For Christmas tree growers, the scouting recommendations for common Christmas tree pests vary with the number of total growing degree-days (GDD) that have accumulated. Based on the GDD, we can estimate the life stages of each of the pests: pine sawfly, pine needle scale, Cooley spruce adelgid, eastern spruce adelgid and gypsy moth.

For example, pine needle scale eggs will begin hatching starting at 250 GDD. In West Olive, Michigan, the first generation of crawlers are out and will continue for 134 more GDD. At 984 GDD, the second generation of crawlers are expected to be active; this usually occurs in July. Cooley spruce adelgid and Eastern spruce adelgid first adults were active throughout the middle of April in west central Michigan and their feeding will continue until 300 GDD. The spring foliar treatment window is closed for both of these pests. The best time to control these adelgids is to target sprays at the overwintering females. In fields that have had problems with adelgids, timing is between 25 and 120 GDD, which is much earlier in spring.

Do not spray for gypsy month until at least 150 GDD. From 150 to 200 GDD, treat with Dimilin. Treat with all other registered compounds for gypsy moth from 200 to 700 GDD, which will be throughout the next few weeks.

Of the five Michigan State University Enviroweather stations that were examined (Arlene, Allegan, Cassopolis, Commerce Township and West Olive), Allegan had the greatest number of current GDD at 365. Arlene has the least GDD of 216. With the recent warm weather of around 80 degrees Fahrenheit, all of the locations have met or exceeded the 30-year average number of GDD.

Below are the current degree-day totals at those stations and the life stages of common Christmas tree pests.

Arlene, Michigan

GDD total: 216 on May 17, 2017
GDD normal*: 175
Number of GDD ahead of normal: 44
GDD forecast: 249 on May 22, 2017

• Pine sawfly eggs are hatching. The first larvae are present.
• Cooley spruce adelgid eggs are hatching and nymphs have started to feed.
• Eastern spruce adelgid eggs are hatching and nymphs have started to feed.
• Gypsy moth treatment for all compounds is recommended now (between 200 and 700 GDD).

Allegan, southwest Michigan

GDD total: 365 on May 17, 2017
GDD normal*: 272
Number of GDD ahead of normal: 93
GDD forecast: 433 on May 22, 2017

• Pine sawfly eggs have hatched. The first generation of pine needle scale crawlers are active.
• Galls are visible from Cooley spruce adelgid and Eastern spruce adelgid. Spring foliar treatment window is closed.
• Gypsy moth treatment for all compounds is recommended now (between 200 and 700 GDD).

Cassopolis, Michigan

GDD total: 344 on May 17, 2017
GDD normal*: 316
Number of GDD ahead of normal: 28
GDD forecast: 405 on May 22, 2017

• Pine sawfly eggs have hatched. The first generation of pine needle scale crawlers are active.
• Galls have started to form from Cooley spruce adelgid. Spring foliar treatment window is closed.
• Eastern spruce gall adelgid eggs are hatching, the nymphs are starting to feed and the galls will be visible soon.
• Gypsy moth treatment for all compounds is recommended now (between 200 and 700 GDD).

Commerce Township, southeast Michigan

GDD total: 264 on May 17, 2017
GDD normal*: 253
Number of GDD ahead of normal: 11
GDD forecast: 323 on May 22, 2017

• Pine sawfly eggs have hatched. Pine needle scale eggs have not hatched.
• Galls have started to form from Cooley spruce adelgid. Spring foliar treatment window is closed.
• Eastern spruce gall adelgid eggs have hatched and the nymphs are starting to feed.
• Gypsy moth treatment for all compounds is recommended now (between 200 and 700 GDD).

West Olive, west Lower Michigan

GDD total: 294 on May 15, 2017
GDD normal*: 264
Number of GDD ahead of normal: 29
GDD forecast: 356 on May 22, 2017

• Pine sawfly eggs have hatched. The first generation of pine needle scale crawlers are expected.
• Galls have started to form from Cooley spruce adelgid. Spring foliar treatment window is closed.
• Eastern spruce gall adelgid eggs have hatched and the nymphs are starting to feed.
• Gypsy moth treatment for all compounds is recommended now (between 200 and 700 GDD).
• 646 GDD in 50 percent white polyhut (approximately one month ahead).

*The normal GDD are based on averages from 1981 to 2010, which are accumulated at temperatures over 50 F with the Baskerville-Emin method.

If you don’t see a station close to you

1. Go to Enviroweather.
2. Click on the station within the network closest to you.
3. Click on the “Landscape and Nursery Tab.”
4. Open the folder “Christmas Trees” on the left-hand side of your screen.
5. Look at the data for each of the common pests in the station closest to you.

For more information

• European pine sawfly
• Pine needle scale
• Cooley spruce adelgid
• Eastern spruce gall adelgid
• Gypsy moth

Also, check out the “2017 Michigan Christmas Tree Pest Management Guide” by MSU Extension for treatment of insects and diseases.

Thanks to Jeffrey Andresen for providing the 30-year normal comparison data. Thank you to Jill O’Donnell for her review.