Guide to Pond Leeches & The Best Leech Treatments 2020 (updated)
An animal with a name enough to make most peoples skin crawl, and one that can quickly become a concern in garden ponds – the leech!
Although unpleasant for humans, most leeches are actually harmless to ponds so long as their populations are controlled. Only a few species actually suck blood, and most fish, especially koi, love to eat them as a snack! Even so, certain species of leech are more dangerous to fish than others, and it’s not always easy to determine the best course of action if you find a leech swimming around your waters. In this article, we cover some common questions regarding pond leeches, as well as list the best leech removal methods and how you can help prevent future leeches from calling your pond their new home.
Pond Leech Facts
Where do Pond Leeches come from?
Finding leeches in your pond isn’t common, and you can go an entire lifetime without ever seeing when visiting your waters. Depending on your location, the climate, and where you purchase your pond supplies, you may never have to worry about these creepy crawlies.
Although leeches can be found naturally in various locations, the single most common way a leech will find its way into your pond is by hitching a ride on plants, fish, or rocks you introduce to your system. Leech eggs are tiny, and can go hidden within the substrate of pond plants or become attached to new fish you add over time. Pond plants are often the biggest culprit for bringing leeches to ponds, but luckily, these kind of leeches are not usually the bloodsucking variety and instead feed on the bottom sludge and organic matter on the pond floor. Even so, the added bio-load of an increasing leech population, whether they’re thirsty for blood or not, can cause all sorts of issues on the eco-system if left unchecked.
The best way to prevent leeches from arriving is to fully inspect new pond supplies you buy, and even quarantining plants and new fish before introducing them to your main system. Adult leeches are easy to spot, but eggs aren’t always that easy and their hatching time is different for each species. Quarantining new arrivals for 2-4 weeks is good practice, and as many common leech species have fairly short incubation periods, you may be able to catch the hatching offspring before they’re added to the main pond.
What do Pond Leeches Eat? Are all blood suckers?
Most of us will associate leeches with the bloodsucking variety also used historically in medicine for removing blood and treating a variety of illness. Although you can still get bloodsuckers in your pond, and some are even specifically evolved for fish, the most common type are plant leeches which feed primarily off bottom sludge and organic waste. The reason these are the most common is due to the fact that pond plants are easiest way for leeches to get into the pond in the first place; and where you have plants, you have plant leeches! These types of leeches will happily feed away at the bottom sludge and dying plant matter in the pond, often hiding away in substrate as they move about the pond liner.
Other types of leeches, which actually account for most leech species, are predatory in nature and will hunt other small invertebrates, such as snails and slugs. These can also find their way into your pond, although they’re less common in comparison to their vegetarian cousins. Predatory leeches can arrive the same way as plant leeches, and although they won’t harm your fish, they’ll certainly reduce your natural snail, slug, and worm populations over time.
The final type, and the one we all know best, are the bloodsucking variety of leeches. This sub-group of parasitic leech feed directly from the skin by attaching themselves to their host with small hooks around the mouth. They secret a substance to numb the pain receptors, and produce toxins to prevent blood from clotting. Although unpleasant, most blood-sucking leeches can’t survive in ponds for long, but there is a special species which can, and this is the one to watch out for if you have koi or goldfish!
Are all Pond Leeches harmful to fish?
Plant leeches and predatory leeches are not harmful to pond fish, and not all blood sucking species are actually capable of using fish as their hosts. With that said, there is one species of leech to watch out for if you have goldfish and koi, and that is the fish leech (piscicola geometra).
These long worm-like leeches are capable of attaching themselves to fish and sucking blood from their host, which can lead to dangerous infections and stress. The adult leeches are around 2.5 cm long, and often arrive to the pond attached to fish or inside pond plants. They’re difficult to catch as they can attach themselves to any part of the fish to feed, and often like to hide away within the gills and under the fins. The leeches, although unpleasant for the fish, are not deadly by themselves. The problem is the open wounds they leave which can lead to nasty infections, both viral and bacterial.
In terms of a leeches’ life cycle, this varies depending on the species, but the most common fish leech, piscicola geometra, has a 30 day life cycle. This relatively short cycle can be used to our advantage to help starve the leech and prevent the eggs from hatching and finding a meal – hence why a quarantine period is very effective.
How do I know if I have blood sucking leeches or not?
A simple test to see if you have hungry blood suckers or friendly vegetarians is to tie a small piece of raw meat, such as liver, to string and place this in your pond close to the surface. As the smell of the meat spreads, any blood sucking leeches you have will be attracted to it and start swimming over. If there are no worms after a few hours, it’s safe to assume you have plant leeches feeding on bottom muck. If you find the piece of meat has become an active feast after a few hours, then you have the blood sucking variety. An alternative to the string method is to make use of a professional pest trap, such as the one we reviewed below in our treatments section!
