Hemlock browning after Imidacloprid treatment.

Let me preface this by saying, I'm new to the PHC game but have been thrown into the deep end. To carry the metaphor, since y'all seem to be the lifeguards on duty, I'm hoping you can keep me from drowning.

We did soil injections of Imidacloprid in March to control for HWA. Just returned to the trees after the owner complained about browning. "Browning" being a couple branches have needles that turn yellow, then brown, and finally fall off. Generally, the trees look pretty healthy (see pic 1).

The neighbor recently put up a wooden fence less than 2 feet from these trees, but the fence didn't appear to damage any of the roots (yet).

After inspecting a number of the affected branches, I found a very small population of needles have some sort of scale(?) (see pics 2-3).

Is this browning normal? Might it be related to the Imidacloprid, the scale, or even the fence?



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I would agree with the mite population spike. We have seen it on probably 30% of imid suggested treatments (customer applied). @TsugaPHC would probably be able to shed light on why as he worked closely with someone that did research on why.


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Sorry, Didn't catch the scale comment in the my first reading of the op. That is likely hemlock elongate scale and can be as much of a problem as HWA. The imid should handle that as well in most cases from what we have seen. When we look at hemlock we always look on top for adelgid and flip for scale and determine if treatments are required. Again Mike can add so much more to the hemlock conversation than I, hopefully he will chime in.


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Didn't see the scale comment either. Imid may help control it, dinotefuron is better with scales.

Neonic insecticides cause increases in populations if spider mites either by somehow negatively affecting predatory and parasitic bugs or perhaps the chemical causes changes to occur to the mites themselves.
Sorry, I’m late to the hemlock party, haven’t been on the Buzz in a few weeks.

That damage looks more to me like either some kind of needle blight, or possibly a root rot issue. Seems counterintuitive since hemlocks like stream banks in the wild, but I’ve seen them with phytophthora when they’re in heavy soils with poor drainage. Elongate hemlock scale also causes needle yellowing like that, but in populations way higher than what appears in the photos.

JD is correct, imidacloprid has little to no efficacy on elongate hemlock scale. Imid isn’t particularly water soluble, so it stays in the xylem/phloem. That’s fine for soft scales, since that’s where they feed. EHS and other armored scales feed cell to cell in the outer layers of tissue where imid doesn’t move. Dinotefuron, being much more water soluble, can diffuse out to these cells, which equals a degree of armored scale control—still not as good as a properly timed oil/insect growth regulator foliar spray, in my opinion.

As for the mechanism of why imid flares spider mites, the physiology is waaaay more intense then I can understand. Bottom line is that studies showed that imid stimulates spider mites to lay more eggs—around 25% more eggs. I think it was Dr. Cliff Sadof at Purdue that authored the study around 15 years ago, but it may have been someone else. There have also been studies that found imid makes predators lazy. The PhD that I used to work with called it “bong hits for beneficials”. Not toxic enough to kill them, but enough to make ‘em dumb and lethargic. Both factors contribute to mite flare back.
Thank you all for your replies! Your information is very valuable to me and I still have a job in PHC (for the time being)! I'm still quite green, but I've been trying to absorb as much information as possible about PHC and so have begun using the UT Extension agency as another resource. The agent showed me some tiny, rust mites on a different sample I'd brought him. I couldn't believe it was there as I could see no definition to any of the abnormalities on the needle with my naked eye! There's a whole world going on that we can't even see - Mind Blown!

Anyway, regarding phytophora, are there any telltale symptoms I can look for if I go back to this clients trees? Soft spots on the roots? Should I try to get the soil sampled? I'd suggested we wait till spring and then look to use fert to reinvigorate the trees and replace the fallen needled branches.

Regarding properly timed oil foliar spray, what time is best? I've started using it - perhaps unsuccessfully - to treat severe elongate scale and rust mite damaged areas.

I look up to you guys as the experts I hope to become. Thanks for your time!


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Conifer spider mites are active now but eriophyids are more active in late winter and early spring but oil now may still help. Not sure about the elongate scale but oils can be hit or miss on armored scales depending on where they are in their lifecycle. Try Googling "elongate hemlock scale pdf" for sources of info.

Attached is some info on phytophthora. I look for bleeding cankers, discoloration in wood and roots, and dieback but that one is pretty nonspecific as far as symptoms go.