Are Grubs in Compost Good or Bad? Expert Views

Composting is a great way to turn food scraps and yard waste into a nutrient-rich soil amendment called compost. When you turn compost, you may see grubs and many other organisms.

The process of composting relies on organisms, big and tiny, including bacteria, fungi, mites, snails, beetles, ants, worms, grubs, and many more. Therefore, it is normal to see grubs in compost piles, bins, and tumblers. They have a role to play (as you’ll find out below).

Are grubs in compost good or bad?

Compost grubs are good detritivores because they eat dead and decaying organic matter and break it down into nutrient-rich compost. In addition, their burrowing action aerates the compost pile, making the composting process more efficient.

Grubs are physical decomposers in compost piles.^[1] They tear, grind, and chew organic matter into smaller pieces that would otherwise be difficult for microorganisms to break down. The activity of grubs and other macro-organisms sets the composting process in motion.

While grubs can be unsightly to some people, they have a positive effect on compost, especially when they’re not in excess.

Laura Locker Bell, a composting expert from Sacramento, California, says: “Compost is not sterile and needs decomposers like grubs to make it happen. They eat only decaying and dead material and make excellent compost in a short space of time.”

However, too many grubs in a compost pile can easily consume most of the food scraps and yard waste before it gets a chance to decompose. We discourage applying finished compost full of grubs in gardens because the beetle larvae cut and eat the roots of desired plants such as turfgrass.

According to Laura, controlling their population in compost piles is important since the “white grubs soon pupate, and the adults could be destructive depending on what they are.” 

See a short story version about grubworms in compost piles here.

Benefits of compost grubs

There are many species of grubs. Most of those in compost piles and lawns belong to the scarab beetle family. Teoh Huat, a seasoned composting expert based in San Diego, explains that “the larval stage of these beetles are quite harmless, even beneficial, at this stage. When they become adults, they can damage desired plants.

Meanwhile, here’s a list of the benefits of having grubs in your compost:

• Grubs break down organic matter in compost, making it easier for microorganisms to continue the process of decomposition and release of nutrients. 
• Grub activity makes the process of composting much faster since they make the mechanical breakdown easier.
• They produce waste known as castings which improve compost’s nutrient content.
• Their burrowing helps aerate the compost and allows oxygen to reach the organic matter, which is essential for decomposition.

Adding grubs helps break down manure, and reduces manure’s odor and the chance of disease in areas with animal manure. For instance, composting cow manure is a much faster process with beetles and grubs.

Identifying the white grubs in compost?

Grubs are the larval stage of various beetles, including Japanese beetles, June beetles, and chafers. Most grubs are C-shaped, soft-bodied, and white or cream-colored.

Bumble flower beetle larvae

Bumble flower beetle (Euphoria inda) larvae are white grubs with a dark brown head capsule and dark gray terminal body segment. They have six legs and are easily mistaken for other scarab beetle larvae, or white grubs.

The larvae of bumble flower beetle resemble June beetle grubs, and both feed on organic matter and compost to help with the decomposition process.

According to  Ryan S. Davis, a Utah University Cooperative Extension Arthropod Diagnostician, the bumble flower beetle is ”Widely distributed in the United States from Connecticut to Florida and westward to Oregon and Arizona.” ^[2]

June beetle larvae 

June beetle (Cotinis nitida) larvae are white grubs with brown heads and six prominent legs. They’re 1/4″ to over 1 inch long, and their bodies are curved into a C-shape. The hind portion of their abdomen is slightly enlarged and darker as it is filled with fecal matter and soil from their feeding activity.

June beetle larvae are distinguished from other similar-looking larvae by two parallel rows of spines on the underside of their last abdominal segment.

Japanese beetle larvae

The larvae of Japanese beetles (Popillia japonica) are white grubs with a grayish abdomen due to the accumulation of fecal matter and soil in their lower gut as they feed on decomposing matter. They’re typically C-shaped with a dark brown head with 3 pairs of legs.

Green fig beetle (Cotinis mutabilis)

Also known as the figeater beetle or the green fruit beetle, the green fig beetle larvae are the larval stage of the iridescent green fig beetle insect. They are white, thick-bodied grubs about 2 inches long with dark heads. They have 6 legs, but they prefer to move by rolling their bodies using their stiff hairs.

The green fig beetle grub can inhabit compost and piles of manure as it prefers to feed on decaying organic matter.

Pepper weevil grubs

They are present in the southern parts of the United States. They have white or light-grey bodies and brownish-orange heads. They are small, measuring ⅕ inches when fully mature. Unlike white grubs, they are not C-shaped.

Vine weevil grubs

They grow from tinny eggs and hatch into 10mm long C-shaped creatures with cream-colored bodies and brown heads. They are pretty resistant to sprays.

Manual removal of the grubs from compost bins is recommended if they present a problem. You can also drench the compost with a systemic insecticide if their population is too high.

Author note: It is also possible to find black soldier fly larvae in compost. The larvae are attracted to organic waste, such as manure and food scraps. They will quickly colonize and decompose this waste, making them a valuable tool for composting and waste management.

How to control grub populations in compost

An exploding grub population can be detrimental to your compost and plants. These larvae are innocent and harmless in this stage, but will soon become a pesky pest in your garden if their population is not controlled.

Natural grub control measures include nematodes, neem oil, and attracting birds and other predators to feed on them.

Introduce nematodes

Nematodes that kill insects like white grubs are called entomopathogenic nematodes. These are microscopic worms used as a natural control for grubs in lawns. We use them in compost piles that are overpopulated with grubs, especially when the compost is almost finished and is ready to be used in gardens.

 Nematodes are most effective against grubs because once applied to the soil, they seek out and infect grubs. Once inside the grub, the nematode releases a bacteria that kills the grub within a few days. The nematodes then reproduce inside the dead grub, and the cycle continues.

I always use Milky Spore Nematodes in my applications to control Japanese beetle populations in lawns, landscapes, and compost piles.

Attract birds to prey on them

Some birds are natural grubworm predators, so you’ll find them digging on landscapes and lawns to find and eat grubs and other insects.

I encourage attracting birds by exposing a few worms. Their wiggly activity will catch the attention of birds. I simply release my chicken onto the piles to feed on the grubworms.

Small compost piles with grub infestations can benefit from this method. Large piles of compost with grubs deep beneath may require a more robust pest control option to get rid of the grubs. Additionally, birds cannot access sealed composters, so you may have to resort to a different grub control measure.

Spray neem oil over your compost

Grubs hardly dig very deep. They’re typically shallow dwellers, living about 2 inches deep when active. Therefore, spraying neem oil, an effective insecticidal oil, will kill them and reduce the populations of European chafers and other types of grubs.

Mix neem oil with water and spray the affected compost pile once or twice monthly to kill the grubs.

Add brown matter to the compost pile

You can get rid of grubs by adding brown matter to your compost. Grubs prefer eating moist green material like newly added grass clippings. Brown matter like hay and dried grass have little to no moisture content; adding them to your compost will dry it out and discourage grubs.

Seal compost bins

Sealing compost bins can deter grubs. Grubs are the larvae of beetles, and they need moist, cool soil to survive. By sealing your compost bin, you can create an environment that is too hot for grubs to thrive.

Chemical control

Carbaryl and trichlorfon are the two chemicals that are considered curative. Caution must be taken with such insecticides as they can also kill other organisms that help the composting process.