Because you can't wait until you see scarring I would spray now if plums have been difficult to protect in the past. I often wait until last apples have lost their petals, but some sites this is not soon enough for adequate protection. Once petals drop and you get day temps into the 70's there's definitely a likelihood of PC damage at any point past petal fall. Cornell puts it in the '60's.

In my zone you should spray at 75% of petal fall, so you should have sprayed earlier.

Frank:

My Stanley prune and my Balaton cherries are both the
same size as the fruit in your picture. I hope to get out
tonight with Imidan if it is not too windy (every day
seems windy lately).

My issue is that my apple trees nearby are still in bloom
so I am waiting for a really calm day so I don't get any drift on the apples.

Since my 20 tree orchard is of mixed fruits, sometimes I
have to wait a little while some trees at petal fall, yet others not before I break out the insecticide on all of them. Usually I end up waiting to avoid hurting any bees
on trees yet in bloom and hope those nearby that are past petal fall are not yet getting the wrath of the curculio.

Last year the curculio numbers were extremely high in my orchard. Even with Imidan, I had some damage. First
time that happened. Perhaps I waited too long to spray
as worried about drift to nearby trees in bloom.

I envy the commercial scene where everything is planted
in blocks so easier to manage in May. My mixed orchard
is always a challenge as this time of year the prunus need curculio protection yet the apples are still blooming.

It depends on what you are spraying. My schedule was every two nweeks changed to one week once we are in the 80s it will be every 4 days I spray oil and sourwend not to kill bugs I can not see them I am trying to prevent the eggs from hatching so far so good my peach is in the size of large marble, the cherry and nic are in the size of regular marble
No infection noticed cross fingers

I've never like that idea of 75% petal fall, because there will still be late-opening flowers attracting bees.

90% seems more reasonable to me.

I always spray late...its just me! Some PC hits don't bother me...i'm not looking for max production, just want some good tasting fruit. Plus the plums this year have huge fruit set, as do some of my apricots (major thinning needed)... I will spray as soon as the wind lets up..its been so rainy here (over 5 inches of rain this month), windy, cloudy and now cold (40F last nite) that i don't worry too much about PC yet. I've seen a few hits on apricots and just a couple of sweet cherries...but nothing to write home about...

My spraying situation is not good. Lots of kids around, so i mostly try to spray in the late evening and try not to over spray. Calm conditions are a must unless you want a shower of pesticide.

I don't like spraying at 75% either, but I learned my lesson this year. The hard part is when trees don't bloom at the exact same time. It's hard to just spray one or two trees and not zap the bees at the same time. We had a very unusually wet and cold spring, and my trees and the bees didn't know whether to come or go. A lot of blooms, but few bees, and a lot of hit and miss pollinization and cold damage to my peaches.
It makes you appreciate the hardships with which small farmers have to contend.

I just wait for all petal drop on my apples then spray. I need every bee and bumble I have. We've also had so much rain lately that spraying just will not happen. Hope the rain hasn't disturbed pollination! Mrs. G

I'm surprised at the talk of spraying at 75% petal fall as the trees are usually loaded with bees at that time and I've never seen a tree infested with curc at that stage.

I'm also surprised at the talk of so much wind and worry of drift- I guess I live in a relatively calm area where in a five day period there are usually at least two days of calm in the early part of the day.

Here in southeast NY I manage scores of orchards and it is impossible to spray them all at optimum time. We generally only do two insecticide sprays and the orchards I spray take me 5 or 6, 12 hour days to get them all. Some times I go straight from finishing up the first to beginning the second because of lost days to rain.

J. plums will have dropped their petals for 10 days by the time the first spray goes down and E. plums maybe 5. Apricots- forget it. Yet, the only problem I have is losing a significant (say 25%) part of my E plum crops at a minority of sites to PC.

Of course, every region is different, so it's impossible for me to say what's going on here, but I can't help but wonder if you folks are all getting good coverage.

One difference is my spray is coming out of the sprayer at about 200 pounds of pressure (when releasing the spray) and maybe a gallon a minute. It's a small sprayer by commercial standards, pushed by only 4 HP, but it pushes the spray through the canopy with good force and covers everything. I find it very difficult to cover anything well with a back-pack sprayer.

I actually found a PC strike on one of those sized plums above...so man they do hit early. I sprayed about half the tree but man...its just loaded with fruit and a lot of them still haven't dropped their petals

I use a 2 gallon sprayer..i also have a backpack sprayer, but its kind of an overkill, because i spray my trees at different times, so all i need is a gallon or so when i do spray. If i had a large orchard, id go with a big battery powered sprayer..

i usually try to cover the fruit... try not to over do it, and try to limit over spray... lots of kids around here...plus lots of houses... another reason why spraying int he evening is the best for me (calm conditions, kids are inside).

PCs are really out in force this year.

I'm bagging the apples earlier than usual and finding PC scars on them already. I usually don't see them until after they're done ruining all the stone fruit.

The 75% of petal fall is from the entomologist at Texas A&M. It's the most detailed information I've seen about curcs. See the link below.
I have a 5 gallon battery sprayer on rollers, and I spray my trees thoroughly inside and out.

https://insects.tamu.edu/extension/publications/epubs/eee_00014.cfm