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Hort Update for June 1, 2022

Left - boxwood leafminer damage. Right - boxwood psyllid damage. Penn State Department of Plant Pathology & Environmental Microbiology Archives , Penn State University,
Left - boxwood leafminer damage. Right - boxwood psyllid damage. Penn State Department of Plant Pathology & Environmental Microbiology Archives , Penn State University,

Serious ConcernsMajor Symptom
1. Spring environmental damage to trees & shrubs  Environmental, abiotic spring damage
2. June 1 growing degree days (GDD) Several Nebraska sites below, Understanding Growing Degree Days
3. Pest Update Pests to watch for based on growing degree days (GGD)
4. Boxwood leafminer Blisters on leaves, light green to brownish
5. Boxwood psyllid Yellow curled leaves
Timely Topics
6. White grub control Plan applications for turf with a history of infestation by mid to late June
7. Broadleaf weed control  Reduced effectiveness with summer control; increased risk of herbicide drift
8. Yellow nutsedge control  Apple-green, grass-like plant with waxy blades and a triangular-shaped stem

Image of spruce shoot damage due to abnormally high spring temps.

1. Spring environmental damage to trees & shrubsEnvironmental, abiotic spring damage

Nebraska experiences a wide range of spring weather which can damage woody plants after new growth has started, including freezing temperatures and hot windy conditions. Below are problems that may be found across the state this year.

New shoot death on spruce – Areas which experienced hot drying winds after spruces had begun to send out new growth may find shoots browning, drooping and dying. New growth is susceptible to this type of damage until it had begun to harden off later this summer.

Other conifers, such as pine and fir may also be affected, but spruce seems to be the most sensitive to this type of damage.

Deciduous tree which had begun to leaf out may shed leaves due to hot drying winds, too.

Freeze damage to new growth - The very early spring growth on many trees and shrubs can tolerate temperatures in the low 30s and upper 20s. Damage is most likely to appear if temperatures drop into the middle 20s or below. However, as new growth progresses and leaves expand, the soft succulent growth can be damaged by temperatures in the upper 20s. May 21-22 temperatures in western Nebraska were very damaging, killing new growth completely.

Location Dates Min. Temp. F Dates Min. Temp. F
Grand Island 15, 16 21, 22 22 35
Lincoln 9 20 22 34
Omaha 1 22 22 37
Norfolk 1 19 22 33
North Platte 13, 5, 16 14, 19, 19 22 27
Scottsbluff 12, 13 14, 15 16, 13, 9, 18 22 27

Image of spring freeze damage to honeylocust tree in Scottsbluff.

Oak tatters is a sublethal symptom of freeze damage, with symptoms commonly seen on oak and hackberry trees. It is speculated to be the result of freezing conditions when leaves are still in the bud stage or during opening of buds in spring. It is possible herbicides play a role, too, along with physical damage from high winds. Affected leaves appear shredded and lacy, with leaf tissue missing between the leaf veins. It is often assumed to be insect or disease damage.

In both cases, oak tatters and complete shoot death, trees will eventually produce new growth with normal leaves, but early spring leaf damage can significantly stress trees making them more susceptible to drought and disease.

Oak and Hackberry Tatters, Iowa State University

Image of winter desiccation to windbreak following winter drought, 2022.Winter desiccation injury - We are also seeing a higher than normal amounts of winter desiccation injury this spring. Typically, some level of damage is found on boxwood and arborvitae each spring, but this year entire plants have died including older established arborvitae, juniper and other evergreens.

Deciduous trees have been damaged, too, some showing branch dieback, small leaves and poor canopy fill.

Nebraska's dry fall conditions, which extended from September 2021 through April 2022, were very hard on plants. Many of the damaged plants were not watered in fall, to ensure they were well-hydrated going into winter. Dry soil may have caused some root dieback, along with canopy dieback.

Image of oak tatters damage due to spring freezing temperatures.Care of Damaged Plants – Dead plants should be removed and dead branches pruned back to living growth. Good basic care – mulch and summer/fall watering - will help trees recover vigor. Nitrogen fertilizers, in particular, should be avoided because the nitrogen promotes shoot growth over root growth, and re-establishment of the root system is required on stressed trees before damaged trees can adequately support new top growth. Sites with very poor soil or where construction activities have altered the soil composition may be deficient in certain nutrients. In such cases, each tree should be assessed individually for nutrient needs.

How to Care for Trees, Nebraska Forest Service


2. June 1st growing degree days (GDD)

Location Accumulated Growing Degree Days
Grand Island, NE - Airport 570
Lincoln, NE - Airport 583
Omaha, NE - Airport 605
Norfolk, NE - Airport 499
North Platte, NE - Airport 419
Scottsbluff, NE - Airport 328


3. Pest updatePests to watch for based on growing degree days (GDD)

Examples of insects that have or may emerge soon in Nebraska based on growing degree days for your area. Understanding and Calculating Growing Degree Days.

