Coco Substrate and Sciaradae (Fungus Gnat, Shore Fly)
Because coco substrate is organic it slowly decomposes in its wet state. This causes the release of nitrates as part of the decomposition process. There’s not much wrong with this as long as the nitrate release is minimal (which it is). However, this nitrate release is what attracts Sciaradae (fungus gnat). Sciaradae are commonly found in organic composting material. Therefore, coco substrate can be a highly effective attractive media for Sciaradae.
Sciaradae, shore flies or fungus gnats are often present in conjunction with pythium. Fungus gnats feed on rotting vegetation and other decomposing organic material. It is not certain whether fungus gnats are drawn to crops that are suffering pythium because of the presence of decomposing organic material or whether fungus gnat is responsible for introducing the pythium. It is more likely that the fungus gnats are attracted to rotting vegetation that is inhabited by pythium fungi (oospores). The gnat larvae (1-2mm white maggots with black heads) can live on a diet of pythium oospores before some of them mature into the flying stage (adults) and carry fungi to other crops. This means the presence of the fungus gnat could be a precursor to a pythium outbreak in your crop.
Other than this, Sciaradae larvae are laid by the adults in the growing medium and their food source largely consists of the roots of the plants and decomposing material.
Adult Sciaradae resemble tiny fruit flies. When they are put under a magnifier their wings can be seen to have, what look like, accentuated veins. Sciaridae have a life cycle of egg, larvae, puparium, and adult. While the adult flies will only live a few days, one female fly can lay as many as 200 eggs. The lifecycle from egg to adult can be estimated at 3-4 weeks. For this reason infestations occur at a rapid rate.
Sciaridae eggs are laid around the soil/media surface. These hatch into glossy, legless larvae with black heads. The larvae are equipped with a sharp pair of mandibles, which are used for sawing and rasping into the soft stems and roots of the plants.
Controlling Sciaradae
Pesticides: I have found that after using numerous approaches and products (biological and pesticide) the Permethrin based products (Coopex, Axe etc) are the most effective treatment for totally eradicating Sciaradae from the crop. Permethrin is non systemic and and degradates (neutralises) quickly which makes it ideal as it is not up taken by the plant in any way.
The product I typically work with is Coopex WP 250g/kg powder which comes in 25gram sachets. Coopex is manufactured by Bayer and is widely available in most countries. Mix one sachet to 10 litres, hand water (drench) the media and leave for an hour. After this, flush with pH adjusted nutrient and you are ready to go.
Yellow sticky traps hung at media height will trap the adult Sciaradae. This will help reduce numbers. More importantly it will allow you to monitor whether Sciaridae are present in the growing environment.
Tip (About Sciaradae)
Look out for:
Signs of the pest through the use of yellow sticky traps
Deformed leaves and generally unwell plants can indicate an infestation of Sciaradae
(I use a product called Gnatrol that works well and is natural)