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Hort Update for July 6, 2021

Hort Update for July 6, 2021, Nebraska Extension,
Green aphids on daisy.
Serious ConcernsMajor Symptom
1. Aphids in shade trees and ornamental plants Sooty mold on plant leaves; sticky stuff falling from trees
2. July growing degree days (GGD) Lincoln Airport 7/5/21 GGD -  1570,  Understanding Growing Degree Days
3. Magnolia scale  Sooty mold on leaves; large shiny tan scales on plant stems
4. Kermes scale Twig and branch dieback in oaks; pale brown hemispherical, tough, gall-like scales present
Minor Issue
5. Brown patch in lawns Roughly circular brownish patches in turf; irregularly shaped tan leaf lesions with red margins
6. Kentucky bluegrass yellowing Weather conditions favor denitrification and yellowing of turfgrass
7. Japanese beetles Feeding damage to lindens, grapes, roses and other plant leaves
Timely Topics
8. White grub management Time for control measures
9. Nimblewill Weedy grass; circular patches of a short-bladed grass with wiry stems
10. Rough bluegrass Weedy grass; declining cool season grass causes patches of brown turf in midsummer
11. Mimosa webworm Browning clumps of honeylocust leaves webbed together
12. Cicada killer wasps Huge yellow and black wasp or small mounds of soil near pavement
13. Slime molds Gray or colorful growths of various forms that appear in turfgrass

1. Aphids in treesSooty mold on plant leaves; sticky stuff falling from trees

This spring, there were many questions about something sticky falling from trees. There are a couple possibilities for this phenomenon, but a common culprit this year seems to be a higher than normal number of aphids.

Aphids are very small soft-bodied insects, typically 1/16 to 1/8 inch in length. They have a mouth like a straw, which is inserted into plant leaves or stems and the liquid contents ingested. They are oval shaped, with a pair of short tail-pipe structures, called cornicles, protruding from the upper surface of the abdomen near the tail. They can be many colors – pale yellow, green, orange and early black; some are even bright red.

There are about 1,350 aphid species in North America, with over 5,000 worldwide. Whitney Cranshaw, Colorado State University entomologist, estimates there are over 350 aphid species in Colorado, so we could expect a similar number in Nebraska. Most aphid species are host specific and feed on only a few types of plants, but others are more generalists feeding on a large variety of plants.

Aphid Damage & Honeydew
Small numbers of aphids are normal, kept under control by natural enemies, cause little to no significant plant damage and don’t require control. But large number of aphids feeding on plant shoots or buds can cause leaf yellowing and curling, followed by wilting, premature leaf browning and death of shoots or buds. In evergreens, large aphid infestations can result in branch dead.

They also secrete honeydew; excess plant sap which contains plant sugars. Honeydew falls to any surface below the infested tree. Lower leaves, as well as cars, patios, tables or chairs, are coated with this sugary liquid, which eventually dries into a shiny, sticky coating. Later in the summer, the honeydew is often colonized by a black sooty mold, turning plants or surfaces black.

Almost all aphids are females. If male insects are produced, it is only in late summer or early fall. Adult females give live birth to genetically identical daughters and are capable of producing 3-5 young per day - talk about cloning yourself! Each immature aphid grows quickly, reaching adulthood in 10-14 days.  

Small numbers of aphids should be ignored. Allow natural predatory insects to do their job. Aphids have many natural enemies, a few of these include ladybug adults; the immature stages of ladybugs, lacewings and flower flies; and parasitic wasps.

Homeowners can use syringing, or knocking aphids off plants with a strong spray of water, which is a good mechanical technique to reduce insect numbers. This will also help wash honeydew and sooty mold off plants. Repeat the syringing as needed to keep aphid numbers under control. Aphids are not strong insects and can easily be washed out of plants by a strong rainstorm. Gardeners may notice more aphids during periods with little rain.

If insecticidal control is needed, protect natural predatory insects and pollinators by first using low risk pesticides like horticultural oil, insecticidal soap or neem (azadirachtin). Even though these are low risk pesticides, they can still cause plant damage if used incorrectly. Don't spray any pesticides when air temperatures are above 85 F. Read and follow all product label directions.

If stronger, residual insecticides are used they can provide control for a week or more, but they will also kill many beneficial insects, such as pollinators and the natural aphid enemies mentioned above. Do not spray these products on trees or shrubs while they are blooming. Traditional insecticides include acephate, permethrin, bifenthrin, lambda-cyhalothrin, cyfluthrin and malathion.

