Ecogrow

Fungus Gnat Bradysia spp
Order Diptera.

Description | Damage | Lifecycle | Spread | Monitoring | Control | Hint

Fungus Gnat Flyer

Description

Adult Larvae
Adult fungus gnats have long antennae and a distinct Y-vein on the wings, they are 2-5 mm long and a greyish /black colour. They may be referred to as Black Fly or Sciarad Fly.

Females mate within a few hours after leaving the pupa and begin laying eggs after a few days. Eggs are 0.09 - 0.15mm and a female can lay 75 - 200 eggs in total.

Larvae (maggots) are translucent white to nearly milky white with shiny blackheads. Their body is almost transparent and internal organs are visible.

Pupae are 3 - 6 mm long and found in the growing media or soil. (Figure 2).

Damage

The Larvae

Damage to growing plants from the larvae often goes unnoticed. This is because because of the fact that:

1. They are relatively difficult to find in the media or within the stem tissue of a plant.
2. The reduction in growth caused to a newly planted crop by feeding on root hairs or to a cutting trying to grow callus is hard to quantify.

There is often cause and effect confusion as to why the plant or cutting has suffered. Damage often attributed to a fungal root disease is often the secondary infection and not the primary cause, the chewing larvae provide sites of entry for disease.
The larvae can also cause mechanical damage in the form of tunnelling that may collapse the stem, or root damage that reduces water and nutrient uptake.

The Adult Fly
Damage caused by the fly itself is often over looked because it acts as an effective vector in carrying pathogens from plant to plant on its feet. The role of the fly in transmitting diseases such as pythium is well documented

Life cycle

Spread

Although the pest can be introduced into the growing area by contamination of growing media or from plant material brought into the growing area It also occurs naturally in many areas and large populations can occur in surrounding drains or properties when environmental conditions are favourable In the protected growing environments of a nursery it is a persistent pest. At temperatures above 24 o C reproduction is continuous. Other factors such as low light levels, high humidity, continually moist media and media containing high levels of organic matter also favour the pest. They are usually found in the same warm, moist, greenhouse environments that favour fungi and algae growth.

Monitoring

Each grower needs to develop their own threshold levels for the flies and or the larvae this will vary with factors such as media composition and types of plants grown. Records compiled over several seasons are the best way to develop control thresholds relevant to the month and stage of growth.

The Adult Fly

Perhaps the best way of monitoring the adult population is using yellow sticky cards as an indication of the potential larval population. Unless this is monitored with vigilance it is easy to allow pest populations to develop very quickly and unnoticed. Allowing the pest population to exceed normal thresholds will obviously negate control and allow damage to quickly occur. Always keeping in mind that for every female adult there is potentially 75-200 eggs laid ready to hatch into larvae.

Additionally the adult population may not seem so great if you are inadvertently knocking some of them down every few weeks when applying routine chemical treatments.

The Larvae

Monitoring the larval population is very difficult since they are very difficult to see within the media and often hide within the plant tissue. Therefore at times it would require emasculation of the plants to locate them, which would be costly and impractical.

Additionally the distribution of the larvae occurs randomly so sampling would be ineffective.

There is a method of sampling the larvae for the purpose of testing for the presence of the larvae in a particular location. It involves using discs of potato placed on the media surface. Use a vegetable type corer to remove some cores from a raw potato. Slice the core into discs approximately 20mm in length and place raw side down onto the media surface. The larvae are attracted to the discs and may be found underneath or tunnelling into the discs. Discs need to be replaced every week since they dry up and can be useful in determining whether controls are killing the larvae.

Control

The first few weeks of a newly planted crop are the most important for fungal gnat control. If your experience has shown that fungal gnat will be a problem with a particular crop then use a preventative approach and inoculate shortly after potting.

Cultural
Avoid storing growing media where it can get wet and attract adult flies.
Avoid excessive irrigation to newly potted plants.
Increase light levels and ventilation wherever possible to reduce favourable breeding conditions for the fly.
Good sanitation practices, remove discarded plant material and growing media.
Control algae.
Remove badly infested containers of plants .

Biological
Entomopathogenic Nematodes. ( EN's )

GNATNEM ®

Entomopathogenic nematodes (ENs) are microscopic, colourless roundworms that kill soil-dwelling fly larvae by entering them and releasing a bacterium that multiplies within the fly larvae. ENs are not plant pests.
The bacterium, harmless to humans and other animals, serves as food for the nematode. The nematodes home in on the larva's carbon dioxide exhalations, travelling in the thin film of moisture that surrounds the particles of the medium.
After approximately 14 days the nematodes have reproduced inside the victim and the larval cadaver ruptures and releases up to 10x the number of ENs as had originally infected and they to go in search of victims.

Nematodes are amongst the best of biological controls:-

They have no odours
They do not injure plants
Do not harm greenhouse workers
Can be stored in refrigeration and used as needed
Can tolerate most commonly used pesticides used to control other pests and diseases.

The nematodes are applied as a drench or spray drench to the growing media after planting and to any areas in the greenhouse that may be a likely source of infestation.
Using nematodes, best control will be achieved with the first application made at planting or shortly thereafter, or if yellow sticky card counts are below 50/trap/week (guide only!).
Two or three subsequent applications at weekly intervals may keep fungus gnat numbers low throughout a 10-12 week crop. Proper application and use of nematodes will likely vary with crop and production system.

Hint

Large populations can occur in surrounding drains or properties when environmental conditions are favourable.
At temperatures above 24 o C reproduction is continuous.
High levels of organic matter also favour the pest.
They are usually found in the same warm, moist, greenhouse environments that favour fungi and algae growth.
Damage often attributed to a fungal root disease is often the secondary infection and not the primary cause. The chewing larvae provide sites of entry for disease.
The role of the fly in transmitting diseases such as pythium is well documented.
Using nematodes, best control will be achieved with the first application made at planting or shortly thereafter, or if yellow sticky card counts are below 50/trap/week (guide only!).
Adult fungus gnats have long antennae and a distinct Y-vein on the wings
Larvae (maggots) are translucent white to nearly milky white with shiny blackheads.