Resource icon

Floating plant pests - a couple of uncommon insect nuisances

  • Author Fishfur
  • Creation date
Sometimes, floating plants bring things with them that you weren’t expecting and they can do a lot of damage by the time you figure out they are there.

I’ve had personal experience with two such insect pests - Duckweed weevils and China Mark moths. Believe me when I say you do not want either of these critters to find their way into any floating plants on your tanks.

Both lay eggs inside or on leaves or thalli in ways that leave nothing much to see in newly purchased or collected plants.


Duckweed weevils are very tiny.

The adults are no more than 1.5mm long and generally green in colour. They move very slowly most of the time and thus don’t catch the eye with movement. You never see their larvae because they’re inside a host leaf tunnelling and eating. The juveniles are a lighter beige-ish colour and of course they’re smaller than the adult form.

They are the species Tanysphyrus lemnae Paykull. The females lay their eggs by making a hole inthe flesh of the thallus, frond or leaf and then plugging the hole. It’s effectively invisible once done.

When the eggs hatch their larvae tunnel and feed on the inner tissues which eventually kills the thallus or leaf or frond.

The adults and juvies rest on the surfaces of leaves and if they fall into the water they simple swim to a leaf and crawl out. Bubbles of air cling to their bodies so they don’t drown. They can last a surprisingly long time when they end up swimming if a fish doesn’t grab them. Their swimming makes a fair bit of disturbance and thus attracts the attention of surface feeding fish.

They do not feed exclusively on duckweeds - they seem to do as well with any floater that has fairly fleshy leaves, fronds or thalli, so they’re quite happy to eat salvinia & frogbit and probably others, though those are the plants I found them on. I am unsure if they’ll bother Azolla but I would not count on them not infesting it.

I didn’t grow Asian watergrass (Hygrorhiza aristata) at the time, but it’s possible they might get into the buoyant petioles of the leaves, as they are quite fleshy even though the leaves are not. I don’t think they’d bother floating lily leaves but they might, along with water lettuce and perhaps even water hyacinth.


The image shows adults and juveniles on giant duckweed thalli and there’s another macro shot of an adult as well.

They don’t fly but I found that they easily spread from one tank to the next even without any plants being transferred. Initially, I thought they might be good to culture as live food, as my fish eagerly ate any that I knocked off the leaves into the water but they proved to be too difficult to control. I think it can be done but you’d need a separate room and a few simple biosecurity measures to keep them from spreading outside the room.

I was forced to trash all my floaters to get rid of them in the end. Very disappointing. But there is no control for them you can use safely around fish and you can’t tell if eggs are inside a leaf or larvae either.

If you collect wild duckweed or buy it from a nursery where it’s growing outside, inspect the source carefully to see if there are any weevils on it. The risk of there being weevils is quite high if the source plants are growing outside and there is always some risk to buying duckweed and even salvinia as they could harbour eggs which you won’t be able to see. Fortunately, though these beetles are abundant in the wild, they’re not all that common to find in store bought plants, but the risk is certainly not zero.


The second insect pest I wrangled with was the innocent looking China Mark moth, though I never did identify which species it was that I had. I doubt you’d have trouble with them unless you do what I did to get them or have water lilies growing outside and some moths find their way to floating plants indoors.

There are several species including the Small CMM (Cataclysta lemnata) found in Europe, Brown CMM, (Elophila nymphaeata), also European, the Beautiful CMM (Nymphula nitidulata), found all across Eurasia and Nymphuliella daeckealis, a North American species which has no common name.

I originally thought that the very small moths I was seeing fluttering around as the sun went down were pantry moths, as the size matched. I never did manage to get a good image of the adult moths, unfortunately but if you see a small moth like a pantry moth fluttering at the surface of your tank as the sun is sinking, be very suspicious that it’s not a pantry moth!

The adults don’t do any harm but their larvae are highly destructive leaf cutters and they are a big problem for those who cultivate water lilies, which are their preferred food. But they’ll cut any floating leaf, such as frogbit and they feed on them as well.

They’re clever little larvae - they make boats out of cut leaf pieces. The cut pierce are a half moon shape more or less and they take them out of the leaf edges. Then they use silk to glue them together with the larvae safely dry inside and float around looking like perfectly harmless bits of leaf. There is an opening at one end of the ’boat’ they use so they nibble any leaf they bump into. They can do unbelievable damage to floating plants if their numbers are high and even if numbers are very small, in a fish tank they can wreak havoc.

Female moths lay eggs underneath the edges of leaves where they’re not visible so it’s very difficult to tell if there are eggs present on any given leaf.

The larvae are surprisingly large by the time they’re ready to pupate but before that they’ll have made five different boats from cut leaves, one for each instar or moult of their larval life cycle. Then they take the fifth boat about ten or so inches down the stem of a water plant and bite into it deeply to anchor the boat and pupate there.

The new adult moth will climb up to the surface to dry off and then go off to find a mate. The only way I found to control these pests was, again, to sacrifice all my floating plants.

I had grown frogbit outside that summer, on my balcony. Even though I was on the tenth floor and there were no water lilies to attract them, they found my frogbit and came inside with them in the

That’s the pupa on what I think was some watersprite. You can see that the outer walls of the pupa are sections cut from a leaf. Other pics show one of the floating ‘boats’ they construct and a pupa showing the hooked mouthparts that anchored it to a stem below water.
  • 1692554037900.jpeg
    244.6 KB · Views: 11
  • 1692554323027.jpeg
    304.4 KB · Views: 16
  • 1692554356032.jpeg
    257.3 KB · Views: 15
  • 1692554394614.jpeg
    276.7 KB · Views: 15


First release
Last update
0.00 star(s) 0 ratings

More resources from Fishfur

Top Bottom