Ah, Japanese beetles – 2 questions no less. Yes, they can be a problem on Christmas trees – I have seen them on the leaders and they can chow down because they are big.
The pheromone (which is what is in those hanging beetle traps for sale everywhere) is good at attracting the adults but not good at getting them all in the bag so don’t bother with traps as a control measure unless you can blanket the whole area- or attract them to some other area where they aren’t doing damage to plants you want. They are useful to monitor when the adults arrive and then take it away.
If just a few trees have adults – knock them off into soapy water (I can hear your eyes rolling) (A note from a reader –
Knocking the beetles off into soapy water is actually pretty quick and easy if you do it in the morning. They fly off if you wait until later. When you crush them you call in more according to Brian Nault (Cornell Entomology faculty).)
For treating adults 1029-2154 GDD for foliar sprays and it may take several applications. They tend to be localized where I have seen them – so you wouldn’t have to spray everything – but since you are scouting your fields you will know where they are. And write it down for next year – to learn if they tend to appear in the same place every year. (I don’t know the answer to that. One would think they would lay eggs near where they feed, but they have preferences for egg laying sites – grassy areas – and can fly.)
There are quite a few active ingredients listed in the Guidelines for adult management (get your own copy here so you don’t have to wait for me – https://www.cornellstore.com/product/149958)
Check the labels to make sure that Christmas trees or conifers are mentioned as well as Japanese beetles. Pyrethroids, like bifenthrin, cyfluthrin, lambda cyhalothrin and permethrin last 2-3 weeks. There are some imidacloprid soil applied pesticides that are systemic. All of these pesticides are toxic to bees and other pollinators so use appropriately. I need to ask some questions on some other active ingredients so more information soon.
Grub control is in the fall when the larvae are fairly small and before they have migrated lower in the soil profile. You really should sample for grubs before applying anything to find out where they are – which makes it a little more difficult. Check in the areas where you have seen adults, sandy and grassy areas seem preferred (for all white grubs – and id is a little complex). There are some biocontrols – nematodes, Bacillus popilliae or milky spore disease – which have variable results and are often better in warmer parts of the state.
Drier soils are less preferred – some good out of this rain-free period. But adults will often move from irrigated grassy areas like lawns to other areas to feed.
Timing is important for both grubs and adults