An insect sighting and a damaged lawn are two observations to connect, not a diagnosis by themselves. Prepare to record what you find: grass condition, roots, larvae and the location of each check. Compare a damaged edge with nearby healthy turf.
Inspect the root zone
Use a small inspection area and replace lifted turf after the check. Photograph larvae clearly and note the sampled area so observations can be compared. Do not label every soil insect a harmful grub. Ask extension for identification and a locally appropriate sampling method if you are unsure.
Keep species and life stage together
Minnesota Extension explains that Japanese beetle larvae live in soil and that management depends on the situation and life stage. Adult beetles can arrive from elsewhere, so treating your lawn does not guarantee protection from adult feeding on nearby plants. Other white grubs can have different seasonal patterns.
Make the treatment decision locally
Use identified species, the number found, turf damage and local guidance to decide whether action is warranted. Avoid copying a calendar or threshold from a different region without checking applicability. If treatment is justified, match the current product label to the target and stage, then save the record for follow-up.
A brown patch or a visiting adult beetle does not establish that the entire lawn needs insecticide.