Hello leharr19,

The insect that “devours” yellow loosestrife (Lysimachia spp.) in July in Michigan is almost certainly the oil‑collecting bee in the genus Macropis, NOT a pest insect. More information at the link below:

Oil-Collecting Bees | Department of Entomology

The culprit: Macropis oil‑collecting bees
These bees are specialists that rely entirely on yellow loosestrife for their reproduction.
Macropis bees depend completely on yellow loosestrife (Lysimachia) for floral oils
They are found specifically on yellow loosestrife in June and July
Females collect floral oils and pollen from yellow loosestrife to feed their larvae
They are rare and ecologically important
These bees do not eat the leaves — they collect oils from the flowers.
But their activity can make the plant look “chewed up” or “damaged” during peak bloom.

So what is actually damaging your plant?
Not a pest
Yellow loosestrife is not known to have a leaf‑devouring specialist insect in Michigan.
The bees are harmless
They do not harm the plant in the long term.
They only take floral oils — the plant continues to bloom normally.
Timing matches perfectly
Your damage occurs in July, exactly when Macropis bees are active on yellow loosestrife.

Should you control them?
No.
These bees are:

Rare
Non‑aggressive
Important pollinators
Dependent on your plant for survival
They are considered a species of conservation interest.

What to do
If you want to help the bees (recommended):
Keep the yellow loosestrife
Avoid insecticides
Maintain some moist soil nearby (they nest in soil)
If you want to reduce bee activity:
Remove some of the yellow loosestrife flowers
Thin the patch
Plant alternative ornamentals nearby
But again — they are beneficial, not harmful.

I hope this helps!