When Insects See Yellow

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Yellow sticky traps catch harmful insects, yet pollinating insects are not drawn to them. Source: http://www.amazon.com

Many harmful insects are attracted to the color yellow. It’s the color of plants under stress and millions of years of evolution have taught them to head directly to yellowing plants whose defenses are probably weakened.

You can find yellow sticky traps in most garden centers. They are simply pieces of yellow plastic or cardboard covered with a sticky, non-drying glue. Hang or clip one on or near a plant that is susceptible to infestations from flying insects (whiteflies, fungus gnats, aphids, thrips, leafminers, leafhoppers, moths, etc.)… then keep your eyes peeled. At the beginning of an infestation, you will quickly notice the presence of a lot of winged prisoners on the trap, a sign it’s time to think seriously about a pesticide treatment (insecticidal soap, neem, etc.).

And if you’re very lucky, the trap can even nip the infestation in the bud: if the very first whitefly arriving in your garden or your houseplant lands straight away on the sticky trap, it may be game over for whiteflies (or aphids or leafhoppers, etc.) for the year!

Grow Your Own Living Sticky Trap!

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Pinguicula gigantea with the usual contingent of fungus gnats. Source: Noah Elhardt, Wikimedia Commons

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?

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One of the Mexican butterworts, Pinguicula moranensis, a parent of many of the hybrid varieties Source: worldofsucculents.com

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.

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The flowers (here, Pinguicula moranensis) look a lot like violet flowers. Source: Noah Elhardt, Wikimedia Commons

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.

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Butterwort leaves (here, Pinguicula colimensis) are covered with shimmering droplets of mucus. Source: Barry Rice, http://www.sarracenia.com

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

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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

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You can grow Mexican butterworts on a window ledge like you would almost any small houseplant. Source: ChrisLamb, www.flytrapcare.com

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.

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This Pinguicula esseriana is going ito dormancy. The new leaves in the center are winter leaves: smaller, thicker and are not covered in sticky dew. Source: http://www.thecarnivoregirl.com

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

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Butterworts only identified as “starfish plants” (note plant 3) show up in garden centers with other insectivorous plants. Source: www.boomanfloral.com

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)


Grow your own sticky traps? Why not!1200px-Pinguicula_ne1

Have A Ball Stopping the Apple Maggot

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The main apple insect pest in most home gardens is the apple maggot (Rhagoletis pomonella), a small fly whose larva burrows into the fruit and makes it inedible. But you can control this pest if you know it weakness. You see, the female fly loves red! Given the choice of tiny green apples and a nice big red apple, she will almost certainly choose the red one. And the home gardener can turn this affection to his advantage.

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Red ball trap covered with apple maggot flies.

Just take a red ball (or paint a ball red) and hang it from the tree. Do this after the petals have fallen, but before the unripe apples are visible, usually before the end of June. Now apply a non-drying glue (“Tanglefoot”, widely available in garden centers, is the usual choice) to the outside of the ball. The trap is now set.

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Commercial trap with attractant.

If you’re not interested in making your own red ball trap, you can also purchase commercial traps. They too are shaped like red balls and come with their own non-drying glue And they’re easy to find: most garden centers and even some hardware stores offer them. Some are even equipped with an attractant that makes them more effective: a fruit essence that attracts the apple maggot flies.

Or use a real apple! Simply coat a apple from the supermarket with Tanglefoot or a similar glue and hang it from the tree.

How the Trap Works

When the apple maggot fly sees the sticky red ball, it will tend to visit it rather than the real apples nearby which are still small and green… and it remains stuck on the trap. In years where the apple maggot population is low (it varies widely from season to season), a single trap will see that up to 98% of the fruit is free of maggots. When the population is high, however, you may need to multiply the traps, placing up to five red balls in each tree… and also, you’ll have to clean the balls when as they become covered in flies. Just apply another layer of glue and hang them back in the tree.

Double Punch

Several studies show that the trap red ball will be even more effective if you accompany it with yellow sticky traps, also known to attract insect pests. Place the yellow trap on the outer edge of the tree, in full sunlight, and the red ball trap in among the foliage, but still well exposed, and the poor pest won’t know what hit it!

Ladd Trap
There is also trap that combines both methods. The Ladd trap, named for its manufacturer (www.laddresearch.com), consists of a red ball surrounded by a yellow trap and includes an attractant as well. It is, apparently extremely effective: truly the Cadillac of apple maggot traps. However, it is also sold at Cadillac prices: $29.95 US per trap on the Gardens Alive website.

Hang a few red balls from your apple trees this summer. It can save you a lot of disappointment.