Several species of white grubs are present at this time as larvae or adults. Although these species are primarily a problem in turf, May beetles cause feeding damage to trees.
True white grubs, also known as 3-year white grubs and May beetles, consist of many species in the genus Phyllophaga in Illinois. Their life cycle varies from 1 to 3 years. At least one 3-year species is present as adults at this time in central Illinois. These 1-inch-long, dark brown, stocky May beetles are active at night, feeding on the foliage of oak, ash, crabapple, and other deciduous trees. They eat the edges of the leaves, occasionally removing leaf tissue to the mid-vein. Because the feeding occurs at night, no pest is seen during the day: They are hiding in the thatch of the lawn. Scouting the trees after 9 p.m. usually reveals the feeding beetles. Several insecticides are effective in controlling these leaf-feeding beetles, but the damage is usually not heavy enough to warrant treatment. Having a 3-year life cycle, other May beetles are likely to be found as grubs during tree planting and other soil digging, Many of these are 1 to 1-1/4 inches long. They will pupate during the summer, emerge into the soil in early fall, and spend the winter underground as adult beetles before emerging next spring to feed on tree foliage and reproduce.
Black turfgrass ataenius adults have been reported in early May on golf course turf in central Illinois. These 1/4-inch, cylindrical, black or brown beetles are obvious in the clipping baskets of greens mowers. If adult beetles are numerous, applying imidacloprid (Merit) or halofenozide (Mach 2) 2 weeks after the adult beetles are seen is recommended for control.
Japanese beetle larvae have successfully survived the winter in central Illinois and are in the turf root zone feeding. White grub activity in turf has been reported from other locations in central and northern Illinois as well. These are in areas where Japanese beetle is prevalent and are probably also Japanese beetle grubs. Research has shown that Japanese beetle grubs do not migrate deeper than 11 inches into the soil for the winter. They die if the temperature reaches 15°F or if they are subjected to freezing temperatures (32°F and below) for 2 months. With the previous cold winter in which landscapers reported the soil being frozen to 15 inches deep in central Illinois and 30 inches deep in northern Illinois for several weeks, it was anticipated that there may have been little to no survival of this species.
It may be that under colder conditions, Japanese beetle grubs migrate deeper than 11 inches or that the soil was not frozen long enough. Not only does it appear that Japanese beetle adults will be present in Illinois this summer, but that their numbers will be large. I am still interested in whether Japanese beetle grubs are being found in northern Illinois. Let me know what grub species are being found in northern Illinois or send me some grubs to identify.
Realize that Japanese beetle grubs pupate in early to mid-June and that these mature grubs are very hard to control. Irrigation can keep the turf growing faster than the grubs can eat it at this time of year, greatly reducing obvious damage. If grub numbers are very high or if the raccoons, skunks, and birds are feeding on the grubs and damaging the turf, try triclorfon (Dylox) or Heterorhabditis bacteriophora (Hb) nematodes for control. Neither trichlorfon nor other insecticides are reliably effective in controlling large grubs. Hb nematodes are effective against large grubs but will probably provide only about 60% control.