The science bit: sciarid fly larvae feed on algae and fungi in damp compost, meaning they need moist conditions to survive.
Prevention: Simply reduce your watering regime to reduce the chances of flies taking up residence. Your plant will thank you for it too as they don’t tend to like having their roots in wet compost anyway.
Cure: If you have flies already then stop watering the plant altogether. Leave the compost to dry out completely, which could take several weeks. Once dried out, leave it for another few days to make sure there is no chance of fly larvae survival. When you do they resume watering, make sure you don’t revert to over-watering.
The science bit: fungus gnats lay their eggs in the top few centimetres of compost.
Prevention: Cover the surface of the compost, known as top dressing, in a thick layer of gravel, grit or pebbles. Around 1cm of mulch will be sufficient, this layer will stop the flies from being able to lay eggs in the compost and, hey presto, no houseplant flies. At calathology, we top-dress interior plants as the pebbles make the plant look ‘finished’ and even more attractive as well as reducing the chance of a flies. Also, most commercially available composts are sterilised however if you don’t use compost immediately, then never store it exposed or outside as you may unintentionally bring fly larvae into your home when you are re-potting any houseplants.
Cure: If the flies have already taken up residence, then once you’ve followed step one and allowed the compost to completely dry out, add a gravel or grit mulch before resuming watering again.
The science bit: The brilliance of nature is there are other organisms that love to eat fungus gnats, a process known as biological control.
Cure: Assuming you have purchased your houseplant from a reputable source and have used sterilised compost and a top-dressing then we don’t see the need to use organic control in a preventative way. However, it works brilliantly as a quick, safe and easy cure should you get a fly outbreak. Our recommendation is to apply nematodes; steinernema feltiae. These microscopic, soil dwelling parasites actively hunt, penetrate and destroy over 230 different pest insects, including common houseplant flies. Nematodes are easily available online and are completely safe to use in the home. They are normally added into water and then simply watered into the compost, though care should be taken to follow the specific instructions on the pack. They get to work quickly and will naturally die out themselves once they have eaten all their black fly food source.
An effective alternative to nematodes is to invest in a few carnivorous plants. Carnivorous plants are mesmerising to watch, provide the same air improving benefits of all plant species and love to eat houseflies. We recommend growing sundew (Drosera) or, the ever popular, venus fly trap.
The science bit: fungus gnats hover a few centimetres from the top of compost.
Cure: As they don’t travel far from the compost, the use of yellow sticky traps is an effective way of capturing the adult flies and breaking the insect’s lifecycle. The traps can be bought online and should be hung just above compost level on bamboo canes near affected plants.
If you want to extend your houseplant collection, contact the team at Calathology to start a monthly plant subscription. Each plant will arrive in a unique pot or basket, will be potted in sterilised compost specifically chosen for your plant needs and top dressed