GDD update 9.5.19

Sun on the lake!  I haven’t been in it yet as it is a little chilly…..but soon.  Sorry – that has nothing to do with Christmas trees.


Doing this email on GDD and pests has taught me a lot.  One thing is how confusing all the information can be.  My 2 main sources are the Cornell Tree and Shrub Guidelines (as we call it for short) and Pennsylvania IPM’s IPM for Christmas Tree Production.  And even with only 2 sources, there is confusion. Cooley Spruce gall adelgid fall treatment is a good example.  Admittedly, we are dealing with a pest with 2 hosts on which the insects behave somewhat differently and with a variety of pesticides.


Tree and Shrub says 1850-1950 GDD for fall control on spruce and 1500-1775 on Douglas fir, late fall for soil applications.

PA says 2800-3000 GDD for fall control of overwintering stages on their calendar.  In the specific chapters, it notes a single treatment in late September to early October for both Douglas fir and spruce to control exposed nymphs and immature females.  For spruce, it is recommended to treat after the first frost and that this is the most reliable time to treat.  Dormant oil for either species should be after the first frost when there is no active growth and the temperature is above freezing.   Depending on where you are, late Sept/early Oct and 2800-3000 might be the same time, but Long Island is predicted to break 3000 about Sept 10.


How do you figure out which is the right answer?  Trial and error sometimes.  And a little more research on my part.


The increase in GDD over the 5 day forecast is getting smaller it seems.  So there are not many changes in pest management.


Spruce spider mite – 2375- 2806 GDD

Doug fir needle midge – remove heavily infested trees (carefully so you don’t spread the pest) before larvae exit the needles (needles will turn brown). Mark infested trees for placing traps in the spring

Elongate hemlock scale – keep scouting for crawlers.

Cooley spruce gall adelgid – prune out galls on spruce before they turn brown (about 1850) – see other information on Cooley above

Cryptomeria scale crawlers – 1750-2130 GDD

Japanese beetle adults – 1029-2154 GDD

Two spotted spider mites– 1300-2000 GDD

Pine needle scale 2nd generation– 1290-1917 GDD


Have a great week!