Sitka Mountain Ash being eaten

Hi,

Wondering if others in the Pacific Northwest have seen this before. I came out into the garden this morning and seemingly overnight, half of the Sitka Mountain Ash had been eaten. I found a relatively fast moving bright orange caterpillar on one of the victim's branches.

It looks as if the whole tree will fall to this. Any solutions?

Thanks!

 

Photo? Otherwise, no way to know if the chewing was insect damage or if the insect was a plant eating species.

 

Yes, share a photo.

We've had a mild infestation of apple hawthorn leaf skeletonizer on mountain ashes in the Garden this year, so that might be worth investigating.

 

Thanks - here are a number of photos, some just with paper behind to see definition of the infestation.

 

Yep, that's the same one that we have. We're controlling it on one prominent tree at the front entrance by removal and drowning or simply squishing them. Other plants in the garden that have infestations we are leaving as-is.

The tree should be able to withstand one year of defoliation.

 

Same damage being seen on S. aucuparia seedlings here, which is the species that your plants appear to be also in these photos. If you are assuming that yours belong to S. sitchensis because they are growing wild S. aucuparia is a common weed in this region, often pops up on its own. S. sitchensis occurs natively only well up in the mountains and like the other native Sorbus species never has a tree-like habit, with any trunks growing much larger than the others to produce a pointed crown shape, with a single lead shoot extending well above the others. It also does not normally produce so many leaflets in each leaf.

I have seen S. aucuparia offered erroneously as one or more of the native species here, because of the fruits having been gathered from plants encountered growing wild in the region. So if you instead bought and planted these examples that does not establish that they are in fact S. sitchensis. Mislabeling of nursery stock in general is not unusual, so you always have to know what your plant is supposed to look like when assuming it is any particular kind - whether you had it come up on your place, found it growing someplace else or bought it from a dealer.

 

Hmm, I'll have to talk with the colleague who identified the ones at the garden as apple hawthorn leaf skeletonizer. These look far more like mountain ash sawfly (as did the ones in the garden) now that I've seen this identification from this thread.

Apparently a new pest to the Pacific Northwest, first documented in Snohomish County WA in 2009.

 

Good catch, thanks Ron - I didn't plant it so upon your comments, it's S. aucuparia. Thanks for the heads up!

 

I’m very interested in what I can do to save my tree. 2021 was the second year it was attacked, this year being far worse than the first. It happens so quickly if I happen to be away, which I was this year for a funeral in my family, the leaves are gone in a matter of days. Help! What can I do?

Did you ever find a solution?

 

We've had trees attacked multiple years and not die, so I'm not sure how much effort is needed. Their life cycle seems to have larval hatch in mid-summer, so the trees have already been gaining substantial energy. That said, the one mountain-ash near the entrance that gets attacked has a much lower fruit set this year (not sure if it is related).

 

Is there any way to combat this?

 

Bt is not an option: Mountain ash (Sorbus)-Mountain ash sawfly

For cultural control: "Prune out infested branches, though that seems more radical than allowing defoliation to occur with branches releafing."

There is also a long list of chemical controls: https://pnwhandbooks.org/sites/pnwh...nsect21-f-chemicalcontroloflandscapepests.pdf

But, like I said re: saving your tree -- your tree should be fine if it is getting defoliated later in the year. If, however, you want to preserve the look of the tree, then I guess you are looking at the options above. Me, I'd consider an alternative if it was important that a tree in that spot looks perfect unless the tree is special for some reason (sentimental, size, etc.).