US National Arboretum


Pest Management

Pest Management Tips: May

image of a ladybug image of hemlock with wooly adelgids     It is not too late to spray your hemlocks for wooly adelgids. These small, aphid-like insects are covered with a white “wool.” Look for little tufts of cotton on the base of individual needles. The adelgid feeds on the sap of the needle—causing it to turn grey or olive green—and then fall off. New juvenile adelgids, called crawlers, are active during May. Tap an infected branch over a sheet of white paper and, with the aid of magnifying glass, check for this crawling stage. You may have hundreds fall on your page. A 2 percent horticultural oil spray or insecticidal soap will control crawlers.

image of a ladybug     Avoid the temptation to cut off the foliage of spring flowering bulbs as they mature. The foliage is producing the carbohydrates needed for next year’s flowers. Cutting off the foliage—or any other practice that reduces the bulb’s ability to photosynthesize, such as braiding, bunching or tying with a rubber band—will reduce the bulb size and flowering ability next year. You might want to hide the yellowing or floppy foliage by planting bulbs with herbaceous perennials, which emerge after the bulbs have flowered. One good combination is planting daffodils and daylilies in the same bed.

image of a ladybug     Spruce spider mites are also active during May in many parts of the country. Check your Dwarf Alberta Spruce, as you will most likely have spruce mites at some time. They can cause damage on hemlocks, junipers, arborvitae, cryptomeria, and many other species.  A complete generation can occur in about two and a half weeks, so the population can rapidly build. Feeding on the needles, they cause the needles to have a gray or stippled appearance. Young plants with heavy infestations can die; older plants can die after repeated attacks. They do not feed on new growth until it has hardened off, so be sure to check older foliage. Tap an infected branch over a sheet of white paper; these mites appear as black specks. Use a magnifying glass to confirm their presence. If you see more than 20 mites, treat with horticultural oil.

image of a ladybug     Late blooming azaleas, like Satsuki varieties, are susceptible to Ovulinia petal blight. Infected flowers become soft and mushy, and flowers die rapidly. If you have seen these symptoms in the past, you may want to apply a protective fungicide spray just as the flower buds are opening and showing color.

image of a ladybug     May is a great month for planting out annuals of all types. We all have favorites that we plant every year. However, disease and insect populations can build up in the soil over time and may result in poor growth. Think about planting something new this year. The change may surprise you with vigorous growth and an unexpected show of flowers.

image of a ladybug image of rose with black spot fungus     Roses are the nation’s most popular flower, but they can be high maintenance plants. Black spot fungus can cause leaf drop, resulting in spindly, naked plants. Try these methods to keep black spot in check. Disease spores germinate on wet leaves, so try to keep rose leaves as dry as possible, and plant them in sunny areas with good air circulation.

Water early so the leaves have a chance to dry or use drip irrigation or a soaker hose. Old, infected leaves and canes are the source for new infections, so remove and throw away infected leaves throughout the growing season. Clean up around each bush. You still may have to spray with a recommended fungicide on a regular basis. When choosing roses, select varieties that are black spot resistant. Many new cultivars, as well as some old or “antique” roses, show excellent resistance to black spot.

image of a ladybug    Good IPM techniques always involve scouting for problems before they become too difficult to control. Keeping good records and learning from our mistakes are also a part of any IPM program. At the National Arboretum, a cankerworm infestation almost denuded a couple of Trident maples (Acer buergeranum). We noted in our database that next year, an early application of Bacillus thuringiensis will control these caterpillars before they become a problem.

image of a ladybug    May is a wonderful month to be in garden. Enjoy your own yard. Make time in your schedule to visit public parks and botanic gardens. You will be well rewarded with a show of flowering trees and shrubs. At the National Arboretum, the Asian Collection and the native plants in Fern Valley, as well as the late blooming azaleas, are all spectacular this month. What’s more, the peony collection near the Boxwood Collection is a hidden treasure in May.

Pest Management Tips Home

The best way to manage pests is to use a combination of chemical and non-chemical control. Only take action when the problem is serious enough to damage the plant. If we all use Integrated Pest Management (IPM), we can control pests in an environmentally conscious manner.

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Last Updated   June 11, 2009 2:45 PM