GGD (base 50) InsectLifestage present at this GGDType of Damage
325-350 Lilac borer (peak adult emergence at 930, see below) 1st adult emergence stem dieback
350-500 Oystershell scale Peak adult emergence sap feeding
400-500 Pine needle scale 1st generation - hyaline stage (control target) white scales on needles, sap feeders
400-500 Emerald ash borer (peak adult emergence at 1000-2000, see below) 1st adult emergence D-shaped holes on trunk
400-575 Euonymous scale 1st generation white scales on leaves/stems, sap feeding
400-600 Bronze birch borer Adults, eggs, new larvae raised bumps on trunk
440-700 Ash sawfly 1st larvae appear leaf feeding
600-900 Bagworm Larvae appear needle feeding
850-900 Mimosa webworm 1st generation egg hatch leaf webbing/feeding
930 Lilac borer 1st generation hyaline stage stem dieback
950-2150 Japanese beetles Adult emergence leaf feeding
1000-2000 Emerald ash borer Peak adult emergence D-shaped holes on trunk


4. Boxwood leafminerBlisters on leaves, light green to brownish

Damage appears as ‘blisters’ on leaves that turn from light green to brownish. The adult is a fly. The larvae, which causes the damage, is a small yellow maggot that feeds between upper and lower leaf surfaces. Damage that first shows up in spring occurred the previous season but can worsen as larvae resume feeding that causes blister-like leaf spots. If blisters are broken, tiny yellow maggots are found inside. This pest disfigures boxwoods when damaged leaves are shed in mid-summer.

While leafminer may only be an occasional pest, management includes planting resistant boxwoods and removing and destroying infested foliage in spring. If needed, a systemic like Imidacloprid can control larvae inside leaves. Control with insecticide sprays is difficult as timing must coincide with adult emergence. Sprays will not control larvae inside the leaves. Larvae pupate and emerge as adults around GDD 450 base 50.  Confirm the presence of larvae by holding damaged leaves up to the light or breaking open ‘blisters’ to look for maggots before using a pesticide. Boxwood often has winter injury and this damage, or fungal leaf spots, should not be confused for leafminers.

Boxwood leafminer, Michigan State University


5. Boxwood psyllidYellow, curled leaves

Nymphs feed by sucking plant fluids from terminal leaves as they emerge in spring. This feeding causes leaves to yellow, curl, and form a cup, which protects the nymphs. Boxwood psyllids are small (1/16-inch), grayish green insects with a white, waxy secretion that partially covers the body to provide protection for the insect. Boxwood psyllids overwinter as eggs. These hatch into yellowish nymphs that begin feeding as soon as leaf buds start to open. Winged adults appear in late May and June and may be seen flying around plants. Females insert eggs between or under bud scales in early summer. There is one generation per year.

Acephate (Orthene), carbaryl (Sevin), insecticidal soap, and horticultural oil can be used for control. Good coverage within the cupped leaves is needed to be sure nymphs are directly contacted, particularly with insecticide soaps and oils.

Boxwood Psyllids, University of Illinois Extension


6. White grub controlPlan applications for turf with a history of infestation by mid to late June

Control depends on proper timing of the application and moving the insecticide into the root zone where grubs feed. Preventive control applications are made from mid to late June. Curative or rescue treatments are made in August or September.

Preventive - Most of the preventively-applied insecticides are systemic in nature and will be taken up by the plant and translocated to roots. The following products are effective against young grubs. Labels: c = commercial product, h = homeowner product.

  • Chlorantraniliprole – Acelepryn(c), Scotts GrubEx (h)
  • Clothianidin – Arena (c)
  • Halofenozide - Mach 2 (c)
  • Imidacloprid – Merit (c), Bonide Grub Beater (h), BioAdvanced Season Long Grub Control + fertilizer (h)
  • Thiamethoxam – Meridian (c)

Curative (rescue treatments) - Carbaryl (Sevin) or trichlorfon (BioAdvanced Grub and Insect Control) provide the best control due to their higher kill rate against mature white grubs. These products must be watered in for acceptable control. Moving the insecticide into the root zone involves applying ½ inch of water immediately after application. Do not use products that contain ONLY bifenthin, deltamethrin, cyfluthrin, permethrin, lambda-cyhalothrin or gamma-cyhalothrin for soil-applied grub control. These chemicals bind with organic matter at the soil surface and will not move down into the soil to provide effective grub control.

Thatch plays an important role in reducing the efficacy of turf insecticides applied for white grub control. If the thatch layer exceeds ½ inch, light aeration and increased post-treatment irrigation will enhance insecticide penetration and should improve white grub control. In problem areas, such as those with thick thatch layers, repeated irrigations may be necessary every three to four days to continue moving the insecticide into the soil. When white grubs are deeper in soil, curative treatments are more effective if a retreatment irrigation of 1/2 inch is applied 48 hours before the insecticide application. This will encourage grubs to move closer to the soil surface and enhance the level of white grub control.


7. Broadleaf weed controlReduced effectiveness with summer control; increased risk of herbicide drift

Control is not as effective during summer and there is increased risk of injury to non-target plants. Larger weeds are more difficult to control. Herbicide control is less effective and turf safety decreases when weeds are treated under low soil moisture. Air temperatures higher than 85F increase the chance of turf injury as well as herbicide volatization which leads to drifting onto nontargets. Perennial broadleaf weeds often regrow from roots with summer watering and fertilization. September and October are the best months to control perennial broadleaf weeds with herbicides. If weed control is needed during summer, spot treat individual weeds when environmental conditions are cooler and not windy.


8. Yellow nutsedge controlApple-green, grass-like plant with waxy blades and a triangular-shaped stem

This perennial sedge typically emerges from underground tubers in late May in Nebraska. From the first week of June up to June 21 - before plants develop new tubers - is considered prime time for control. To reduce nutsedge, mow tall and avoid overwatering. Hand-pulling is effective where there are fewer plants.  When herbicides are used, sulfentrazone (Dismiss), imazosulfuron (Celero), halosulfuron (SedgeHammer), and mesotrione (Tenacity) are labeled for postemergence control in cool-season turfgrasses and buffalograss.


Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by Nebraska Extension is implied. Use of commercial and trade names does not imply approval or constitute endorsement by Nebraska Extension. Nor does it imply discrimination against other similar products.