Mowing Height and Biological Control of White Grubs in Turf

People's preference for short turfgrass can have a definite impact on biological control of grubs. Turf, especially Kentucky Bluegrass, is most healthy when mowed to a height of 3 inches. Healthy turf means vigorous root growth, and an ability to tolerate some root feeding by white grubs. In contrast, turf that is mowed too short is less vigorous, and less able to replace roots consumed by underground turf pests. Increasing the mowing height of turf improves the chances for biological control by allowing the grass plants to tolerate grub damage while natural enemies such as ants, typhiid wasps, and native milky spore diseases increase their number.

To demonstrate the effects of mowing height on biological control you will need a site that has had white grub problems for at least the past 3 consecutive years and will not be treated for grubs this year. Mow half the site to 3" and the other half to 1.5" all season. Place two pitfall traps (a mason jar or plastic cup buried in the soil so that the lip is flush with the ground surface and filled part way with a non-toxic antifreeze) in each half of the site. Insects and spiders walking over the soil surface fall into the jar and cannot crawl out. You should be able to find at least ants and ground beetles. Check the pitfall traps weekly and keep a record of what you collect each week from the end of June. Are the predators responding to differences in grub populations or just habitat differences?

Note where you first start seeing grub injury. Take out a plug of turf and examine the root growth. Is it more healthy and vigorous at the higher mowing height? How many grubs do you find near the browned out areas in each site? Does the high cut turf really tolerate higher grub populations? What grub species do you find? Do any have a native milky spore disease? You can cut off a leg and examine blood to see if it is milky. (See MBCN Vol 2, No.11 for references.) Don't forget to take a few pictures for use at winter meetings!

- Cliff Sadof and Tim Gibb, Purdue University

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