The Best Pond Treatment & Prevention Methods for Leeches
Step 1) Reduce Bottom Sludge/Muck
If you have plant leeches they’ll be feeding from your bottom sludge, as well as laying their eggs here and raising offspring. Even blood sucking leeches can be controlled to some degree by cleaning muck, as most all leech eggs will end up at the bottom in the sludge, even if they don’t feed from it.
If you have fish in your pond, such as koi, cleaning sludge has a range of benefits, such as helping to improve water quality, clarity, and control other parasites and pests. Sludge builds up as natural debris falls into the pond and sinks over time, which includes leaves, twigs, and even dead insects. As it sinks to the bottom is begins to decompose, it acts as a natural fertilizer to plants, but also raises ammonia and nitrite levels which beneficial bacteria need to break down.
All leeches love to hide in sludge and lay their eggs there as it’s hidden away, dark, and warmer than the upper levels of water. Whatever type of leech you have can probably be reduced by keeping the pond liner clean until the parasite has been culled or starved. The best way to clean a pond liner is with a combination of pond vacuum cleaner and natural sludge eater product. For smaller ponds you could simply use a wide-mouth net to scrap the bottom of sludge, but for larger ponds we recommend investing in a quality vacuum cleaner. A strong vac won’t just remove the sludge where leeches hide, but will also suck up any free-swimming worms trying to wriggle away. After cleaning sludge with a vacuum, you can supplement with a sludge eating bacteria product to help reduce any left over traces of sludge which may become food for hatching leeches.
Step 2) Add Leech Traps to the Pond
Although designed for Aquariums, quality underwater traps can also be used in ponds to catch a variety of pests, including leeches. Almost all traps work with a similar mechanism, providing bait which encourages pests to enter a chamber which they can’t escape. The problem with choosing a leech trap is many are designed for very small scale use, but the Pest Trap (pictured) from Choice Aquatics can also be used for ponds to catch leeches and other bloodsucking insects.
This trap measures 6.5″ in length and features two tunnels for catching both worms, snails, and leech. The tunnels are designed for easy entry and no escape once entered. Bait is placed in the main chamber and pests will enter through one of the two tunnels depending on the species. You’ll have to empty the trap once a day of leeches, and it should be placed on a shallow pond shelve for best results. If you also catch friendly snails and pond insects, you can simply throw these back into your pond after removing the leeches.
Although you can use any kind of meaty bait in the trap, we’ve always found a bit of chopped liver is the best leech attractant. You don’t need much, as even a small amount will attract leeches from all over the pond as the taste travels through the water.
If you have blood sucking pests, you should start catching them within 24 hours of placing the trap. If you haven’t caught anything after a few days you can try a different bait, but this could also mean you don’t have any more biting leeches around (pond vacuum may have been enough!).
Step 3) Remove Adult Leeches from Fish
After cleaning out your bottom muck, you’ll also want to check over fish for any adult leeches already attached to them. Although they could attach themselves anywhere, you’ll commonly see leeches around the gills and under the fins/stomach of fish as this is where they’ll get an easier meal. You can check over goldfish individually by using a basic pond net and carefully guiding them to the surface. Since koi are much larger, we recommend a specialised sock net designed for handling larger pond fish. You can use this net to gently guide your koi to the surface, and you simply close off the open end and lift the koi to inspect them in the net. If you see any pesky leeches, you can use regular tweezers to pull them off carefully.
After inspecting and removing any leeches, you should treat the pond with an antibacterial treatment which will help prevent any infections occurring. These kind of treatments will also promote faster wound healing, so they’re especially useful after removing leeches from the scales.
Fish can be added back to the pond alongside the treatment and should be carefully monitored for a few days to make sure they’re recovering well. You should also perform daily inspections during this period to make sure no further leeches are causing problems for your fish.
Step 4) Quarantine New Plants & Fish
If you want to prevent leeches in future the best way is to quarantine any new fish, plants, or ornaments you add to your pond system. A 2-4 week quarantine is optimal as this falls in line with most aquatic leeches egg incubation periods so you can catch young worms as they hatch. Failing this, a week of quarantine is certainly better than no quarantine, and is always good practice for safe guarding your pond. Not only will this help you identify leeches, it will also help prevent parasites, such as flukes, and stop sick fish spreading new illness to healthy fish in the pond.
Aquatic plants can be quarantined in a submerged container or aquarium tank within their purchased substrate. They should be fine in this environment for a few weeks so long as they’re getting some sunlight. Goldfish and other small fish can be added to a large aquarium tank, which also makes is easier to see any problems (or hitch-hikers) they may be carrying with them.
For ornamental koi it’s a bit more tricky as they’re such large fish, but if you’re serious about combating parasites and leeches you should consider investing in a dedicated quarantine tank for new arrivals. This could simply be a preformed raised pond, or if you’re quarantining several large koi, you can invest in a specialised tank often used by koi breeders, and also called a “show tank” (example pictured).
Although it may seem like a hassle, quarantining plants and fish is the single best way to prevent leeches and other parasites getting into a healthy pond. As the saying goes, “prevention is always better than cure” – it’s easier to stop leeches from getting in compared to removing them when they’ve made the pond their home.