White Grubs - The hidden threat to lawn and garden.

What are the soft, thick, worm-like insects under the surface of the soil?

They are the juvenile life stage of scarab beetles and are known as larvae or grubs. Their life cycle is similar to other insects like butterflies that have a caterpillar as their larval stage. For most insects the larval stage spends its time feeding and gaining energy, while the adult stage is responsible for reproduction. White grubs are white colored, C-shaped and can be from 1/4 inch to 2 inches in length. 

Do white grubs eat my plants? 

Yes, grubs feed on the roots of plants. Although they are notorious for damaging lawns and Turfgrass, they will feed on the roots of vegetables and ornamental plants as well. Damage is more visible in late summer and fall when drought conditions occur and plants  have lost many roots due to feeding. Grubs also feed on decaying plant material and animal manure. 

Do any animals eat the grubs?

Yes, a variety of wildlife feed on grubs. This damage can often be far worse than the actual damage caused by grubs, as raccoons peel back and destroy turf while searching for grubs. Grubs give off a strong scent that attracts raccoons, skunks, bears, moles and opossums as well as crows, starlings and robins. 

Where do grubs come from?

Grubs are larvae from beetles in the scarab family include Japanese beetles, chafer beetles, June beetles, dung beetles and the black turf grass Ataenius beetle. These all produce white grubs that live and feed in the soil for one to three years.  

Adults usually emerge in June to July to mate and lay their eggs in soil. Japanese beetles prefer to lay their eggs in moist, organic rich soil, while chafer beetles can lay their eggs in much drier Turfgrass. Moisture is essential to the development of eggs and newly hatched larvae and they will die if conditions are too dry. 

Beetle ID Chart - Minnesota Dept. of Agriculture

What is a safe way to get rid of white grubs?

Entomopathogenic nematodes are the best option to kill white grubs in the soil. While toxic chemicals can upset the natural balance of the soil system, including the microbiome, nematodes are safe for plants, people and pets and don't contaminate the natural environment. Heterorhabditis is the best beneficial nematode to target white grubs. 

When is the best time to apply nematodes for white grubs?

White grubs are most susceptible to beneficial nematodes in the late summer or early fall while they are young and vulnerable. Use when temperatures are over 60°F and in moist soil. Apply twice, several weeks apart, to ensure good contact with all the grubs. 

How do I keep my lawn and garden safe from grub damage? 

• Maintain healthy plants through proper irrigation and fertilization.
• Mow lawns at a higher level to produce plants with a deeper root system. 
• Monitor soil samples for grubs throughout the season.
• Apply beneficial nematodes if grubs have reached a threshold.
• Plant cultivars that are well adapted to local climate.
• Hand pick adult beetles and drop into a jar of soapy water.

• Encourage natural enemies by planting a variety of flowering plants that produce nectar and pollen.
  

Treatment thresholds for turfgrass 

From Utah State University White Grub Fact Sheet

Treatment is not needed unless obvious feeding damage is visible on the soil surface. Healthy lawns can support a certain threshold of grubs without showing damage. 

• Threshold for masked chafers and Japanese beetle grubs is 8-10/ft2
• Threshold for May/June beetle grubs is 3-5/ft2
• Threshold for black Turfgrass Ataenius beetle grubs is 30-50/ft2

Additional Resources 

White Grub - PNW Pest Management Handbook

White Grubs - Colorado State University 

White Grubs - Utah State University Extension

What are White Grubs? - Purdue University Extension