowers for the winsome fly, don’t drown any Japanese beetles with white spots behind their heads (we want them to play host), and keep your fingers crossed that Brainerd doesn’t get a serious infestation!


Related: Ask the Master Gardener: Girdling roots can be harmful to trees Girdling roots are roots that grow in a circular or spiral pattern around the trunk of a tree at or just below the soil line. They cause the tree to suffer a slow decline in health and gradually strangle the trunk.
Dear Master Gardener: I’m sick of keeping everything watered during this drought. Help!

Answer: Me too! Now that Brainerd Public Utilities has declared watering restrictions (no lawn or garden watering between 9 a.m. and 6 p.m., seven days a week) it’s even harder to keep everything well-watered. Until we get some good soaking rains you need to switch your thinking from keeping everything lush to just keeping things alive. This includes your trees and shrubs, too — they are getting severely stressed.

The best time to water is early in the morning. Lawns and plants can get hydrated for the day and leaves have time to dry off so fungal diseases are minimized. Water deeply so you don’t have to water every day, unless it gets really hot again. Try not to get plant leaves wet or to cause soil to splash up on lower leaves. Soaker hoses are the best — a slow drip that soaks in, minimizes evaporation, and doesn’t cause runoff. And above all, mulch is your friend, keeping the roots cool and cutting down on evaporation. Coarse wood chips are the best, but any organic material will help.

Other hints are to keep weeds pulled — they are sucking up precious water. Shade cloth, or even a sheet, can cut down on the sunlight plants receive which slows down their water needs. Don’t fertilize or prune stressed plants — new growth will require even more water. And don’t worry too much about your grass — it goes dormant when it gets too dry but will usually come back when it rains. Trees and shrubs aren’t as lucky — if they dry out, they might not survive, so water them deeply under the whole drip line, not just by the trunk.

Related: Ask the Master Gardener: Our lawns can survive this dry spell Grass starts turning brown after about seven days without water, and goes into dormancy to survive the drought. The base and roots of the grass are still alive and will green up when it rains again.
Dear Master Gardener: I have had a trumpet vine for eight years and it has never bloomed. It is in full sun, lush, and doing well, but doesn’t bloom. What could be wrong?

Answer: Trumpet vine, with its red-orange, trumpet-shaped flowers, is a great addition to the garden for attracting pollinators. Butterflies, hummingbirds, and bees flock to it and deer typically stay away from it. You may be giving it too much tender loving care. If you feed it too well — even once — it could take several years to deplete enough nitrogen so that the vine will bloom. Nitrogen will give you lush, beautiful foliage, but it is the phosphorus that gives you the blooms. Do not fertilize it this year.

Related: Ask the Master Gardener: Heat can be harmful to hostas Leaves damaged by high temperatures can be pruned, but wait until it's cooler outside.
Dear Master Gardener: I am seeing yellow flowers that look like yarrow in the ditches. Is this a friend or a foe?

Answer: There are two possibilities and they are both foes. One possibility is common tansy, which is prevalent throughout the northern half to upper three-fourths of the state. It is commonly found along roadsides and trails and is highly invasive. The stems are reddish-brown, the leaves have a fern-like appearance, and the flowers are yellow and button-like. They bloom from mid-July to September. They are poisonous if ingested.


The other possibility is wild parsnip. It was first discovered in Minnesota in the 1990s and is also commonly found along roadsides. The biggest concern from this invasive species is that the sap will burn your skin if you come into contact with it in the presence of sunlight. These chemical burns look like a rash, with blistering and discoloration of the skin. Wild parsnip has alternate leaves made up of five to 15 egg-shaped leaflets. On flowering stalks, the upper leaves are smaller than those near the base. The stems are hollow with distinctive grooves. Plants that are blooming have yellow, flat-topped, broad flower clusters that bloom from June to late summer.

Both of these plants are toxic and should not be handled without gloves.

Related: Ask the Master Gardener: How you can keep animals from grazing on your garden No plant or fencing can be said to be completely animal proof, but there are varieties and barriers that can help keep them at bay.
Dear Master Gardener: I planted Asiatic lilies this spring but some of the plants are stunted and look like hedgehogs, just a rosette of leaves. What happened?

Answer: The most likely explanation is that the tips got nipped by the late frost we had in the spring. They should be fine next year.

Related: Master Gardener: Add spectacular color to gardens Phlox subulata are great additions to rock gardens, another plant is underused as a fabulous addition to a sunny garden, how to keep rhubarb healthy and productive and do marigolds really repel rabbits and bugs -- are all part of this week's Master Gardener column.

You may get your garden questions answered by calling the new Master Gardener Help Line at 218-824-1068 and leaving a message. A master gardener will return your call. Or, emailing me at and I will answer you in the column if space allows.
University of Minnesota Extension Master Gardeners are trained and certified volunteers for the University of Minnesota Extension. Information given in this column is based on university research.
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