Hi Suzanne,

Thanks for reaching out to the UVM Extension Master Gardener Helpline.

Hmmm. The photo didn't allow us to get a good look at that fly and since you said it is fast, I'm not sure if it would be helpful to try to submit another one. What would be great, is to get a photo or two of the underside of the leaves. 

Here's why -- There are several possible causes of the cupped/curled leaves on phlox. First, phlox prefer moist, even-soil moisture. Leaves can curl to conserve moisture as a reaction to heat stress and inadequate moisture.

However, spider mites are another possibility. They feed on the underside of the plant’s leaves, sucking sap from the leaf. As the feeding progresses, it causes the leaf to “cup” downward. Leaves may become speckled in appearance, turn yellow/brown, and eventually drop. You may be able to see webbing on the undersides of the leaves. Here’s a really nice description of spider mites, provided by our friends at South Dakota State University Extension: https://extension.sdstate.edu/spider-mites-garden-phlox.

Aphids also feed on the undersides of the leaves and can cause leaf distortion. They usually do not cause little long-term damage, but populations can explode so monitor frequently for them.

And phlox plant bugs are another possibility. They live under the leaves and also feed on plant juices. Their feeding initially causes white or light green spots on the leaves. Continued feeding can result in leaf curling, turning brown, and falling off. Management for aphids and phlox plant bug is the same as spider mites.

Take a look at the undersides of the phlox leaves and look for evidence of the above. You may be able to immediately determine the cause. If not, send along a couple of clear, close-up pics and we will see if we can determine what's going on.

Hi Suzanne,

I'd like our plant pathologist to take a look at your question. Would you mind submitting a few more close up photos of the damage, including the underside of the leaves if you can? Also, I see some white residue on the plants -- is that due to the liquid fence spray or powdery mildew? Do you see anything unusual along the stem of the plant from the base at the soil all the way up to the affected tip? Any holes, discoloration of the leaves, egg masses, etc.? There are a few things that can cause cupping / deformities in plants including heat stress, herbicide drift, etc. and I'm also wondering about aster yellows. 

Hello Suzanne, I reached out to our plant pathologist to take a look at the photos you sent along. We are in agreement that the issue is not the fly. They said it would likely be a plant bug like tarnished plant bug or another true bug (in the Hemiptera family). These insects cause deformed flowers and fruits. For example, tarnished plant bugs and their nymphs feed on the stems, leaves, and blossoms; as they are feeding, they release a toxin into the plant that stunts or otherwise deforms growth. Tarnished plant bugs (TPB) are also very quick movers so they may be difficult to spot. Here is a handy info sheet prepared by our colleagues in NH that describe TPB and control options. I hope that helps!