Hello Bruce,

The only possibilities are fungus, insects, chemical damage, and drought stress.

Based on what you have told me, the most likely cause is insects feeding on your roots.   I am assuming you are having this problem in the summer so it is likely you have bill bug problems.     Adult bill bugs become active in spring and lay their eggs.  The eggs hatch into larvae and the larvae begin feeding on the base of the grass and then crawl down into the soil and feed on the roots.  Your grass dies of drought stress because the grass can't take up the water that is present in the soil.

With bill bugs, it is easier to target the larvae than try to kill the adults because there are systemic insecticides that get taken up by the plant.   When the larvae eat the plant, they also eat the insecticide and they die.  To kill adults, you have to get the insecticide on the adults which is more difficult to do for a couple reasons.  First, not all the adults are active at the same time.  Second, the insecticide gets on the ends of the grass blades and not all the adults will come in contact with it.

To prevent this from happening next summer, you need to apply an insecticide in May.  The best options are the systemic insecticides Aceleprn G (also sold as Scotts Grub Ex), or Imidicloprid 0.5 G., or Meridian 0.33 G.   These products can be applied with a drop spreader but need to be watered in well after application.   

Imidicloprid and Meridian are in a class of insecticides called "neonicitinoids".   Also, there are other neonicitinoids being sold under different names.   These typed of insecticides are harmful to bees.  Aceleprn or Scotts Grub Ex are safer for bees but is more expensive.

If you want to target the adults, you can apply bifenthrin (e.g., Talstar PL Granules).  It is in a different class of insecticides called pyrethroids.  You would time the application at the peak time of adult emergence.   This time is determined by placing linear pitfall traps in the ground.   

These are simply 2" PVC pipes (about 4 feet long) with a 1/2" slit cut in the top of the pipe running most of the way down the pipe.   One end of the pipe has a cap and the other end runs into a plastic jug (e.g. a large yogurt container with a lid).    

You dig a small trench in your lawn and the pipe is mounted flush with the ground so the adult billbugs walking across the ground will fall in the hole and into the bottom of the PVC pipe, and end up in the yogurt container.  Some people put isopropyl alcohol in the yogurt container to better preserve the insects.   It is not necessary if you check the traps frequently.   Look up "linear pitfall trap" on the internet and you will see a good example.

By the way, you can sample for the larvae when your grass starts to turn brown.  The first sign is if you tug on the grass and it pulls out easily.   To confirm pull back the sod, it should remove easily and you will find the bill bug larvae in the soil.  Look up a photo of bill bug larvae.  They are 1/4" long, white bodies with orange heads.  They do not have legs.

I hope this helps.

Good luck.

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Bruce,

I would wait until mid May - I think that is the best timing because it gives you a longer residual.   But there is not a ton of research indicating an exact timing - it depends on the weather each year.   But I did a trial where I applied mid April, mid May and mid June.   Mid April and mid May both worked but mid June was too late. If mid May doesn't work for you, you can apply it now.

The grass will take up the insecticide (so it has a longer residual than a contact insecticide) and when the larvae eats the grass, it will die.  Water it in after the application (30 minutes with a hose and sprinkler), or apply it before a rainfall.

You should not have to make a second application.   But bill bugs will come back every year, so you will likely have to apply every year.    After you have applied it for a couple years, you can try skipping a year and see if you have knocked the population down enough to prevent significant damage.

Good luck.

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Bruce,

I can answer your question if you have one. I am taking a hiatus from Ask Extension because of job responsibility changes. 
 
Email me directly at <personal data hidden> 

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