“Japanese worm” most commonly refers to one of three creatures: the white grubs of Japanese beetles that destroy lawns, the parasitic worms found in raw fish served as sushi, or the invasive hammerhead flatworms native to Asia. Each one causes different problems, and each requires a different response. Here’s what you need to know about all three.
Japanese Beetle Grubs in Your Lawn
Japanese beetle larvae, called white grubs, are among the most destructive lawn pests in the United States. They’re C-shaped, pale-bodied worms that feed on grass roots underground. Newly hatched grubs are about an eighth of an inch long, but by fall they reach roughly one inch. They look similar to other white grub species like May/June beetle larvae, so identification can be tricky without a close look.
Japanese beetles produce one generation per year. Adults emerge in late June or early July, feed on leaves and flowers for six to eight weeks, then mate. Females burrow two to four inches into moist soil, often under turf or mulched beds, to lay eggs. Those eggs hatch in about two weeks, and the tiny grubs immediately begin feeding on roots. By fall they’re nearly full-grown. When soil temperatures drop below 50°F, grubs move six to eight inches deep (sometimes over twelve inches) to overwinter. In spring, once the soil warms back to 50°F, they migrate upward to feed on roots again for several more weeks before pupating into adult beetles.
Signs of Grub Damage
The telltale sign is brown, dying patches of turf that pull up easily from the soil, almost like peeling back a loose carpet. That’s because the roots anchoring the grass have been chewed away. You’ll typically notice damage in late summer or early fall when grubs are largest and feeding most aggressively. Animals like skunks, raccoons, and birds digging up your lawn is another strong clue: they’re hunting the grubs below the surface.
How to Control Grubs
Timing matters more than product choice. The best window for preventive treatment is roughly mid-June to mid-July, before adult beetles finish laying eggs and new grubs hatch. Curative treatments work best in late summer when grubs are small and feeding near the surface.
For a biological approach, milky spore disease is a naturally occurring bacterium that targets Japanese beetle grubs specifically. Applied as a dust to the lawn, it must be ingested by grubs to work. The bacteria multiply inside the grub, turning its internal fluids opaque white before killing it. Spores then disperse into the surrounding soil, creating a self-sustaining colony. The catch: milky spore works best when soil temperatures are between 60°F and 70°F, and it can take three to five years in cooler climates to build up enough in the soil to provide reliable control. It’s a long game, but once established it can suppress grub populations for years.
Chemical insecticides offer faster results. Products containing trichlorfon, halofenozide, or imidacloprid are all highly effective at keeping grub numbers below damaging levels. These are widely available at garden centers and are applied directly to the lawn, then watered in so the active ingredient reaches the root zone where grubs feed.
Sushi Parasites (Anisakis Worms)
The “Japanese worm” people worry about when eating sushi is usually Anisakis, sometimes called herring worm. These parasitic roundworms live in marine fish and squid, and they can infect people who eat raw or undercooked seafood. The worms are up to two centimeters long, roughly the diameter of a nickel, and are whitish or translucent. Despite the common name “herring worm,” they show up in a wide variety of ocean fish species.
The numbers are rising. A University of Washington analysis found that the abundance of these parasites in fish has increased 283-fold over the past 40 years. The worms’ natural lifecycle involves marine mammals: they reproduce in whale and seal intestines, get released into the ocean through feces, then infect tiny crustaceans like krill and copepods. Small fish eat the crustaceans, larger fish eat the small fish, and the worms accumulate up the food chain. Humans are accidental hosts.
What Happens if You Eat One
If you swallow a live Anisakis larva, it can burrow into your stomach wall and cause sharp abdominal pain, nausea, and vomiting within hours. Occasionally the larva is coughed back up into the mouth. In less common cases, the worm reaches the small intestine, where it can trigger an inflammatory reaction that mimics appendicitis or Crohn’s disease, with symptoms developing one to two weeks after the meal.
The good news: these worms cannot reproduce inside a human body and survive only a few days in your intestine. Most cases resolve on their own. When symptoms are severe, an endoscopy to physically remove the larva is curative. The infection is uncomfortable but rarely dangerous.
How Restaurants Prevent It
Virtually all sushi-grade fish sold in the U.S. has been frozen before it reaches your plate. The FDA requires that fish intended for raw consumption be frozen at -4°F (-20°C) for at least seven days, or flash-frozen at -31°F (-35°C) until solid and then stored for at least 15 hours. Both protocols reliably kill parasitic worms. Cooking fish to an internal temperature of 145°F also destroys the larvae completely. The risk comes primarily from fresh, unfrozen, home-caught fish eaten raw, not from reputable sushi restaurants following food safety standards.
Hammerhead Flatworms From Asia
A third creature sometimes called a “Japanese worm” is the hammerhead flatworm, a land-dwelling predator native to Asia that has spread across much of the eastern United States and the Pacific Coast. These flatworms likely arrived in the soil of imported potted plants and continue to spread the same way through the horticulture trade.
They’re easy to recognize. The head is shaped like a hammerhead shark’s, the body is yellowish-brown with stripes, and large specimens can grow well over a foot long. They live in moist soil and organic matter, hunting earthworms and slugs. That predatory behavior is the real concern: earthworms are essential to soil health, and hammerhead flatworms can devastate local earthworm populations over time.
Are Hammerhead Worms Toxic?
Some species produce tetrodotoxin, the same neurotoxin found in pufferfish, in their mucus. They use it to subdue prey. The likelihood of harm to people or pets is low, but the toxin can irritate skin or mucous membranes on contact. The main risk is getting the mucus in your mouth or eyes, or a pet licking or eating one of the worms.
How to Safely Remove Them
The most important rule: do not cut them up. Hammerhead flatworms can regenerate from body fragments, so slicing one in half just creates two worms. Instead, wear gloves and place the worm in a sealed container or bag. Kill it by leaving the container in direct sun for several hours, adding hand sanitizer or soapy water, or placing it in a freezer. Then throw the container in the trash. Wash your hands thoroughly afterward. Don’t spread salt or pesticides on the ground as a preventive measure, as this harms vegetation and beneficial organisms like the very earthworms you’re trying to protect.