A majority of aphids feed on the underside of leaves, so be sure to spray both the surface and undersides of leaves. Pesticides must contact the insects to kill them, so thorough coverage of the plant is required. Repeated applications will be needed when aphid numbers are high.

Systemic insecticides, including imidacloprid and dinotefuran, can be applied to trees and woody plants as a soil drench. The chemical is taken up by plant roots and transported throughout the plant. This process can take 2-4 weeks, depending on the size of the plant. But when the insecticide reachs a toxic level, aphids will be killed as they feed on leaves, stems and branches. Natural aphid predators will not be directly affected by systemic insecticides, but these products do move into the pollen and flower nectar of treated plants, so are toxic to pollinators.

Aphids on Shade Trees and Ornamentals, Colorado State University Extension 


3. Magnolia scaleSooty mold on leaves; large shiny tan scales on plant stems

If clients have a saucer or star magnolia in your landscape, one insect to watch out for is magnolia scale. August through September is a good time of year for control, so it’s worth your time now to scout for potential problems.

Heavy infestations weaken plants, cause leaf yellowing or kill entire branches, but the first symptom usually noticed is a black moldy coating on the magnolia’s leaves. Insects feed by inserting their needle-like mouth into a stem and sucking up plant sap. They secrete excess plant sugars, which drop to foliage and branches beneath. This creates a sticky, shiny coating which is quickly colonized by a black sooty mold. Look for sticky leaves and branches, or plant parts with a black moldy coating.

The insects feed on plant stems, not on the foliage, so that’s where to look for them. Females do not move once they have found a feeding site on a stem, so insects can build up to the point that stems are completely encrusted with scale. Usually at this point the stem dies. But these insects blend into the plant so well, many gardeners overlook them even after the plant starts to have visible symptoms.

Magnolia scale is one of the largest and most conspicuous scale insects found in the United States. Adult females reach up to ½” diameter at maturity in late July and early August. Each female insect is covered by a soft, irregularly-shaped shell, shaped somewhat like a contact lens, which is shiny and light brown. By mid to late-August, the female’s shell turns white as it is covered by a thin coating of wax. Mature males have a similar shell, although smaller. They pupate under their shell in late July and early August then emerge to resemble tiny flies, which fly to the females for mating.

Females give birth to tiny, dark nymphs in mid to late August. These nymphs are called “crawlers” because at this point in their life they have legs and can move around on the plant to find a feeding site. Once nymphs begin feeding, they create a protective shell and stay in place until the males mature or the females die. But they are susceptible to insecticidal control during the crawler stage. If crawlers are not controlled, they will overwinter on plant stems and complete their lifecycle the following summer.

Since magnolias bloom in spring, one of the best ways to control magnolia scale without harming pollinators is to target crawlers in fall with a contact insecticide. Pollinators will not be present on plants in fall since they are not blooming.

Horticultural oils, also known as summer oils, are a good product to use. Since they are not traditional insecticides, but instead are highly refined oils, they are very safe to use around human, pets, wildlife and other beneficial insects. For good control, it’s important to get thorough oil coverage on plant stems. Oils can be applied from mid-August until freezing temperatures occur in fall and again in early spring before the flower buds begin to swell. Commonly available homeowner products include Bonide All Seasons Horticultural & Dormant Oil Spray, Ortho Volck Oil Spray, or SunSpray Ultra Fine Year-Round Pesticidal Oil.

Use caution when applying oils in late summer; they can burn leaves if conditions are too hot or when applied to drought-stressed plants. Never spray landscape plants with a pesticide if air temperatures will reach 85° F or above during that day. Water plants well a day or two before application and wait until moderate temperatures occur to make your applications. Read and follow all pesticide label directions before use.

Magnolia Scale, Morton Arboretum


4. Kermes scaleTwig and branch dieback in oaks; pale brown hemispherical, tough, gall-like scales present

Pale brown, hemispherical scales appear as large growths attached to leaf midribs and twigs. Mature scales are very tough and gall-like. Leaves become stressed, yellow, or withered, and honeydew secretions are evident. Infested trees can suffer serious branch dieback, but infestations are usually isolated to specific trees and are rarely widespread.

This scale produces a profuse amount of honeydew that covers leaves and becomes blackened by sooty mold. Ants and many other insects feed on the honeydew, and there are a number of natural enemies that help restrain populations.

