Fatal Attraction

Fungus gnats that emerge in swarms from soil in plant pots have become the bane of many gardeners' lives. If you grow plants in commercial potting composts on your house window ledge or in a greenhouse or conservatory, it's inevitable that you'll encounter these irritating pests because it seems that all currently available bags of potting compost are infested with them.

These little insects are scientifically known as Bradysia paupera and belong to a group known as sciarid flies. Each female can lay around 200 eggs which hatch into a worm-like, transparent larva that feeds on organic matter in the soil and also on young plant roots. A heavy infestation is capable of killing seedlings. They breed all-year-round, with overlapping generations that take less than a month to progress from egg to adult, so combating them is a constant challenge, but fortunately they have a fatal weakness - the colour yellow. They are attracted to these sticky yellow sheets of plastic that you can buy in garden centres and are glued to them as soon as their feet touch the surface.

Yellow strips of sticky plastic plastered with dead flies are unsightly but there is a more aesthetically attractive alternative - the carnivorous butterwort, Pinguicula sp., whose sticky leaves are like natural flypaper and which produces attractive flowers throughout the year. To see how effective this is, scroll down to the bottom of this post, and to see how it works, click here.

The little club-shaped structures on either side of the insect are halteres - balancing organs which smooth its flight path as its wings beat up and down.

Long-legged sciarid flies spend much of their time running around over the surface of the soil, where they lay their eggs.

Whiteflies caught on the sticky hairs of a butterwort leaf

Sciarid flies trapped on the sticky surface of a butterwort leaf

Botanical Flypaper

The tiny insect in the photo above, just a couple of millimetres long, is doomed. Its body and wings are held fast by the sticky leaf hairs of.....

.... this plant, a butterwort Pinguicula moranensis that originates from Guatemala and Mexico. Like all butterworts, it captures small insects on its leaf surface and then, when they die of exhaustion, slowly digests them.

Almost the whole of the plant surface is covered with these minute stalked hairs, of varying heights for maximum trapping efficiency,each tipped with a droplet of sticky mucilage.

Seen here at higher magnification and in side view, each bottle-shaped hair is composed of a single cell rising from one of the surface epidermal cells, topped with a glandular cap that at higher magnification still...

... is revealed to be made up of eight separate secretory cells, each shaped like a slice of cake, perched on the top of the stalk. Meanwhile, down below and embedded in the leaf surface.......

... there's a different kind of gland, seen here in surface view amongst the jigsaw puzzle-shaped epidermal cells of the leaf. Each leaf upper surface is studded with hundreds of these glands. Once and insect is trapped the glands nearby........

..... like this one, seen here in side view at higher magnification, secrete digestive enzymes. When the insect finally dies....

... it collapses into the pool of digestive enzymes and is slowly dissolved, until only its outer chitin exoskeleton remains, like a ghost of the plant's victim. Then the plant absorbs the resultant 'soup', rich in the essential nitrogen that's lacking in this carnivorous plant's boggy habitat. However, not all insects succumb so easily. The plants in my conservatory almost always host...

... small colonies of to these tiny aphids. Even though they are held fast, they can still use their piecing mouthparts to puncture the plant's cells and feed, and survive long enough to produce the next generation of young, which are born by virgin birth (parthenogenesis) without the need for mating.  If you double-click on this image for a larger view you'll see a pair of minute claws at the tip of each aphid leg. On most host plants these would allow the aphid to grip the plant surface and walk, but the epidermal cells of butterwort are so smooth and slippery that the claws cannot grip. If you watch under a microscope, you can see the claws simply sliding over the plant surface, so the anchored aphid can do nothing other than feed and breed before it eventually dies, leaving a ghostly shell and a clone of itself behind.

Butterworts' flypaper-like properties make them very useful plants to grow if you are troubled by the tiny mushroom flies that emerge from potting composts - a single plant will trap and kill scores of them.