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Beware of summer garden pests

A close-up of white bumps on a magnolia branch.
Magnolia scale
June 26, 2015


In summer, the garden is abuzz with insect life. Most insects are harmless or beneficial to plants, but a few can cause considerable damage, says Sharon Yiesla, Plant Clinic assistant. You should monitor your garden for problems and insects you don’t recognize.

The Plant Clinic can help with identifying pests and diseases and deciding whether, or how, to try to control them. It’s wise to get expert advice before applying any insecticide; using the wrong chemical at the wrong time may be futile, and insecticides may kill many more beneficial insects than harmful ones. 

Here are a few pests to watch out for in July.

Viburnum leaf beetle: This is a relatively new pest of a very popular group of landscape shrubs, Yiesla says. Adult beetles hatch in late June to July. They prefer to feed on species with smooth leaves such as arrowwood viburnum, American cranberrybush viburnum, and European cranberrybush viburnum.

The beetles are about 1/4 inch long and are golden brown with a slight sheen, Yiesla says. Their damage is recognizable because they eat the leaf tissue, leaving a skeleton of veins—the only pest of viburnums with this feeding pattern.

Between July and the first frost, the female beetles lay eggs in twigs, creating rows of small bumps along the twigs’ undersides. Next spring, the eggs will hatch into small larvae that also eat viburnum leaves. Shrubs that are heavily infested for two or three years can die.

Spraying the adult beetles is less effective than attacking the larvae when they hatch in spring, Yiesla says. Contact the Plant Clinic to confirm your identification and get advice on controls.

Or prune out all twigs that have the egg bumps in winter, when the shrub and the eggs are dormant. As with any diseased or infested plant parts, dispose of the branches in the landscape waste, not in your garden or your compost pile.

Scale insects: Populations of scale insects are very high this year, Yiesla says, and may warrant control measures.  These inconspicuous insects spend most of their lives immobile, flattened against the bark of magnolias and some other trees and shrubs, armored by a wax coating, sucking sap. The only time they move is late summer, when the “crawlers” have just hatched from eggs and are searching for a place to settle.

Some insecticides and oil sprays can be effective against the crawlers, Yiesla says, if you time the treatment just right and if you choose the right product. Different insecticides are most effective against different species.

A scale infestation is most often noticed when sticky honeydew, a waste product that drips from some species of scale insects, becomes covered with black mold. The mold itself is harmless. In most years, scale is mainly a cosmetic problem, Yiesla says, but in the long run, a severely infested tree can lose so much sap that it is debilitated and begins to decline.

You can scrape scale insects off the tree any time, if you are careful not to damage the bark. It’s a good idea to prune out branches with a lot of scale insects if that is feasible, Yiesla says.

Japanese beetle: The adult beetles are 1/2 inch long and metallic green with coppery-brown wing covers.  They emerge from late June to August, feeding on more than 300 species of plants but favoring roses, Yiesla says. Females emit a pheromone to attract males, causing the beetles to gather in groups.

Toward the end of summer, females lay eggs in the soil that will hatch to become grubs, which eat the roots of lawn grasses.

When you spot Japanese beetles, hand-pick them and drown them in soapy water. Studies have shown that beetle traps won’t protect plants from damage and may actually attract more beetles.

Neem products containing the active ingredient azadirachtin can be effective as repellents, potentially reducing the damage. These neem products are not the same as neem oil. 

The beetles favor moist soil to lay their eggs, Yiesla says, so “if you water your lawn, you are more likely to have grubs.” Make your lawn less attractive to the egg-laying beetles by not watering and letting it go dormant in the summer, which is natural for grass.

Beneficial nematodes, available through mail order, can reduce the future beetle population by feeding on grubs. They are available from internet sources and in some garden centers.

Bagworms: Caterpillars, the larvae of a moth, begin building their protective bags in late July. Often mistaken for fir or pine cones, the bags usually appear on evergreens but can sometimes be seen on deciduous trees, as seen in the picture on the right.

A bag takes the caterpillar about six weeks to construct from spun silk layered with brown evergreen needles or twigs for camouflage. Several hundred eggs will overwinter in a bag to hatch next May and begin feeding on leaves and needles.

Pluck off any bags you spot and destroy them, Yiesla says. Spraying insecticides at this stage is unlikely to be helpful; the larvae are most vulnerable in June, when they are newly hatched and small. Consult the Plant Clinic about possible control measures.