Nebraska records have confirmed the presence of Kermes Scale since 1921. Hosts affected have been red, pin, and bur oaks, but a wide range of oaks can be infested. Counties involved in the records include Gage, Otoe, Richardson, Pawnee, Lancaster, Douglas, Dodge, Saline, Sarpy, Platte, and Buffalo counties, but this pest is undoubtedly widespread across the state wherever oaks occur. 

There is one generation per season, with females reaching maturity in June. Crawlers emerge in September then migrate to buds being formed for the following year where they spend the winter.  A dormant-season spray oil from March through mid-April is an opportunity to treat. Crawlers are also susceptible to control in September with a topical insecticidal spray.

Kermes Oak Scale, Kansas State Research and Extension


5. Brown patch in lawnsRoughly circular brownish patches in turf; irregularly shaped tan leaf lesions with red margins

This fungal disease appears as reddish-brown patches in lawns. Grass blades within or near affected areas have tan colored, irregular shaped lesions with a reddish margin. Unlike dollar spot lesions, these do not encircle the leaf blade. Fungicides are rarely recommended for home lawns as most turf grows out of this disease. Brown patch is often found in slower growing turf, but can show up in fast growing lawns after fertilization with fast release nitrogen sources. Try to keep grass growing about 1 to 1.5” per week. If it is below that, summer fertilization is recommended; especially if a lawn is less than 10 years old. Use a fertilizer with slow release nitrogen sources or one with at least 50% of the nitrogen being slow release or water insoluble (WIN). Leave grass clippings when mowing. When turfgrass shows signs of needing irrigation, water in the mornings.

Brown Patch in Lawns, Nebraska Extension


6. Kentucky bluegrass yellowingWeather conditions favor denitrification and yellowing of turfgrass

While summer yellowing of Kentucky bluegrass is often due to iron chlorosis, the chlorosis is believed to be caused not only by high pH soil but also by a root dysfunction from hot and/or wet soils. Iron chlorosis will NOT respond to an application of nitrogen. Yellowing often does not harm turfgrass as cooler temperatures eventually mitigate it. When yellowing occurs, a foliar only application of iron, such as iron sulfate, can aid green up. Do not water it in.  It is important to discuss with clients correct irrigation practices that avoid consistently wet soils from excess or too frequent irrigation.


7. Japanese beetlesFeeding damage to lindens, grapes, roses and other plant leaves

Adult Japanese beetles begin appearing at about 1,030 Growing Degree Days (GDD). They feed on leaves, flowers and fruits of more than 300 plant species. Linden trees, roses, grapes and soybeans are favorites. The beetles feed 4 to 6 weeks beginning in late June; eating green tissue between leaf veins and causing leaves to appear lacy. Severe defoliation can stress trees and reduce yields in orchards and crops. The larvae feed in soil on grass roots and can damage lawns. Controlling the larvae will not prevent beetle damage.

Low-risk management methods includes collecting beetles (7 PM is a good time) and placing them in a bucket of soapy water or using plant covers to exclude them where feasible. For example, fine mesh nets can be placed over roses. Two organic sprays, Neem (containing azadirachtin) and pyola, will protect plants for 3-7 days. Other products that protect plants against Japanese beetle adults include products containing active ingredients of permethrin, cyfluthrin, bifenthrin or carbaryl. These all provide about 2 weeks of protection for foliage and flowers after thorough treatment. Because insecticides affect pollinators, try to spray in the evening only and after trees have finished blooming. Follow label instructions explicitly to avoid harming pollinators and damaging plants.

How to ID and Manage Japanese Beetles, UNL GRO Big Red Blog


8. White grub managementTime for control measures

June to mid-July is the time to apply preventive insecticides to lawns that had white grub damage last summer. If a lawn did not have confirmed white grub damage last season, preventive insecticides do not need to be applied. Instead, encourage monitoring of lawns during August for signs of white grubs. These signs include dry or brown spots, flocks of birds (particularly starlings) feeding in turf, and animals like skunks and raccoons foraging in an area. In August, if 8 to 10 masked chafer or Japanese beetle grubs can be found per square foot, or 3 to 5 June beetle grubs per square foot; then an application of Dylox or carbaryl (Sevin) is justified at that time.

