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Over time you'll start to notice an increasingly large amount of minute, black flies appear all over your home. They'll parade on the foliage, windows or light shades and will fly away when under threat. Although a large proportion of them are actually trying to escape, others will begin to deposit small eggs in the top layers of the soil, slowly developing into white larvae within a few days. These pesky critters are what will cause the damage to the plants - digesting the root systems, spreading diseases (like Pythium or Botrytis blight) and wreaking havoc around the home.
Although most active during the colder months, they can attack throughout the year. Fungus Gnats are in search of a dark, moist breeding ground to lay their eggs. There are three main ways for the Gnats to enter your home - most will enter the home through open windows or doors, but a few will be in the soil already. When purchasing a new plant from a store, always be sure to replace the top 3cm (2 inches) with a fresh batch of compost to eliminate introducing new critters. The final way is via a transplant when compost is used from a poorly-stored bag. When hoarding soil for later use, keep the lid tightly enclosed in a dark, dry location - a great example would be in a garage or shed.
Although the plants may be seen on the foliage, their larvae will only attack the roots and soil fungi. Although smaller infestations won't affect established plants, large numbers in one pot may cause a reduction of roots which in turn can cause yellowing leaves (rare).
Plants - Most plants will be affected, especially with those located in dark settings. A continuation of moist soil will also promote an attack, as they thrive best in dark, humid environments with poor soil aeration. They'll rarely attack plants that require dry soil, for example, cacti or succulents. In some cases, they'll also feed off the nectar of flowering plants, spreading potential diseases across your plant collection.
There are several methods of eradicating the Gnats and their larvae. The tips that ukhouseplants will provide have worked a treat over the last few years, so don't worry if there are other methods online.
As with Vine Weevils, there are two stages to eliminate Fungus Gnats. The first stage is to remove the larvae and THEN the airborne specimens. There's little point in only removing the adults, as the eggs or larvae will only pick up where the predecessors left off.
1. Prepare a room (ideally small) where there aren't any Gnats or plants present. Providing a 'neutral' zone where you can confidently say that there aren't bugs will aid pest regulation and the eradication process. Keep the doors shut and regularly check for any Gnats that may be hovering around the windows or lampshades.
2. Take all of your plants outside and replace the top 2cm (1 inch) of soil with the appropriate potting mix. Click on the link before continuing with this article if you're unsure as to which compost is right for your plant.
Repot all of your plants at the same time as this will reduce the chance of larvae hatching at different times. Removing the eggs and larvae before they mature will nip the infestation in the bud. Spray the new soil and foliage with garlic water; the herb's scent will deter females from the compost and therefore potential egg-laying. If you don't want to use garlic, you can sprinkle a fine layer of Diatomaceous Earth (also known as D.E. or DE) across its new soil to prevent new females from laying their eggs. The white powder-like pesticide is made up of grounded diatomic mantels (think loosely of powdered seashells) that'll act as a sharp, abrasive texture which aggravates and weakens the pregnant females before they lay. Their bodies begin to be scrap and punctured by the powder, causing dehydration and death to slow the generation-cycle of the Fungus Gnats. Sprinkle a fine layer onto the soil of every plant and leave the powder there for several weeks. DE isn't harmful to plants, pets or humans, so there are no health risks to consider. Remove the powder once you feel the infestation has elapsed.
4. Place all of the plants into the prepared room. Keep them here for at least a week to check if any more Gnats are present. The larvae will develop into adults within a week, so keeping the plants under a watchful eye will show up any live bugs.
5. Get a handheld hoover and suck each airborne pest that you crosses your path throughout the house, or crush the airborne critters by hand. Keep an eye out for the next seven to ten days, regularly checking the hard-hit areas of windows and lampshades.
6. After this period, check the soil and windows for any signs of larvae and adults in the room. Once there are no airborne pests in the rest of the house, you can safely re-introduce them back into their original locations.
7. Always keep an eye out for more Gnats and kill or hoover each one that you see. This can be a long process to eradicate, so doing small hunts each day will make a big difference long term. If another infestation arises, try replacing the top two inches of the soil for the second time and consider purchasing Diatomaceous Earth if you haven't already. Always remember that the Gnats are indeed harmless, so never get too worked up about your plants' overall health!
At the local plant shop or garden centre, regulate and inspect any plants that you wish to buy. Check around the hard-hit areas, for instance, the soil and lower stems, before considering the purchase. Most Gnats will come from already-affected plants, so always keep this in mind when increasing your plant-collection.
Keep the doors and windows shut during autumn and winter as this is the time for an infestation to strike.
Change the top layer of the soil when bringing any new plant from a shop. Replacing the top layer of the compost will remove any eggs & larvae that may have fallen in, or been deposited by a mature adult. We do this religiously, each time a new plant is purchased!
Regularly check for pests on your own plants. Although this may sound patronising, many gardeners forget to inspect their indoor specimens. As soon as you see a symptom, keep it quarantined and follow the steps above!
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