Least-Toxic Control of Grubs Choose a different pests
Factsheet: Grounding Out Grubs: Managing Grubs with Prevention and Least-toxic Strategies
Pest type: Insects
Grubs, or more specifically white grubs, are the larvae of scarab beetles, Japanese beetles, June beetles, chafers, and others. They are one of the nation’s most destructive lawn pests. These organisms are C-shaped, off-white in color with a characteristic dark brown head. The larvae feed on grass, plant roots and organic matter in the soil. As a result, grubs can be found at the root zones of damaged areas of the lawn. It is important to identify grubs as the source of your browning lawn before utilizing biological treatments highlighted below after trying preventative methods. Other factors, such as drought, disease, excessive fertilizer, poor soil or even another pest, may be the cause of your lawn’s brown spot.
Is it a problem?
- Sample your lawn in early to mid August, at the start of the grub’s life cycle. Early sampling of your lawn is one way to identify young grubs before they are capable of seriously affecting your lawn. With a spade or shovel, cut three sides of a square into your turf and peel back the turf like you would a carpet. Look for c-shaped grubs on the exposed soil and under the sod mat. Repeat this every 20-30 feet.
- For an otherwise healthy lawn, a couple grubs per square foot (0-5 grubs per sq ft) is not considered to be a problem. If there are 6-9 grubs per sq ft, you may want to take into consideration the overall health of your lawn. If your lawn is healthy, has a robust root system and is dense, it can probably withstand a few grubs. Otherwise, you may want to consider treating your lawn. For more than 10 grubs per sq ft, treatment should be carried out.
- Know when to use biological controls! Treating your lawn for grubs is most effective in late summer or early fall, when grubs are most susceptible. This is because grubs are small and near the soil surface while the temperature is warm. Treatment done at other times may not be as effective once the grubs have grown bigger.
Pest prevention practices
Check the mowing height of your lawn mower. Adult beetles prefer to lay their eggs in short grass. Cutting your grass tall – minimum of 2 inches high – may discourage egg laying, and reduce future grub populations.
Aerate your lawn. Cultivate a healthy lawn by encouraging deep grass roots. Deep roots have a greater chance of surviving a grub infestation. Grubs that may be feeding on roots deeper into the soil are spread out over a larger area, making their damage less discernable. Aerate your soil, either by hand or aerating equipment, in the spring and fall to promote deeper roots.
Consider your watering frequency. Lawns that are heavily managed and watered regularly, especially during the summer months, may actually attract beetles. Eggs require moist soil conditions in order to hatch and prevent the larvae from drying out. Therefore, deep periodic soaking of the turf is more beneficial than frequent, light watering. Infrequent watering also encourages roots to grow deeper into the soil. If there is moderate grub infestation, watering in late August or September, can promote tolerance and recovery.
Monitoring and record-keeping
Be sure to continuously monitor your lawn every year, sampling in early to mid August, at the start of the grub’s life cycle. For an otherwise healthy lawn, a couple grubs per square foot (0-5 grubs per sq ft) is not considered to be a problem. If there are 6-9 grubs per sq ft, you may want to take into consideration the overall health of your lawn. If your lawn is healthy, has a robust root system and is dense, it can probably withstand a few grubs. Otherwise, you may want to consider treating your lawn. For more than 10 grubs per sq ft, treatment should be carried out.
Non-chemical and mechanical controls
Mechanical traps that lure adult beetles (with food type lures or pheromones) can be placed around the borders of your property and can capture around 75 percent of beetles that approach it. Setting up traps should coincide with the emergence of beetles in your area. Since these traps attract more beetles than they can catch, it is advised that traps be placed away from plants susceptible to beetle damage. However, do not use traps if you currently do not have beetles visiting your property! Traps can be obtained from many garden centers.
- Milky Spore is a naturally occurring host specific bacterium (Bacillus popillae-Dutky) that once applied to the lawn, releases spores that are swallowed by the feeding grubs. The ingested bacterium then begins to cripple and kill the grubs within a period of 7-21 days. The build up of spores in the grubs causes them to take on a characteristic milky appearance. Once the grubs are dead, new spores are released into the soil, providing years of protection. Milky spore has been effective in the mid-Atlantic region of the U.S., but is generally not as effective in areas below garden zone 5, including New England states and the Midwest, due to low soil temperatures. This treatment is recommended for long term rather than short-term control. Note: Milky spore targets the Japanese beetle species of grub only.
- Nematodes These microscopic worms live and breed in the soil and infect and kill feeding grubs. Commercially available nematodes for grubtreatment can be obtained at local supply stores, and the strains Steinernema carpocapsaeand Heterorhabdis spp seem to be the most effective against grubs. When applying nematodes to your lawn, it is important to irrigate before AND after application, since nematodes require moist soil conditions. It is recommended to treat the entire lawn.
- Bacillus Thuringiensis (Bt) Though not as popular as milky spore, Bt can also be used to control grubs. Bt is a naturally occurring soil bacterium that, when ingested, acts as a stomach poison that interrupts feeding, and eventually leads to death. Bt is a microbial pesticide and is available at local garden shops. There are several strains of Bt used to control various types of pests, but the most effective grub killer is likely Bt galleriae, which is the active ingredient in products like grubHALT!
Least-toxic chemical options as a last resort
Few least-toxic chemical options are effective at grub management, see above for cultural, mechanical, and biological controls for grubs.
Chemicals to Avoid
Look at your product labels and try to avoid products containing those chemicals listed below:
(A = acute health effects, C = chronic health effects, SW = surface water contaminant, GW = ground water contaminant, W = wildlife poison, B = bee poison, LT = long-range transport)
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