This is the question I asked myself for three years. The first year I noticed the brown-gray “knitted” look on my leaves I thought it was wind damage. In 2014 we’d had a spring wind storm after the leaves had emerged, and I thought that’s what caused the damage I was seeing. But when it returned in 2015 and 2016, I knew this wasn’t the case. I hadn’t used any products on or around these shrubs – no herbicides, insecticides, or even fertilizers – so I doubted that the damage had anything to do with these or the weather. Finally, in the fall of 2016, I sent the samples off to the University of Massachusetts Diagnostics Lab for analysis.
Tawny Simisky, Entomology Specialist with UMass Extension, contacted me a few weeks later with her initial diagnosis of an insect I’d never heard of: chilli thrips, aka Scirtothrips dorsalis. She asked for more samples, and I sent some as did Russell Norton, Extension Educator with Cape Cod Cooperative Extension, who had seen similar damage on other hydrangeas in the region. After having her initial diagnosis confirmed by several other Extension specialists and the USDA, she confirmed that this was the first time that chilli thrips had been seen in Massachusetts. Oh great. Not a first I really wanted to be part of!
But here we are, and I post these shots of my plant, along with this link to the fact sheet from UMASS that has more information about this pest and possible treatments, so that if your hydrangea leaves start to have a similar look, you’ll have someplace to start for information.
So, the question, of course, is what am I going to do. Russ Norton, at the Cape Cod Cooperative Extension, tells me that although the ingredient spinosad is recommended as a treatment, the problem with using this is that thrips can become immune to it over time. So for the moment I’ll refrain from using that on my shrubs. Last fall I cleaned all the fallen leaves around these plants and bagged them for disposal. I don’t normally do that around hydrangeas, so it will be interesting to see if this helps. Secondly, once these plants break dormancy I’m planning on spraying repeatedly with horticultural oil, just to see if that can suppress their populations…it can’t hurt, is my thinking.
UPDATE 8/2018: Cleaning up fallen foliage around the plants in autumn seems to help but it’s not a cure. I haven’t kept up with the sprays, but I will do so next season as this year I see quite a bit of damage. I plan to try Bonide Mite X which is labeled for thrips.
Although the fact sheet lists the many other plants this non-native species of thrips is known to attack, including many agricultural crops, right now in MA it’s only been seen on hydrangeas. Just our luck, Cape Cod…our signature plant! Well, as Gilda Radner’s character, Rosanne Rosannadanna, used to say, “It’s always something!”