I consider myself an optimist: I work hard to find the silver lining in nearly any gray cloud. As someone who has spent the last 25 years looking at plant diseases and insect pests, however, I can’t help but see the pests when I see a gorgeous garden. It’s not that I’m trying to discourage the gardener; the truth is I find the interplay between plants and all those who consider them dinner to be fascinating.
When spring finally breaks, and the star magnolias come into bloom, my attention turns to the diseases and pests. From the sawflies that kill the top of white pine trees to the fungal disease that can cause crab apples to drop all their leaves, these spring pests are busy in our gardens now, or will be in a few short weeks.
White pine weevil
This is a native pest that attacks the leader, or topmost growth, of pine and spruce trees. Adult weevils spend the winter in the duff around the base of trees, and emerge in spring around the time star magnolias are just beginning to bloom. The female weevil climbs up the tree to lay her eggs on the leader just underneath the bark.
Immature beetles hatch out and begin to feed on the cambium layer of the tree, protected beneath the bark. By summer, the leader begins to wilt and eventually turns brown and dies. The beetles continue their life cycle in the dead tissue, emerging as adults in autumn.
To manage this pest, a protective pesticide application can be made directly to the leader in late winter, before the adults lay eggs. Later in summer, cut and destroy any wilted or dead leaders — don’t just cut them and leave them on the ground, because the beetles will survive. Trees can be protected from infestation next year by using a soil drench of the insecticide imidacloprid in autumn.
European pine sawfly
The European pine sawfly hatched out about two weeks ago, and is now busy eating the needles of mugo pine. While this insect looks like a caterpillar, it is more closely related to bees and wasps. This pest overwinters as eggs, laid in needles last year. The needles have yellow bands where eggs have been laid, and can be removed if noticed in winter.
Once the eggs hatch (around the time flowering pear comes into full bloom), the young larvae begin to feed. These small larvae are unable to eat all the way through the needle, so they leave brown strips of uneaten tissue. These brown needles, just under this year’s new growth, help to detect the presence of this pest. Look now for brown needles on mugo pines to locate and limit sawfly damage.
European pine sawflies mass together to eat, and will raise their bodies in unison when disturbed. Otherwise they blend in with the plant, making them difficult to see. Larvae will completely strip the plant’s older needles, leaving new needles untouched, giving the plant a “poodle cut” look. Sawflies are easily killed with horticultural oils, insecticidal soaps, or traditional insecticides, or simply knock them into a bucket of soapy water.
Viburnum leaf beetle
This pest was first introduced to North America from Europe in 1947 in Ontario, Canada. From its initial introduction, the beetle has spread across the northeast, reaching Ohio in 2002. Both the adult and larvae damage viburnum shrubs by eating leaves.
In spring, around the time Koreanspice viburnums are in full bloom, beetle larvae hatch out of eggs laid in twigs the previous fall and begin to chew through leaves. Shrubs can be completely defoliated by larval feeding.
Beetles pupate in the soil, then emerge as adults in summer. Adults also eat leaves, often defoliating shrubs that tried to push out new leaves.
Local populations can build up quickly, since each adult female can lay up to 500 eggs in twig ends. This pest can be managed by pruning twigs with egg damage in fall or winter, treating with systemic insecticide applications after bloom, or applying foliar insecticides in spring as leaves unfold. Arrowwood and cranberrybush viburnum are very susceptible to this pest.
Several species of roseslug sawflies feed on rose leaves. This pest isn’t a slug, it’s a relative of the European pine sawfly. The immature larvae feed on rose leaves (often on leaf undersides), leaving a layer of tissue that causes a “window pane” look. These areas can drop out, creating holes in the leaves.
Most damage is seen in spring, but damage can also occur through the summer. Roseslugs overwinter in the soil under rose plants. Larvae are easily controlled with insecticidal soaps, oils or labeled insecticides — just be sure to coat the undersides of leaves. Note that Bt insecticide products that kill caterpillars are not effective on sawflies.
This is a fungal leaf disease of apples and crab apples. The fungus overwinters in fallen, diseased leaves. In spring, the fungus produces spores, which are carried in the wind to infect emerging leaves. Within a few weeks, olive-green lesions appear on leaf surfaces, eventually turning brown. Infected leaves turn yellow and drop prematurely. In some seasons, susceptible trees can lose nearly all their leaves, yet they manage to come back in subsequent seasons without lasting effects. Fruit lesions can significantly reduce apple quality.
Fungicides may be applied on susceptible trees in key locations to prevent the disease, but they must be applied continually from bud break through leaf expansion to protect the leaf tissue. Many scab-resistant crab apple cultivars are available for the home garden, such as ‘Adirondack,’ ‘Strawberry Parfait’ and the sargent crab apple. Scab-resistant apples include ‘Freedom,’ ‘Liberty’ and ‘Macfree.’
Denise Ellsworth directs the honeybee and native pollinator education program for Ohio State University. If you have questions about caring for your garden, contact her at 330-263-3700 or click on the Ask Denise link on her blog at www.osugarden.com.