US National Arboretum


Pest Management

Pest Management Tips: July

image of a ladybug     Bagworms can be a real nuisance on many trees, especially conifers like junipers, arborvitae, and Leyland cypress. Young caterpillars, weaving a small silky bag with leaf and stick pieces attached, are active throughout July. They can be controlled now with Bacillus thuringiensis (Bt) when the bag is less than 1/2 inch. If the bags have grown larger than this before you notice them, you can treat them with a spray containing spinosad. Bags will eventually reach two inches and if left to mature, male moths emerge from the bag later in the season, mating with females who never leave their bag. Each female can lay up to a thousand eggs, which remain in the bag until they hatch in the spring. It is a very good idea to remove and destroy bags any time of the year.

image of a ladybug     Black spot, caused by the fungus, Diplocarpon rosae, continues to be a problem on roses through the summer months. This disease is easily diagnosed by the black circular spots on leaves. Infected leaves turn yellow and drop prematurely. Infected plants can lose most of their leaves. Because disease spores are splashed around during periods of rain, clean up fallen leaves and prune off infected branches to reduce inoculum. Favorite roses must be sprayed with a preventative fungicide every few weeks throughout the summer. We strongly advise planting cultivars that are resistant to black spot. Many old rose varieties, as well as new landscape roses, are not susceptible to black spot.

image of a ladybug image of an azalea infested with lacebugs    Lacebugs continue to be a problem in July. Commonly found on azaleas and pieris, they will attack some trees, including oak and sycamore. These bugs suck sap from the underside of leaves, causing the leaves to turn light green and become speckled with white.





image of an azalea infested with lacebugs

In severe infestations, the whole plant will appear light in color with whitish or pinky overtones.  





image of lacebug


Lacebugs are 1/8 inch long, whitish, with clear wings. The wingless nymphs are darker. They also leave tarry excrement on the leaf undersides. Because there are several generations per season, new, healthy leaves will also become infected.

Horticultural soap or oil will control this pest.


image of a ladybug    Hemlock woolly adelgids, small, aphid-like insects, are a major pest of Eastern hemlocks. Look for little tufts of cotton on the base on individual needles. The adelgid feeds on the sap of the needle, causing it to turn grey or olive green, and then fall off. Juvenile adelgids, called crawlers, are feeding throughout the summer. Tap an infected branch over a sheet of white paper and with the aid of magnifying glass, check for this crawling stage. A horticultural oil spray or insecticidal soap will control crawlers. Repeated, heavy infestations can kill young trees and weaken mature trees.

image of a ladybug    The importance of well drained soils and properly prepared planting beds are highlighted during times of heavy and prolonged rainfall. Seemingly healthy plants succumb to a wilt or root rot almost overnight. Young rhododendron plants, garden perennials, and annuals collapse, with little chance of recovery. Phythium and phytophthora are many times responsible. Wet, water soaked conditions and poorly drained, heavy soils favor these fungi. Once infected, it is difficult to control these pathogens. Improving soil drainage with the addition of organic matter is the best way to prevent these fungi. Raised beds with copious amounts of organic matter (barks, peat, compost) will provide a well-aerated environment, encouraging healthy root growth.

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 2:59 PM