Question
Our grass is dying and we suspect a worm/grub problem since we have seen some yellowish worms come up when we have raked the thatch layer. When is the best time to treat to kill the worms? What is best to kill them if we don't know exactly what species they are? How long after treatment can we prepare and plant new grass seed?
Answer(s)
You may be seeing larvae of lawn pests, most probably billbug. Here are some hints on how to scout and diagnose for billbugs and other lawn pests: Dig up a 6 by 6 inch square of your turfgrass, including 2 inches of soil. Put it in a bucket and pull it apart, looking for any insects or larvae. Collect any/all you find. Identify the insects by referring to these USU Extension FactSheets: Billbugs, at
http://extension.usu.edu/files/publications/factsheet/ENT-106-07.pdf
White grubs, at
http://extension.usu.edu/files/publications/factsheet/ENT-104-07.pdf
Sod webworm, at
http://extension.usu.edu/files/publications/factsheet/ENT-44-07.pdf
Cranberry girdler, at
http://extension.usu.edu/files/publications/factsheet/ENT-42-07.pdf
There's only one generation of billbug per year. They may be emerging as adults about now. Most of the damage is already done for the year, so if you don't find any pests in the turf sample, wait until next May to treat for the pests. Once summer cools off (highs in the 80's), you should fertilize the affected areas to promote new growth and vigor, to repair the damage. Also, be sure to check the thickness of the thatch layer in your turf. If it's thicker than one-half inch, plan to have your lawn core aerated in early fall. Aeration helps promote thatch decomposition. Thatch is a protective habitat for the billbug, so it's best to minimize it.
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