Slug bait pellets are small granules scattered on soil to kill slugs and snails before they damage plants. They come in two main types based on their active ingredient: metaldehyde and iron phosphate. Both work, but they differ significantly in how fast they kill, how safe they are around pets and wildlife, and where they’re legally available.
How Each Type Works
Metaldehyde pellets are the traditional option. When a slug eats metaldehyde, it destroys the slug’s ability to produce protective mucus. The slug dehydrates rapidly and dies, often within hours. Metaldehyde is fast and effective, but it’s also highly toxic to mammals and birds, which is why the UK banned it entirely in 2022 after government advisors concluded it posed unacceptable risks to wildlife. It remains available in the United States and some other countries.
Iron phosphate pellets work differently. After a slug eats the bait, it stops feeding almost immediately, but death takes much longer, sometimes up to seven days. The iron disrupts the slug’s digestive system, and while the slugs don’t die on contact the way they do with metaldehyde, they stop damaging your plants right away. Iron phosphate products are widely marketed as “pet safe,” though that label is somewhat misleading.
How to Apply Pellets Effectively
The key to slug bait working well is pellet density, not total volume. Research from Oregon State University found that the optimal distribution is about five to six pellets per square foot. You can hit this density with surprisingly little product: roughly 5 pounds per acre of a standard 2.5mm pellet, or 8 to 10 pounds per acre of mini-pellet formulations. Broadcasting (scattering evenly) rather than piling pellets in clumps gives the most uniform coverage.
Timing matters too. Slugs are most active in cool, damp conditions, so applying pellets in the evening or before rain is ideal. One thing to be aware of: earthworms also eat slug pellets. USDA research found that half of bait pellets disappeared in less than 2.5 days, with earthworms consuming them about 20% of the time they encountered one. The worms weren’t harmed, but the bait was wasted. This means you may need to reapply more often than the label suggests, especially in soils with healthy worm populations.
Risks to Dogs and Cats
Metaldehyde Poisoning
Metaldehyde is extremely dangerous to pets. Dogs are drawn to the pellets because many formulations contain grain-based attractants that smell like food. Once eaten, metaldehyde breaks down in the stomach into a toxic byproduct that is rapidly absorbed. It lowers levels of an inhibitory brain chemical, effectively removing the brain’s braking system. The result is uncontrolled nervous system excitation.
Symptoms in dogs appear within 30 minutes to 6 hours of ingestion. Early signs include muscle tremors, anxiety, uncoordinated movement, drooling, and a rapid heartbeat. Body temperature can spike above 106°F. As poisoning progresses, dogs develop continuous seizures and become unresponsive to their surroundings. Vomiting, diarrhea, and abdominal pain are common. In cats, involuntary eye movements tend to be more prominent, and external stimuli like noise or touch can trigger seizures. Without emergency veterinary treatment, metaldehyde poisoning is frequently fatal.
Iron Phosphate Risks
Iron phosphate is far less toxic to pets, but “pet safe” on the label doesn’t mean harmless. A study published in the Journal of the American Veterinary Medical Association reviewed cases of dogs exposed to iron phosphate baits and found that 71% of reported cases involved vomiting, 43% involved diarrhea, and 25% involved lethargy. Some dogs had blood in their vomit or stool. Less common signs included loss of appetite, dehydration, and uncoordinated movement. One dog had a seizure.
The danger comes from iron overload. When a dog eats a large amount of iron phosphate bait, the body’s normal iron-handling systems become saturated, and free iron enters the bloodstream where it damages cells directly. Doses of 20 to 60 mg of elemental iron per kilogram of body weight can cause mild to moderate toxicity, and doses above 100 mg/kg can be fatal without prompt treatment. Iron poisoning follows a deceptive pattern: initial vomiting and distress in the first six hours, then a period of apparent recovery between 6 and 24 hours, followed by a dangerous return of symptoms including internal bleeding, shock, and potential cardiovascular collapse between 12 and 96 hours after ingestion.
If your dog eats a significant quantity of either type of slug bait, treat it as an emergency. The misleading recovery period with iron phosphate is especially dangerous because pet owners may assume their dog is fine.
Wildlife and Environmental Concerns
The UK’s decision to ban metaldehyde was driven by its secondary toxicity to birds and mammals. When birds or hedgehogs eat poisoned slugs, or eat the pellets directly, they’re exposed to the same nervous system effects that make metaldehyde lethal to pets. Iron phosphate is generally considered safer for wildlife, though its impact on the broader ecosystem is less dramatic than sometimes claimed. Earthworms eat both types of bait readily but appear to suffer no ill effects, which is good news for soil health even if it means some of your bait gets diverted.
Choosing the Right Product
If you have pets, children, or wildlife visiting your garden, iron phosphate pellets are the clearly safer choice. They stop slug feeding immediately, they break down into iron and phosphate (both naturally occurring soil nutrients), and they carry far lower poisoning risk. The tradeoff is speed: you won’t see dead slugs littering the ground the next morning the way you might with metaldehyde. The slugs crawl away and die over several days, which can make it feel like the product isn’t working even when it is.
For those in the UK, the choice is already made. Metaldehyde has been illegal to sell or use since April 2022. In countries where both options remain available, metaldehyde works faster but demands careful storage and application away from anywhere pets or children can reach. Pellets should never be piled in heaps, which concentrates the dose and attracts curious animals. Scatter them thinly at that five to six pellets per square foot density, and consider applying them under slug traps or barriers where pets can’t access them.