Both insects cause irregular to round patches of yellowing, dead grass. It may seem difficult to distinguish between the two but there are few factors that will help you figure out what pest you are looking to control.
When is the damage occurring? Early Spring and Fall or Mid-summer during drought?
White grubs are the juvenile form of the Japanese beetle. The life cycle of the Japanese beetle starts in the early spring. The beetles are present as grubs in the soil, these grubs feed on roots, causing the irregular brown patches you find on some lawns. As the summer progresses to late June/ Early July, the juvenile beetles morph into adults who can fly. The adult Japanese beetle does not feed on roots but on plant leaves, often leaving large holes and leaf veins behind on our most precious plants. In late August/early September, the adults lay eggs that once again will feed on roots causing the brown spots in grass areas. The best method for adult and juvenile Japanese beetle control is using soil nematodes. These nematodes are naturally occurring organisms in the soil that feed on the beetle’s juveniles. You spray nematodes in early spring or late fall to prevent lawn damage and prevent future plant damage caused by adults. Before you spray the nematodes water your lawn thoroughly (min. 3 inches) so that the little organisms can move freely throughout the plant rooting zone. Nematodes are most effective in the fall when the juveniles are the smallest, but in problematic areas a fall and spring application may be required to keep populations at bay.
Chinch bugs feed as adults on the stems of the grass, but they are very small so the damage does appear to be very similar to that of the Japanese beetle. But chinch bugs love drought and really do not like wet weather. So you are much more likely to find chinch bugs causing damage in late July and August when the weather is hot and dry. You are also likely to find that chinch bug damage will begin on the parts of your lawn that are most drought susceptible – higher land and sunnier places. If you believe you may have a chinch bug problem it is best to get it under control while the patches are small, meaning the population is low enough to keep a handle on. The best method for control of chinch bugs is keeping a healthy lawn; re-seeding and topdressing in areas where grass density is low, fertilizing, watering and ensuring you are not mowing the lawn too short. If with these practices you are still having a problem try this method: Saturate soil with a hose in the evening or early morning, soak grass with an insecticidal soap, cover the area with a flannel blanket or a few sheets of burlap for about 20 minutes. The water and insecticidal soap will drive the insects out of the grass and they will crawl into the fabric where their feet will get caught. Now remove the fabric. You can drown the insects on the fabric in a bucket of water or throw out the fabric to ensure no pests end up back on the lawn. You may repeat this every 4-6 days.