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Hort Update for January 21, 2021

Hort Update for January 21, 2021, Nebraska Extension,
Serious ConcernsMajor Symptom
1. Understanding Growing Degree Days A tool to predict insect emergence and development; increasing pesticide effectivenss and efficiency. Omaha GDD, 1/20/21 = GGD 2
Timely Topics
2. Dormant oils Apply in late winter before bud break
3. Brown marmorated stink bug Warm days result in active insects; stink bugs become an indoor nuisance
4. Green industry trends Understanding changes in consumer horticulture preferences
5. Emerald ash borer quarantine update There is no longer a state or federal quarantine. 
6. Application of antidesiccants When temperatures are above 40 degrees F., apply to evergreens and broadleaf evergreens at high risk of winter desiccation.
7. Avoid foot traffic on frozen turf Minimize winter traffic on any turf area and especially when frost is present on green turf. 

1. Understanding Growing Degree DaysA tool to predict insect emergence and development; increasing pesticide effectivenss and efficiency

Insects are cold-blooded creatures, relying on external temperatures to advance in growth and development. Knowing when insects will reach a specific development stage, and are most susceptible to control, is difficult for several reasons. First, weather varies from year to year. Unusually cool spring weather slows insect development, making correct pesticide timing later than it would be in a warm year.

Second, there is great variation in weather between locations. Typically eastern Nebraska is 2-4 weeks ahead of western Nebraska in both plant and insect development. But even within the same county, urban locations are typically warmer with faster plant and insect development, than rural locations.   

When pesticide applications are needed, all this variation makes is difficult to pinpoint the best timing. Use of growing degree days is a tool landscape managers use to identify the best time for their location, enabling them to make highly effective pesticide applications and minimize total pesticide needed for good control.

What Are Growing Degree Days?
Growing degree days (GGD) are a measure of accumulated heat. A heat value is assigned to each day based on temperature. Each day’s heat value is added together to give an estimate of insect growth stage. One degree day results when the average day’s temperature is one degree over the insect’s threshold temperature.

Begin with the insect pest needing control and determine its base or threshold temperature threshold - the temperature below which no growth or development occurs.  Many horticulture and forestry insect GDD calculations are made using the base temperature of 50° F, although some cool season insects may be active at 38° F or 43° F. Below is a short list of common landscape insects and their GDD. For a more complete list, visit Michigan State University GGD of Landscape Insects or GGD of Conifer Insects.  

Insect Pest Life StageGGD50
American plum borer   adult flight and egg laying  245-440
2nd generation  1375-1500 
Bagworm  caterpillar emergence  600-900 
Banded ash clearwing    adult emergence 1800-2200
Bronze birch borer    adults; eggs; new grubs 400-600
Codling moth   1st generation control stage 250 
2nd generation control stage   1250
Eastern tent caterpillar    egg hatch 45-100
tents apparent  150 
pupation  450
Emerald ash borer   1st adult emergence  400-500
peak adult activity  1000-1200
European pine sawfly  1st larvae  100-195 
Fall webworm    egg hatch 850-900
caterpillars feeding   1200-1800
tents become apparent  1850-2050 
Greater peach tree borer    adult emergence 575-710
Honeylocust spider mite    egg hatch 220-250
Japanese beetle  adults emerge and feed  950-2150
Lesser peach tree borer  adult flight  350-375 
Lilac borer    adult flight 325-350
Magnolia scale    egg hatch  1925-1950
Mimosa webworm   1st generation egg hatch 850-900 
Oystershell scale   egg hatch  350-500
Pine needle scale  1st generation control stage  400-500 
2nd generation control stage  1500 
Spruce spider mite    1st egg hatch 150-175
Zimmerman pine moth   1st larvae  25-100
adult flight   1200

Insects also have an upper temperature threshold value, 86° F, above which no significant increase in insect growth rates occurs. 

Calculating GGD
When temperatures fall between the base and upper temperature threshold, the easiest way to calculate GDD is to add the low temperature and high temperature for the day, then divide by 2. Subtract the threshold temperature from the insect.

For example, if the day’s minimum temperature was 55° F and the high was 75° F, the average temperature is (55+75)/2=65° F. If the insect’s threshold temperature is 50° F, then 15 degree days accumulated in that 24-hour period. 

