Thank you for reaching out to OSU Extension with your quesition about the worm-like insect on your hydrangeas. What type of hydrangeas do you have?  Is it possible to get a picture? If not, how long is the insect and have you notice it mutating into a caterpillar or moth?

My research so far has found that it could be Hydrangea Leaf-tiers  

1.  Hydrangea leaf-tiers (Exartema ferriferanum), are a small moth whose larvae, (a tiny green caterpillar with a black head), sew terminal leaves together with a silk thread. Once inside this enclosure they feed on the leaves and flower head. These leaf enclosures become wrinkled and puckered and are easy to spot on the plant. They are native to Canada and U.S.A with the majority of sightings coming from the southern tip of Ontario and Quebec and the North/East states of the U.S. ( iNaturalist.ca (n.d.)). Hydrangea leaf-tiers attack various hydrangea species but their preferred hosts are the smooth hydrangeas like Hydrangea aborescens ‘Annabelle’ and Hydrangea aborescens Incrediball

Control

• Physical Control: Prune all stems to 10 cm (4″) from the ground in spring after egg laying has occurred. Pry open the cupped leaves and destroy the caterpillar, or remove the affected leaves and destroy.
• Chemical Control: single application of Bacillus thuringiensis kurstaki (Dipel, Thuricide, others) or a labeled pyrethroid will also be effective (Nixon P. (2012)).

https://horticultureforhomegardeners.ca/2021/05/16/hydrangea-problems-leaf-tiers/

This link has more pictures that may help you decide if this is the  problem:  https://bygl.osu.edu/index.php/node/1979

Please let me know if you have any questions or need anymore information.  Again, if I can get a picture it would help me to be as accurate as possible.

Thank you!

Happy gardening!



Thank you for your response.

I have escalated your case to a specialist. 

You should have an answer within a few days.

Hello,

 Thank you for reaching out to the Ask an Expert system.

I have read through your description of the problem and the discussion with the Master Gardener Volunteer you have already had.

Based on this information, I believe it is time for a sample of your hydrangea containing the arthropod in question to be sent to the Plant and Pest Diagnostic Clinic in Wooster, Ohio.  

Without a photo we cannot make a recommendation and given the described size,  I believe a photo would also be of little help with an online diagnosis.  Given that you have already tried several insecticides without success, I believe a definitive identification and guidance are required.

The Plant and Pest Diagnostic Clinic (http://ppdc.osu.edu ) charges a $20 fee that they will invoice you for.  You would mail a sample of a leaf showing symptoms and containing the insects you are describing.  Put them into a ziplock bag and overnight ship it to the address below.  A staff entomologist will use microscopy to try and identify your arthropod if one is present and offer suggestions.  

Physical Samples may be mailed:
C. Wayne Ellett Plant and Pest Plant Diagnostic Clinic
Ohio State CFAES Wooster Campus
c/o Dr. Francesca Rotondo
234 Selby Hall, 1680 Madison Ave., Wooster, OH 44691

Physcial Samples may be hand-delivered:
Requires coordination with Dr. Rotondo:<personal data hidden>) | <personal data hidden>

You can contact the plant and pest diagnostic clinic at the number above if you have further questions about submitting a sample. 

Apologies that we cannot identify your problem further through the online system. I hope you will consider the clinic as your next step for assistance.