Sunday, March 17, 2013

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


  1. Thanks for this. It looks like one solution to focus stacking insects.

    1. Good idea Adrian, there's no way I could have photographed them if they hadn't been glued down!

  2. I've started sterilizing my commercial soil in the oven. That stops the infestation before it starts.


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