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Identifying a Plant Hitchhiker

Learn about the aquatic moth and how to deal with it if you find a larva in your aquarium.

By Cassandra Radcliff |

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Aquatic moth caterpillar
This aquatic moth caterpillar has a camouflaged casing that protects it from predators. Photo credit: Cassandra Radcliff

Most aquatic plantkeepers know about the different types of snails and other common critters that can hitchhike in on live aquatic plants. With quarantine, these pests can be avoided in a display aquarium. And even if they were accidentally introduced into an aquarium, most plantkeepers know or can easily find out how to control the numbers of these creatures. But what would you do if you saw a hitchhiker in your quarantine or display aquarium and had no clue what it was?

Several days after adding a hornwort bunch to a small container to quarantine it for a short period, I noticed a strange, half-inch-long creature attached to a hornwort stem. At first I thought it was an odd type of snail coming out of a papery-looking shell. But that wasn’t right — it wasn’t a snail. Looking at it from a different angle, I could see through the casing. Inside was what looked like a caterpillar. It grabbed onto a hornwort stem with one end, and it extended out of its casing at the other end. It could retract into this case and move around on the hornwort stem.

I was stumped. Luckily I was able to discuss this creature with Aquarium Fish International plant expert Stephen G. Noble, and he gave me a great tip: “It might be an aquatic moth larva.” Because neither of us is an entomologist, we can’t be absolutely sure, but we think it is probably an aquatic moth caterpillar.

The aquatic moth is a short-lived moth (depending on the species, the aquatic moth’s adult life span ranges from a day to two months). The aquatic moth spends much of its life underwater. Adult females lay eggs underwater on a rock or plant. The eggs hatch, and the larvae live in the water, going through several larval stages before they each create a cocoon to pupate. During the aquatic moth’s larval (caterpillar) stage, it will breathe through gills (which are visible underneath a magnifying glass) and eat aquatic plants, algae or diatoms. Some aquatic moth caterpillars create protective casings (like the one I saw) out of silk thread that the caterpillar secretes, as well as bits of aquatic plants. These cases allow the caterpillars to blend in with their surroundings and avoid predators.

After completing its larval stages, the caterpillar creates its cocoon out of the same materials it uses to build its protective casing. In less than a month, it will break free of the cocoon as an adult. The adult climbs out of the water, finds a safe place to dry off and then repeats the cycle.

The best way to deal with aquatic moths in a quarantine or display aquarium is to remove them right away. They shouldn’t be dangerous to your fish, but they will eat plants — and you don’t want moths flying out of your aquarium! I removed my aquatic moth caterpillar with a pair of tongs and squashed it.

Whenever you don’t know what a hitchhiker is, it’s best to get it out of your tank as soon as possible. It could possibly multiply to plague proportions, harm your fish, eat your plants or do other damage. Mentors who know more than you do about aquariumkeeping can also help. If you don’t know anyone, you can always use the forums to talk to other fishkeepers. But the best thing to do is always quarantine new additions to your tanks! That way, no hitchhiker can do damage to your display aquarium.

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Reader Comments

Daniel    Aurora, CO

3/21/2012 12:58:08 PM

I wonder if that was what I had on my Vallisneria gigantea. I noticed some squiggly lines on a couple of the leaves. One of them had something at the end of the line.

Bill    Beckley,, WV

12/14/2011 10:57:57 AM

I tried an idea that I had read online that has worked very well for me against algae, snails, and any other creature that may hitchhike into my aquariums from the LFS.
Using a clean dedicated 5 gallon bucket with a resealable lid. Mix 3 TBSP of Alum per gallon into 3 gallons of dechlorinated water and stir to dissolve. Let stand overnight and stir again the next morning, now you are ready to use.
When you buy a new plant from your LFS, soak the plant in this solution for three hours, rinse in dechlorinated water for a few minutes and you're ready to place into youir aquarium.
If you already have a snail problem in your aquarium, remove all plants and ornaments from your aquarium and place them into this solution for three hours. NEVER pour the Alum into your aquarium.
While your plants and ornaments are soaking would be a good time to vacuum your aquarium gravel. Crush any snails you see on the glass and in the filter. Wipe the glass clean with a clean cloth rag.
Rinse the plants and ornaments for a few minutes before resetting them and you're good to go.
You'll see the blobs of snail eggs on the plant leaves as an opaque blob with tiny white dots embedded in them, not to worry, bacteria will take care of these in a few days.
Alum is a pickling spice and is available in the spice section of any grocery store and is reasonably priced. You'll need to buy four containers in order to have enough to be able to have three TBSPs per gallon for a total of nine TBSPs.
Reseal the bucket in order to have this solution available the next time you buy a new plant, and you and I know that we will probably see a plant at the LFS that we believe must have.

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