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How many grubs are too many? Here’s a guide to treatment thresholds for European chafers, Japanese beetles, and June beetles, the most common grubs in home lawns. Numbers are based on grubs/sq.ft.
0-5 grubs: rest easy. Fewer than five grubs per square foot is a low population. You don’t need to treat
6-9 grubs: think about your lawn. Is your grass dense, with a healthy, robust root system? If so, it can probably withstand grub populations of 6-8 per square foot, or more. On the other hand, if animals such as skunks, raccoons, birds, and moles are digging up the turf to feed on the grubs and this bothers you, consider treating highly populated areas.
10 or more: they may cause damage. Ten or more grubs per square foot will likely cause damage, especially if the lawn is otherwise stressed. In most circumstances, you’d be justified treating where populations are this high. Several weeks after treating, sample in a few locations to determine whether treatments were effective.
In the soil, microscopic worms known as nematodes live and breed. Some nematodes infect and kill grubs, thereby reducing populations. You can also purchase and release nematodes that will kill grubs, if used properly.
Eggs
Most eggs are a creamy-white in color, about 1/16-inch (1.5mm) long and slightly oval when first laid in the soil. These absorb water from the soil and swell slightly, becoming more round.
Larvae
The C-shaped white grubs are thick bodied, creamy-white with brown head capsules and short legs. All species have three instars, that is, the larvae molt three times.
Pupae
The pupae are often slightly longer than the adults and are formed in chambers one to two inches in the soil. The pupae are first cream colored and darken before the adults emerge.
Adults
The adults are typical scarabs - robust, oval beetles with the antennae ending in a large club of flattened plates. Most genera are easy to identify by sight but species identification of May/June beetles and masked chafers require a specialist.
White grubs seem to be periodic pests, attacking turf areas irregularly from year to year. The major factor influencing development of damaging numbers of grubs is soil moisture and rainfall. In general, in years with normal or above normal rainfall, grub populations increase. Well maintained turf next to ornamental plants favored by the adults seems to be more commonly attacked. However, masked and European chafer adults do not feed as adults and these pests build up in well watered and maintained turf.
Cultural Controls
Certain species of scarab adults prefer specific host plants. Where Japanese beetles are common, do not plant roses, grapes and lindens around high maintenance turf areas. May/June beetles prefer oaks. The fine and tall fescues are not as severely attacked as Kentucky bluegrass and perennial ryegrass.
Practically all white grub species require moist soil for their eggs to hatch. The young larvae are also very susceptible to drying out... In areas where turf can stand some moisture stress, do not water in July and early-August when white grub eggs and young larvae are present. On the other hand, moderate grub infestations can be out grown if adequate water and fertilizer is applied in August through September and again in May when the grubs are feeding. This latter strategy is not preferred because mammals may dig up the turf or irrigation bans may occur.
Since white grub occurrence is rather sporadic, applying pesticides for control of anticipated grub populations is not recommended. However, in areas where adult activity has been observed or perennial infestations have occurred, preventive applications may be warranted. Chemical applications generally perform best when applied before mid-August, or when white grub egg laying is underway.
Most of the modern soil insecticides have short active residual periods (three weeks or less) and must be used when the grubs are actively feeding. No registered insecticide is 100% effective; they usually kill 75 to 90% of the grubs present in any given area. This is why re-applications may be necessary when grub populations get very high. Timing of treatments is critical for success. You should apply the pesticide when the grubs are small and actively feeding yet late enough to catch all of the population. In general, reducing thatch and using good irrigation after making a pesticide application will increase control.
As with the late-fall pesticide applications, spring treatments are often ineffective. Though the grubs feed during the spring, they are quite large and the span of time for treatment is short. If a spring application is deemed necessary, check to make sure that the grubs are actively feeding at the soil/thatch level.
Populations of annual grub species that are less than six grubs per square foot can usually be masked by water and fertilizers. Populations between 10 and 15 per square foot can cause significant turf damage in September and October. Of course, populations occasionally reach 40 to 60 grubs per square foot and these levels can cause damage by late-August.
In general, irrigating after an insecticide application is made will improve performance for soil insect control. It is also generally recommended that grass clippings be returned to the lawn for one to two mowings after a grub insecticide application. Do not wait more than 30 days to recheck the grub infestation, especially if the original population was high. If the grub population has not been reduced below six grubs per square foot consider reapplication of another pesticide. Remember, the smaller the grubs the easier they are to kill with insecticides. |
