Grow Your Own Living Sticky Trap!

Yellow sticky traps have been around for over a century (flypaper was patented in 1910) and gardeners know them well. You put them out near plants susceptible to attack by flying insects and bif, boom, bang!, the annoying little creatures stick to the trap rather than eating your plants.

However, Mother Nature has been making sticky traps for millions of years in the form of plants with sticky foliage. There are many plants that capture insects that way. The more efficient ones are said to be carnivorous, or more correctly, insectivorous plants. Wouldn’t it be wonderful if you could just set out pots of sticky trap plants and watch insect pests fall prey to them? Well, you can. But not just any insectivorous plant.

I’d suggest using butterworts.

What the Heck is a Butterwort?

Butterworts are plants in the genus Pinguicula. There are about 80 species found mostly in the Northern Hemisphere, but also South America, where they grow from the Arctic to the tropics, and from near sea level to above the tree line. Unless you live in Africa or Oceania, there are probably wild butterworts growing not far from where you live.

The name Pinguicula means “small greasy one,” as they’re quite small plants (most measure less than 8 inches/20 cm in diameter) with apparently smooth pale green leaves that seem to be covered in a thin, shiny layer of fat. They’re called butterworts for the same reason. If you look closely, though, you’ll see the leaves are neither smooth (in fact, they’re covered with tiny hairs) nor greasy: it’s not a layer of “fat” that covers the leaves, but rather many fine droplets of clear mucilage.

For indoor use, you’ll want one of the tropical Mexican species (P. moranenis, P. esseriana, P. gigantea and various hybrids), as they are adapted to conditions very similar to those found in the average home. Most will produce dense rosettes of sessile (lacking a petiole), spoon-shaped leaves, looking rather like small, smooth-leaved African violets (Streptocarpus sect. Saintpaulia). Note the leaves often roll up at the edges to keep digested insect liquids from dripping off.

Even the flowers, usually purple or pink, more rarely red , yellow or white, look rather like the blooms of species African violets, with five petals (two smaller upper ones, three larger lower ones), but borne on taller, thinner stalks. Many of the hybrids, especially, bloom sporadically much of the year … again, much like African violets.

However, in spite of these similarities, butterworts are not related to African violets: the latter belong to the Gesneriaceae family, while butterworts are in the Lentibulariaceae family (the bladderwort family).

How Butterworts Trap Insects

I’ve grown many insectivorous plants indoors over the years—Venus flytraps, nepenthes, tropical sundews, etc.—and if they ever caught any insects, it was only because I supplied them. Not so with butterworts.

If there are any small flying insects in your home, your butterwort’s leaves will soon be dotted with them. Fungus gnats seem to be fatally attracted to them. I haven’t had a true fungus gnat infestation since I began growing butterworts: any fungus gnats that manage to work their way into my plant collection end up stuck on their leaves. And I’m not the only one who’s had success with butterworts as living sticky traps. Some commercial orchid nurseries also use them to control fungus gnats.

Why are the leaves so attractive to small insects? The principal theory is that the shimmering appearance of the leaf seems to suggest water. It always strikes me that the color of butterwort leaves is such a pale, bright lime green, almost yellow, a color that seems to attract many insects (if most commercial sticky traps are yellow, it’s not for nothing!), so that might also be a factor. And it’s possible the leaves give off some sort of chemical attractant.

Whatever the reason, when the insect lands, it becomes stuck on the mucilage. The more it moves, the more is produced, soon covering and smothering the insect. Then the leaf produces digestive enzymes that dissolve the insect into fluids the leaf can absorb. Note that the leaves often roll upwards along the edges: this is to keep the “bug juice” from washing off in the rain. Butterworts obtain all their minerals from trapped insects: their roots absorb only water.

Fun Fact

Tätmjölk is the Swedish name used for the yoghurt like product made by curdling milk with butterwort leaves.. Source: Kristofer2, Wikimedia Commons

Adding a butterwort leaf to milk will cause it to curdle, resulting in a fermented milk product notably consumed in Scandanavia.

Reasonably Easy to Grow

You can grow tropical butterworts under average household conditions. I grow mine alongside my African violets, as they have similar needs. In other words, give them good light with some morning sun, such as an east window (or, as I do, a spot under grow lights), reasonable air humidity, normal indoor temperatures, etc. They prefer water with a low mineral content, such as rainwater or distilled water. In the winter, when rainwater is scarce, just use water from your dehumidifier.

Keep the potting mix evenly moist throughout the growing season, never letting it dry out, as butterworts have a very limited root system and simply don’t tolerate dry soil. Some people wick water them or leave the pots soaking in a thin layer of water during the growing season, but you can also water from above as you would any houseplant as long as you keep the soil moist.

Never apply fertilizer: like other insectivorous plants, butterworts evolved to grow in soils or other substrates (often moss) that contain no minerals. Thus, fertilizer can kill them. They’ll get their “food” from the insects they catch.

Nor do they seem to tolerate pesticides of any kind, either fungicides or insecticides. If you need to spray your other plants, move your butterworts somewhere else first.

With most butterworts, there is a “dormant” period. The tropical types rarely go fully dormant (the temperate ones do, though), but instead start to produce smaller, thicker “winter leaves” that are not insectivorous. During this period, which usually occurs from September/October to March/April, you can grow them cooler, although this is not strictly necessary. However, it is very important that you allow their growing mix approach dryness before watering again. When new, longer summer leaves start to appear, start watering more abundantly again.

That said, I find some of the Mexican butterworts, especially the hybrids, continue to grow throughout the year, especially when kept under lights. If so, just follow the plants’ lead and keep watering them well.

You can try multiplying butterworts by leaf cuttings (tricky) or seed (a bit easier) and commercially they’re produced by tissue culture, but most species will produce offsets you can remove and pot up, something most easily done during their dormant period.

It’s best not to repot butterworts into regular commercial potting mix, because it contains fertilizer. Instead, prepare your own blend, say half peat moss, half perlite or vermiculite, or sphagnum moss with or without a share of perlite or vermiculite. If the mix contains a pinch of lime, that’s not a bad thing either, because unlike other insectivorous plants, mostly from acid growing conditions, butterworts prefer slightly alkaline soil. However, adding lime to the mix  doesn’t seem to be essential.

In spite of the above, some people do use ordinary peat-based potting mix and claim to get wonderful results.

Limited Local Choice

Butterworts often show up in garden centers among shipments of other insectivorous plants (one company calls them “starfish plants” for reasons unknown) in little plastic terrariums. These are usually hybrids of unverifiable origin and are the cheapest way of getting started with butterworts. Do not leave the plants in these transport terrariums: they’re for shipping purposes only and cut off air circulation, harming the plant if it’s left in one for too long a time.

If you want a wider choice, a better-quality plant or proper identification, you can order butterworts from a specialist insectivorous plant nursery.

Here are a few nurseries that sell them by mail order:

Carnivorous Plant Store (Canada)
Carnivorous Plant Nursery (USA)
Cascade Carnivores (USA)
Exotik.fr (France)
Triffid Carnivorous Plants (Great Britain)

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Grow your own sticky traps? Why not!