Grubs as well as other insects are found on every farm and in every yard in the world. Some are beneficial and some are destructive. However, none seem to get homeowners working on their lawns more excited than White Grubs which most of the time and most of the year are not a detrimental problem.
Most white grubs that are found in turf are Japanese beetle larva. To know if the grub population is a problem you have to understand the life cycle of the Japanese beetle.
During the late spring and early summer you will see the largest accumulation of the ½ inch long, shiny, metallic green and bronze colored adult beetles. This is the period they are feeding on the trees, shrubs, and flowering plants in your yard. During this feeding, they are not affecting or eating your turf. What is happening is that the females will intermittently leave the plants they are feeding on to burrow about 3 inches into the soil, lay a few eggs and return to feeding on the leaves. Each female continues this process until they have laid 40-60 eggs. They usually lay these eggs in the turf because the soil contains more moisture and it is easier to dig.
By midsummer (late June into July), the eggs hatch and the young grubs begin to feed. This is the period that we need to be concerned about potential damage from Japanese beetle grubs. By late fall the grubs will be fully grown and will be about an inch long. To prepare for winter the grubs will burrow 4 to 8 inches below your turf to remain inactive all winter. This insect spends about 10 months of the year under ground.
In early spring, the grubs return to the root zone of the turf and continue to feed on the roots until late spring when they change into pupae. In about 2 weeks the pupae become adults and emerge from the ground to start the cycle again.
This early spring feeding is rarely a cause for concern, as the grubs emerge slowly thus their numbers per square foot are small (usually less than a couple per square foot of sod) and what little feeding that does occur is not damaging due to the rapid growth of your turf during this time of year.
As stated earlier, the time we need to be vigilant and watchful for large grub infestation is midsummer (late June into July); however we still do not want to apply insecticides if they are not needed. The insecticides, along with killing what grubs might be present, also kill the beneficial insects and present a hazard to our environment. Therefore what we need to do is a little scouting to determine if pesticides or treatments are necessary.
Using a shovel cut a hole in your turf 8 x 8 x 3 inches deep. Turn the turf over onto a piece of newspaper and search the root area and soil for any grubs. Multiple the numbers of grubs found by 2.25, the resulting number will be the number of grubs per square foot. Generally you should consider treating your lawn if you determine you have 10-12 grubs per square foot. Replace the soil and the turf and water heavily for a couple days to allow your turf to mend itself.
If you decide that chemical treatment is necessary, you must read and follow all label directions exactly. Remember pesticides (including insecticides) are toxic materials.
At this time, perhaps the most effective chemical insecticide for Japanese beetle grubs is Merit. It is available in both granular and wettable powder formulations. It must be applied when the Japanese beetles are in the larva or grub stage and it must be watered into the soil with plenty of water within 2 days of application. If not watered in, the ultraviolet rays of the sun will begin to break down the active ingredient.
However, there is more and more concern over the serious hazards that insecticides can pose to people, pets, wildlife, and the environment. There is also increasing concern about the fate of insecticides in the environment and the potential for pesticide runoff causing water contamination. Because of these concerns, it is recommended that homeowners consider biological controls for the suppression of insects.
The 2 most common biological controls are bacteria that are not harmful to humans, wildlife, or the environment. In fact these bacteria often are naturally occurring, in limited quantities, in most soils already.
The first bacterium has effectiveness very similar to that of the chemical insecticides. Bacillus thuringiensis (BT) has strains which are selective for Japanese beetles and must be applied while the beetles are in the larva or grub stage. BT is a stomach poison to the grubs they ingest it while they are feeding. It is applied as you would chemical insecticides, including the need for watering.
The other bacterium is used more as a preventative than an immediate control measure. Milky Spore has been used on turf in the United States since the late 1940’s. Just like BT, the grub ingests the milky spore. The spores germinate in the grub’s gut infects the gut cells and enters the grub’s blood stream, where they multiply. The build up of the spores in the blood causes the grub to take on a characteristic milky appearance.
Milky spore bacteria build up in the turf slowly over 2 to 4 years. As the grub ingests the spores, become infected, and die, each releases 1 to 2 billion spores back into the soil. Milky spore is very effective in suppressing large beetle populations but it works best when applied in a community wide treatment program.
Communication and Scouting are probably the two best tools in controlling Japanese beetles. Talk to your neighbors regarding possible solutions for the problem pest. Organize a neighborhood turf and garden group and develop a cooperative watch program utilizing adult traps, resistant plants, and biological controls. Invite local experts to speak, such as extension agents, or representatives from a horticultural society, or personnel from a nearby University or College.
Japanese beetles can be destructive pests of trees, plants, and turf. Elimination of this pest from just your property is not practical or possible and the chances of them returning next year are almost certain. However, following the management options discussed in this “Guideline for Homeowners” will help you reduce the damage that can be inflicted by this pest.