These are in the true bug group and have the apt common name of Phlox Plant Bug, and Phlox is their only host plant, though related and similar-looking insects in the same genus can feed on other species. They overwinter as eggs laid in plant debris, so removing all dead Phlox stems and foliage during winter is important to reduce the population for the following year. In-season, you can either knock the bugs off of foliage into a tray or bucket of soapy water to drown, or treat heavily-infested plants with a lower-toxicity insecticide like horticultural oil or insecticidal soap. Take note, though, that either of those topical sprays will only impact the insects they come into direct contact with; residues of oil or soap on foliage without bugs present will not have any impact on suppressing them.

Fortunately, heavy damage might reduce the ability of the plants to bloom normally for this season but otherwise don't threaten the long-term health of the plant, especially if growth was largely unaffected in prior years. If your plants have already flowered or you don't mind losing or delaying some bloom this year, cut back the Phlox as a more immediate way to reduce the bug population and make any regrowth easier to treat if the insects return. We do not recommend the use of any longer-lasting pesticide ingredients on native plants that are important to our local ecosystem, though several other ingredients in those higher-toxicity tiers would work to kill or repel them.

If Bishop's-weed (also called Goutweed, Aegopodium podagraria) is the companion plant you're referring to alongside the Phlox, we recommend its removal because it is an invasive species and seedlings have escaped into natural areas across about half the state. The plant is usually so vigorous that no treatment should be needed (it's not a host plant for this specific insect anyway) and a complete cut-back and removal and disposal of the debris should greatly help reduce a pest population if it too is being damaged; it should regrow just fine on its own, if not this summer, at least next spring.

Miri

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Goutweed, being perennial, would need either repeated pulling/cutting-down until it stops regrowing, smothering (if not too close to desirable plants), or spot-treatment with a systemic herbicide that will kill the roots. If you wish to avoid herbicide, the most practical, though tedious, method is to just keep removing all above-ground Goutweed growth every time you spot it. Be vigilant and remove foliage promptly to deprive the roots of an energy source; every time the plant is forced to resprout it's using-up root energy stores. Eventually, it will exhaust itself and die out, but that might take a while if it's well-established or if any regrowth is missed and is allowed to photosynthesize undetected for a while, recharging those root "batteries." Even if you do use a herbicide, one or more re-treatments might be needed if the plant rebounds after first succumbing. Check label directions for the timing between sprays and give the treatment plenty of time to start working before removing any Goutweed debris after it shows symptoms of dying back.

Miri