Extension Educator, Horticulture
It's time for our annual discussion about lawn grubs. Want to put fear into the heart of a lawn connoisseur? Just point to a brown patch in the lawn and yell grubs!
Grubs are the immature stages of some beetles. In our area the highest populations are annual white grubs. Grubs are whitish, "c" shaped and wormlike. The adult is the tan 3/4 inch June beetle. Each morning I find June beetles doing the back stroke in the dog's water bowl (or is it the dog paddle).
Some lawn areas are more attractive to June bugs laying eggs. More eggs are deposited at night in warm soil areas such as next to sidewalks and driveways and often near outside lights. Open areas free of trees and shrubs are preferred and moist soil is favored over dry soil. This year egg laying is likely to be concentrated in watered lawns since most unirrigated lawn areas are dry. The ultimate irony - in most years highly managed turfs are more likely to get hit with grubs.
Eggs will start hatching the first of August. Damage will usually occur in August through October. Grubs feed on grass roots until the grass dries out and dies. The turf can easily be pulled back like a carpet. Turf can survive some feeding. Less heavily watered turf should be scouted for grubs in early August. Peel back the grass. Just a couple grubs per square foot are not a problem to an otherwise healthy lawn. Ten or more per square foot are necessary to justify treatment.
Home gardeners have some options for grub control: Don't water during July and August. If the weather is hot and dry, cool season grasses such as Kentucky bluegrass will go dormant. If a few grubs are detected, water and fertilize appropriately so turf can recover from some feeding then reseed or resod areas damaged.
Several pesticides are available for grub control. Diazinon has been the home gardener's standby for many years. Diazinon is long lasting and effective, but it can commonly take 3 weeks to kill treated grubs. Although treated grubs do little feeding, it can be upsetting to still see live grubs after treatment.
Trichlorfon sold as Dylox is an effective pesticide, but it is short lived. Grubs need to be present when this insecticide is applied, but it kills grubs quickly. Dylox is good for rescue situations when damage is ocurring.
Imidicloprid sold as Merit or Grubex is fairly new to the home gardener market. It is very long lasting. However imidicloprid takes three weeks to kill grubs in many situations. Although it is sometimes marketed for spring application, it should be applied in July once it is determined that numbers of adults are high enough and that the soil moisture like this year is low enough to concentrate heavy egg laying in irrigated turf. In other words, if you see large numbers of beetles eyeing your lawn and no one else's.
With all home gardener applied products it is recommended that granular formulations be applied and then irrigated in through the thatch to the root zone with at least two inches of water. Be sure to read and follow all label directions.
Parasitic nematodes are showing mixed results. Adequate soil moisture is critical to their effectiveness and survival.
Spikes O' Death, usually sold as lawn aerator sandals, actually showed fairly good results in killing lawn grubs in a Colorado State study. Researchers had to walk the lawn three to five times to achieve an average of two nail insertions per inch providing entertainment for the neighbors and aerobic exercise.
The biological insecticides containing milky disease sold as Japdemic, Doom and Grub Attack, while effective on Japanese beetle grubs, do not control annual white grubs.