White Grub Management, Nebraska Extension


9. NimblewillGrassy weed; circular patches of a short-bladed grass with wiry stems

Nimblewill is a warm season perennial grass with wiry stems, short blades and a fine texture. It is most noticeable in spring as tan patches since it greens up later than bluegrass. It is best controlled when young and actively growing. Control should begin shortly after green up in summer and stop once plants go dormant in late summer. For selective control, the herbicide mesotrione (Tenacity) has been shown to work. Three applications made 7 to 10 days apart are needed. This product has a yearly maximum use rate so read and follow label directions. For non-selective control, two to three applications of glyphosate may be needed. Follow label directions for frequency of application and the number of days to wait prior to reseeding bare areas. Know that control of perennial grassy weeds is difficult and time-consuming and clients may choose to tolerate these weeds.

Perennial Grassy Weed Control, UNL Turf iNfo


10. Rough bluegrassGrassy weed; declining cool season grass causes patches of brown turf in midsummer

Rough bluegrass, Poa trivialis, appears as a finer bladed, yellowish-green grass in spring and fall. As a cool season grass, it begins to decline at this time of year and appears as unsightly patches of brown turf. These may be mistaken for summer diseases. There is no selective control. To manage rough bluegrass, mow high (3 to 3.5”); grow turf on the drier end of the soil-moisture spectrum meaning turf off automatic irrigation systems and only water when lawns show signs of needing irrigation; and overseed areas where rough bluegrass declines to increase competition.

Controlling Poa trivialis in Sports Fields, Purdue University


11. Mimosa webwormBrowning clumps of honeylocust leaves webbed together

Mimosa webworms are web building caterpillars that infest honeylocust and mimosa trees. Caterpillars web foliage together to create a silken home from where they skeletonize leaves (eat the green part and leave behind veins). As they feed, patches of leaves turn brown. Caterpillars are about an inch long when mature and gray to dark brown with five white stripes and a brown head. If disturbed, they drop from trees on a silken line.

Although unsightly, feeding rarely causes serious damage to large trees; however, several years of defoliation on young or small ornamental trees can weaken them. The web impedes most insecticides from reaching the insects, unless caught early. Proactive tree owners can scout trees and use a pole pruner to remove visible nests in June or July. Insecticidal options include bifenthrin, carbaryl, cyfluthrin, chlorantraniliprole and malathion. Organic control options include spinosad and Bt products applied in June to control small caterpillars. Thoroughly cover leaves next to the nests so larvae ingest the insecticide while feeding. Sprays for first generation larvae should have been applied in June. For second generation larvae, applications are made in August.


12. Cicada killer waspHuge yellow and black wasp or small mounds of soil near pavement

Cicada killer wasps are up to two inches long and boldly marked with yellow stripes on a black body. Females are larger than males. Cicada killer wasps create underground burrows near sidewalks, driveways, and retaining walls, creating small mounds of soil around a 1/2-inch entrance hole. Cicada killers are most abundant in midsummer when their prey - cicada, grasshoppers and crickets - are active. They paralyze their prey and drag it to a burrow, then lay an egg on it. After hatching, the larval wasp feeds on the paralyzed insect. Larvae develop in the soil until they emerge as adults next summer.

These wasps are solitary but  will create clusters of nests in preferred areas. Solitary wasps are not aggressive and would only sting when handled or accidentally caught in clothing. Male cicada killer wasps will sometimes aggressively fly towards people to defend their territory, but males cannot sting. Females, which do possess a stinger, are docile in nature. Due to this and the fact they are beneficial predators, solitary wasps are best tolerated when possible. They are only active for a short time in mid- to late-summer. Since they prefer to dig in areas of dry soil, homeowners can discourage their nest building by running a sprinkler where they are trying to nest. This may have to be done a couple times a day to keep the soil moist until the wasp finds another location.

If control is necessary, an application of carbaryl dust (Sevin) or cyflutrhin (Tempo) made directly into the burrow entrances is effective. Make applications at dusk when wasps are the least active. Don't broadcast applications of liquid insecticide over the area where solitary wasps are nesting. This method of application is unlikely to reduce their populations.

Remain Calm, It’s Just a Cicada Killer Wasp!, UNL GRO Big Red Blog


13. Slime moldsGray or colorful growths of various forms that appear in turfgrass

Slime molds can appear suddenly on any turf species during warm, wet weather (especially in shaded or overcast conditions). The fruiting bodies of these organisms grow on turfgrass stems and leaf blades. They are gray to black but may be white, yellow, or pink to purple. They do not infect the turfgrass to cause a disease. They only use grass plants for support to grow on. Dense slime molds may cause some yellowing of turfgrass as the fruiting structures can last a few weeks if not removed from turf with mowing, brushing, raking, or by running water over affected areas.

Slime Mold in Turf, NC State Extension


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