Description and Life Cycle
White grubs are soil-dwelling larvae of certain scarab beetles. They feed on the roots of turfgrass and other plants, destroying the plants ability to absorb and transport water and nutrients. White grubs are pudgy, off-white larvae with a brown head and typically are bent in the shape of the letter "C." Full-grown length varies from 3/4 to 1 inch. The white grubs that routinely damage lawns in Iowa are called annual white grubs because they have one generation per year and take one year to complete their life cycle of egg, larva, pupa, and adult.
The adult beetles of our annual white grubs are either Japanese beetles or masked chafers. Japanese beetles are 3/8 inch long. The head and thorax are shiny metallic green and the wing covers are coppery red. Masked chafer beetles are tan or straw brown in color and as the name implies, have a black stripe across the eyes and face.
Japanese beetles and masked chafers emerge and begin flying in late June and lay eggs in the turf during July. The eggs hatch in 2 to 3 weeks and tiny white grub larvae begin to eat the grass roots. The grubs grow rapidly and are fully grown at by late August or September. Feeding by the grubs prunes the roots from the plants and causes the grass to wilt and fade. Extreme feeding by populations of 10 or more annual white grubs per square foot will cause the grass to die. White grubs move several inches deep in the soil to spend the winter. The move back near the surface in the spring time but cause little additional damage as they wait to pupate in June, emerge as adults and start the cycle over.
Extent of Damage Due to White Grubs
Damage from white grubs in lawns can show up anytime after mid-August though damage may not become obvious until September or even into October. Damage from white grubs is usually localized. It is typical to have severe damage in irregular and isolated spots.
White grub damage may first appear as drought stress (gray-green discoloration and wilting in the hot sun). More severe damage causes the turf to die in large irregular patches that can be rolled back like a loose carpet. High populations of grubs may go unnoticed until discovery by raccoons or skunks. Raccoons, skunks and crows will turn over large patches of loose turf, eat the grubs and leave behind a torn-up mess.
Rainfall and soil moisture are critical factors affecting the extent of grub damage. Adequate moisture in mid-summer will favor beetle activity and grub development. If plentiful rainfall or irrigation continues through August and September (when grubs are actively feeding) damage may not be noticeable because the grass continues to grow and masks the root injury symptoms. Healthy turf can sometimes tolerate 20 or more grubs per square foot before showing signs of injury. The onset of dry weather can lead to “sudden” appearance of grub damage symptoms.
Treatment using an Integrated Pest Management (IPM) approach
It is very difficult to use an IPM approach to white grubs in turfgrass. Turfgrass often has a very high value – either monetary value (golf courses and sod farms) or aesthetic/emotional value (home lawns). White grub infestations are highly variable from year to year and from place to place. Damage is spotty, localized and impossible to predict.
Monitoring, one of the keystones of IPM is not practical for white grubs under most circumstances. Inspection for grubs requires cutting and lifting flaps of turf and looking for grubs below the thatch level. This is difficult, time consuming and potentially damaging. Research has indicated that the number of samples necessary is too large to be practical.
Studies at Cornell University have shown that over 70 percent of all grub control treatments were applied needlessly because there were no grubs in the lawn. Many homeowners are frightened into applying grub controls because of advertisements on TV, in plant centers, or because of horror stories they have heard about grub damage. Most grub treatments are not only expensive but hard to justify from an environmental standpoint. There are 3 approaches to grub management in the home lawn, depending on your tolerance for damage, comfort with pesticides and willingness to spend the cash.
Grub Management Techniques
The Golf Course Approach
If the potential for ANY turfgrass damage by white grubs is unacceptable, special, long-residual insecticides can be applied to prevent white grub growth and feeding. With this approach you treat every part of the lawn, every year because use of insecticide is preferable to ANY white grub damage. Preventive insecticide treatments should be applied from late May to early August. The insecticide must be watered in with irrigation ror rainfall to be effective (0.5 inch minimum). Applications after mid-August and in the spring time, when grubs are fully grown, are not effective.
The insecticides available to COMMERCIAL applicators for for grub prevention are Chlorantraniliprole (Acelepryn); Clothianidin (Arena); Clothianidin + bifenthrin (Aloft); Cyantraniliprole (Ference); Imidacloprid (Merit, etc.); Imidacloprid + befenthrin (Allectus); and Thiamethroxam (Meridian). A recently-introduced biological insecticide is Bacillus thuringiensis galleriae (grubGONE).
Insecticides available to homeowners for white grub prevention include Imidacloprid (Merit, etc.); Chlorantraniliprole (Grub-Ex); and Bacillus thuringiensis galleriae (grubGONE).
A compromise modification of the Golf-Course Approach is to treat only those areas of previous damage. Grubs tend to return to the same areas in successive years, so it is logical to treat the areas where you had grubs last year or the year before.
If you choose to apply insecticides, read and carefully follow directions.
In most years careful monitoring of the turfgrass in August - September may alert you to early signs of damage (wilting, turning brown). At that time your could apply a fast-acting, short-lived, curative insecticide to reduce the number of white grubs. Curative insecticides are not as good as preventive insecticides at reducing the number of grubs, though risk of severe damage can be greatly reduced. You also have the advantage of only applying insecticide where and when it is needed. Unfortunately, you might still lose some sod, especially if summer rainfall or irrigation keeps the grass growing and vigorous through July and August but is discontinued in September.
In some years damage symptoms may not appear until after it is too late for effective treatment (late September through late October). Insecticide treatments after early October are not effective and are not recommended. Unfortunately, raccoons and skunks are much better at locating grub populations than we are and the first hint of a grub problem in your turf is likely to be that your lawn was "plowed" by varmints overnight.
Insecticides for curative grub control include Trichlorfon (Dylox, Bayer Advanced 24-Hour Grub Control) and Carbaryl (Sevin, etc.). One-half inch of irrigation immediately after insecticide application is essential for good results and to promote recovery of damaged turf.
The Do-Nothing Approach
Count up how many years you DID NOT have grub damage. Divide the cost of replaced sod by that number of years. If the yearly-averaged cost of sod is less than the price of insecticide, do nothing and take your lumps in the occasional year when damage occurs. This approach is much easier to follow if your attitude is "it's just grass, anyway."
White grub management decisions are difficult and frustrating. There is no one right answer for everyone.