Remember - 1 GGD results when the average day’s temperature is 1 degree over the insect’s threshold temperature. So if the day’s average temperature is 15 degrees above the threshold temperature, then 15 GDDs have accumulated in that day.  

When temperatures are below or above the insect’s threshold values - GGD is calculated as below using 50° F as the base threshold and 86° F as the upper threshold.   

GGD = (Temp max + Temp min) / 2- Temp min  

Temp max is the actual daily high temperature or is set to the upper threshold value, 86° F, when the day’s temperature goes above this value. Temp min is the daily low temperature or is set to the base threshold value, 50° F, when the day’s temperature goes below this value.   

Example 1 - Temperature drops below the insect’s lower temperature threshold, 50° F. Day’s low temperature 30° F and high 65° F.
Calculation - ((65 + 50)/2) - 50= 7.5 GGD 

Example 2 - Temperature goes above the insect’s high temperature threshold, 86° F.  Day’s low temperature 68° F and high 95° F.
Calculation - ((86+68)/2) - 50 = 27 GGD 

Each monthly HortUpdate will now give a list of accumulated GDD for Omaha, NE and a list of insects reaching a control stage. For your location, use a minimum/maximum thermometer to keep track of each day’s temperatures and keep a running tally of GDDs to determine the exact time for control of your customer's pests.


2. Dormant oil use and phytotoxicityApply in late winter before bud break

Dormant oils are applied to dormant plants to help manage pests that overwinter on plants, like scales and mites. Dormant oils reduce a wide range of insects and mites, but not all. For example, they control spruce spider mite but not two-spotted spider mite which tend to overwinter in plant debris rather than on plants. Dormant oils are effective against some eggs, but less so on those that are stacked on top of one another. The risk of pests developing resistance to oils is low, however resistance has been found in pine needle scale. While oils can be applied any time during dormancy, they will likely be more effective in mid to late winter when pests are in a weakened state.

There is a risk of phytotoxicity from dormant oils. To avoid plant damage, read label directions for use, application timing, and a list of plants products should not be applied to. Always apply dormant oils to plants before bud break to avoid tender plant tissue being damaged. Make applications when temperatures will remain above freezing, ideally above 40 degrees F., for 24 hours and continually agitate the spray solution. Avoid applying dormant oils to stressed plants as these are at higher risk of phytotoxicity.

Dormant Oils, Kansas State University


3. Brown marmorated stink bug (BMSB)Warm days result in active insects; stink bugs become an indoor nuisance

 If clients are complaining about a brown, shield-shaped insect in their homes this winter, it’s likely BMSB who invade homes in fall to overwinter. While they won’t breed in the home or damage the structure, they’re annoying and smelly. BMSB, like many stink bugs, have a shield-like shape. They are a little over ½ an inch in length and brown in color. Marmorated means they have a marbled or spotted appearance. If stink bugs are found indoors, recommend vacuuming and disposal. Insecticide foggers should not be used as these provide little control. To reduce future invasions, entry points into homes should be sealed and screens on doors and windows checked for holes. Seal cracks around windows, doors, pipes, and chimneys with silicone or silicone-latex caulk. Repair or replace entire screens. Foundation insecticide sprays can provide some protection if applied at the correct time of year prior to home invasion.  

Brown Marmorated Stink Bug, Nebraska Extension


4. Green Industry trendsUnderstanding changes in consumer horticulture preferences

Keeping up with garden and landscape trends helps preparation and planning for consumer horticulture. Check out these articles on a few 2021 gardening trends.


5. Emerald ash borer quarantine updateThere is no longer a state or federal quarantine. 

The federal quarantine officially ended on January 14, 2021. Continue to encourage people not to move firewood. Also, new EAB infestations still need to be monitored in new locations, so continue to report possible infestations to your local Extension office or the Nebraska Forest Service.

APHIS Changes Approach to Fight Emerald Ash Borer, 12/14/2020 


7. Avoid foot traffic on frozen turfMinimize winter traffic on any turf area and especially when frost is present on green turf.

If ice crystals (frost) have formed and foot or vehicle traffic occurs, the physical abrasion can damage turfgrass. Winter traffic can cause cosmetic damage, physical abrasion, and/or soil damage depending on the situation. Too much traffic on turfgrass at a time when it cannot recover also leads to winter injury. Winter golf or over-using soccer fields during winter are examples.


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.