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Grub Scouting Watch

July 21, 2004

Japanese beetle adults have emerged throughout the state. They were reported in Will County on June 22. They should be present in damaging numbers until about mid-August. Carbaryl (Sevin), cyfluthrin (Tempo), and other labeled pyrethroids should give about 2 weeks of control, so three treatments will be needed for control on plants that are treated.

Masked chafer adults emerged during the last week of June in central Illinois. Their numbers appear to be relatively small. These are both the southern masked chafer and northern masked chafer, whose larvae are annual white grubs. Both species are 1/2-inch-long, tan June beetles with black heads. They are strongly attracted to lights at night, particularly from about 10:30 to 11:30 p.m. They are present until mid-July. Observe numbers of adult Japanese beetle and masked chafers to determine the need for grub appli-cations. Also observe the state of non-irrigated turf. If non-irrigated turf is green and actively growing through mid-July, the beetles will lay their eggs in both it and watered turf. Unless the adult flight is very large, this will result in low grub numbers as a result of adultsí being spread out. Some areas of Illinois have received enough rainfall for non-irrigated turf to still be attractive for egg-laying. However, a week or two of hot, dry weather can quickly turn lush green turf dry, brown, and dormant. In these situations, the beetles will lay most of their eggs in irrigated turf, resulting in grub injury this fall in those areas.

If high grub numbers are anticipated, application of imidacloprid (Merit) or halofenozide (Mach 2) is warranted during July. Both insecticides take about 3 weeks to kill the grubs, so they need to be applied before early August to avoid grub damage in late August. Another option is to wait until early August when the grubs have hatched. At that time, the grubs can be scouted, and trichlorfon (Dylox) or insecticidal nematodes can be applied to high-population areas. Scout for grubs by cutting through the turf with a heavy knife, then pulling it up to expose the grubs in the root zone. In drier soil, the grubs may be 3 to 6 inches deep, but they will be near the surface if the soil is moist.

This is when first-generation black turfgrass ataenius grubs are full-grown and can cause the most damage. Be watchful for wilting and brown turf in the dampest areas of golf courses. These will be greens and tees as well as locations where water tends to stand or run to after irrigation, such as along the lower side of green aprons and in fairway swales. Fifty or more grubs per square foot is the damage threshold. The grubs will be in the root zone and will look like other grubs, white and C-shaped, but they will be only about 1/4 inch long when fully grown. It is very hard to kill mature grubs with insecticides. Heterorhabditis bacteriophora, an insecticidal nematode, should be effective, but it provides only about 60% control. This level usually reduces the impact of the grubs on turf enough for the turf to recover. Commonly the best way to cope with mature grubs is to irrigate to get the grass to grow roots quicker than the grubs can eat them. As the grubs pupate, their feeding on turf roots will lessen, and the turf should improve.


Author: Phil Nixon

 

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