What is killing my grass?
We bought a new-build house and placed sod in the spring of 2016. It looked great for the first two years but not quite as good last year. A small area (that sometimes dries out) was attacked by something last year and eventually died in June or July. It became straw colored and I could easily pull up clumps of the grass like there wasn't good attachment to roots. I dug a patch up and checked for grubs and didn't find any. The birds were really hitting this area hard. The only thing I ever noticed was a dark mass of tiny larvae of some sort while mowing. I made a pass and came down on the next pass and noticed a large mass of very small and writhing larvae (brownish gray in color) where I had just cut the grass. I went to get my wife to show her and by the time I came back (couple minutes) it was gone. Now this spring I have a large area of very pale straw colored and dry/dead grass in the front yard and there are several birds on it every morning. The grass pulls up easily and if I rake it with a leaf rake, there is pretty much nothing but soil left with sparse grass seedlings coming up. Everything else is nice and green, although it looks like it is spreading into those areas. I dug up a patch and no grubs and I never see the birds with grubs in their beaks. Is it lawn mites? If so, how do I control those? Is there something I need to do before trying to seed or place new sod?
Weld County Colorado
The "sometimes dries out" mention suggests that there is an irrigation coverage problem. The timing of the larvae appearance is confusing in your narrative? You were mowing - so the grass was alive and growing - when you saw the larvae? But the grass was dead last June/July? The timing of the larvae you saw was before then? In the spring?
The grass in the photo doesn't look like grass that has been dead since last July - because the leaves are still intact. Was this grass alive last summer, then died later in the fall? Or it was OK last fall and is not greening up this spring?
As for mites, the recent late winter/early spring snows and rain have essentially eliminated mites as problems in lawns this spring along the Front Range.
My best guess is that this is a "hotspot" irrigation coverage problem. When you have turned your system on, place some containers (yogurt containers, a few extra coffee cups, mason jars, cat food or tuna cans, similar-sized food storage containers, etc.) out on the lawn in this area. Place a few in the brown area(s) and others in the healthy, green part of the lawn - but in the vicinity of the brown area. Run the irrigation system and measure the depth (use any ruler or tape measure) of water in the containers. The water depth in the containers from the brown areas is likely to be less. Even a tenth of an inch or so less than what is collected on the green parts of the lawn translates to large amounts of water over an entire growing season - which results in brown spots in the lawn. This can be due to heads that don't pop up perfectly vertical, are too low or high, don't turn (if you have rotors), are plugged or damaged, etc.
If you check the irrigation and there are no differences between the green areas and brown spot, then other causes can be investigated.
If you want to clarify the above (when these spots became a problem, and when you saw the larvae relative to the problem beginning), I might have a different diagnosis.
The small area that sometimes dries out is in the backyard and is not the same area in the photos that is a problem this spring. There is an irrigation coverage issue with the small area in the backyard. However, the weak/damaged area spread into surrounding once healthy grass that does receive adequate water. The mass of larvae appeared while mowing the backyard last summer. The dried out area was relatively small surrounded by green grass that needed mowing. After I made a pass with the mower, the larvae appeared in the cut swath I had just made. The larvae were in the green grass at the edge of the damaged area.
The large dead area in the photos is in the front yard facing east. The area was not dead last summer but it was starting to look unhealthy - thinned out, shorter, not as much dark green as the rest. We have 4 overlapping rotary sprinklers that cover the front yard. I periodically check irrigation amounts with small rain/sprinkler gauges I stick in the lawn. It does not appear there are any irrigation coverage problems in the front yard. This area was ok (not as healthy as the rest) last fall but did not green up this spring. I have since raked and removed most of the dead, overseeded, and have been watering. After 2 1/2 weeks there is finally a decent amount of green coming up. However, it is very sparse to almost non-existent n some smaller areas still. The better areas have quite a bit of green now. albeit very short and slow growing at this point.