Fungus Gnats! What do we know about these flies?

Jane Smith

Fungus-gnats-1 Well, they are also known as sciarid flies; they are small, dark, two-winged (Dipteran) insects; they love compost, dung, rotting vegetation – in fact anything where there’s a rich source of nitrogen for them to feed on. There are thousands of species of sciarids throughout the world – for example there are over 150 species in Britain, 172 species in East Germany (Menzel et al, 1990) and in the South Americas, 193 species (Dalton de Souza Amorin, 1992). In nature, they play an important part in the breakdown of dead and decaying matter alongside the various fungi that are also involved with this process. In a natural habitat, this attribute makes them a useful part of the decomposition cycle but where protected crops are grown, the species that have adapted to these conditions have become a real pest. In fact, conditions for growing glasshouse crops and mushrooms are close to ideal for the fungus gnats’ development – a source of nutrition, high humidity, warmth, protection – what more could a little fly ask for?

So, why are they such a pest?

A gravid female sciarid – although only a few millimetres long – can lay up to 200 eggs. She chooses a suitable site for them such as moist, warm compost and after two or three days the eggs develop into larvae. These are easily recognised by their black heads on a thin white body. At first, they are tiny – only one or two millimetres long – but they pass through four moults over a period of two to three weeks (depending on the temperature) growing fatter and longer with each moult. As larvae, they feed voraciously on protein within the compost and they browse on fungal mycelium although some mycelial exudates inhibit their development. After the fourth moult they pupate and a few days later emerge as adult flies ready to go forth and repeat the whole cycle. The increase in fly numbers can be astronomical and in a protected crop environment there are few natural predators to keep the numbers in check. The larvae produce a waste matter called frass which renders the compost unsuitable for mushroom mycelial growth, they reduce yield by damaging the mycelium and they can burrow into the stipes of mature mushrooms rendering them unsaleable. The flies spread disease from crop to crop, they are a nuisance to pickers and can cause crop rejection at market.

And why is control so difficult?

Their control is a problem partly because there is a limited number of insecticides that can be used safely on mushrooms and other food crops; the flies seem to be able to build up resistance to chemicals over a period of time; and some insecticides seem to work on one mushroom farm but not in another. This phenomenon begs the question ‘why?’ There may be several answers:- the grower may have misused the chemical; the flies may have built up resistance; or it is possible that the sciarid is a different species – one that does not respond to a particular insecticide in the same way. To investigate this latter theory we need to be confident of the identity of the species we are breeding in the laboratory and we need to be able to identify correctly the different species that may occur on commercial farms.

How to do this?

Dr Menzel showing the male genitalia of a sciarid fly

Dr Menzel showing the male genitalia of a sciarid fly

To start, I began to study the taxonomy of sciarids. Whenever I had a few spare moments I would remove some specimens that I had earlier preserved in alcohol, dissect them, boil them, rinse them and finally mount them on microscope slides ready for examination. Eventually, it was decided that I needed some formal training to verify my identifications and thus enable us to take this work forward.

The Deutsches Entomologisches Institut, Eberswalde

The Deutsches Entomologisches Institut, Eberswalde

Only a very few scientists in the world have studied in depth the taxonomy of the sciarid fly. In the past, when we have wanted to get flies identified, we have sent them to Dr Frank Menzel, a sciarid fly taxonomist working at the Deutsches Entomologisches Institut in Eberswalde, Germany. He agreed to give me a short course that would give me the necessary expertise to identify the most common sciarids that infest mushrooms and glasshouse crops. Having completed a crash course in German, I went to Eberswalde in October this year, taking with me specimens that had been sent to me from the USA and from Brazil – both for verification of the identification I had been doing and to provide samples on which to practise.

In the laboratory, DEI

In the laboratory, DEI

The institute was housed in a beautiful, old, red-brick building separated from a lovely tree-lined park by a quaint cobbled street. It has had a chequered career and has suffered several moves since 1886 when it was started as a private foundation in Berlin by a beetle expert. It has become world famous as an international information centre for entomology and has an amazing collection of preserved insects. Climbing to the second floor, I was introduced to the other scientists (experts on beetles, grasshoppers, fleas and other insects) before settling down to work.

The male sciarid is most important for purposes of identification as the female has only a few characteristics that tell her apart from another species. The wing shape and the arrangement of the veins, arrangements of bristles on the front legs, the colour of the legs and ‘false wings’, the length to width ratio of the antennal segments, the arrangement of the claws and bristles at the tip of the male styles – used to grasp the female during copulation – are all characteristics used to tell one species from another. After two very intensive weeks under the excellent tutelage of Dr Menzel, I returned to England inspired to put into practice my new knowledge.

In England, our major mushroom-infesting sciarid used to be Lycoriella castanescens (formerly L. auripila) and in the United States it was Lycoriella ingenua (formerly L. mali). However, in a survey of British mushroom farms carried out in 1996, we found both of the Lycoriella species (identified by Frank Menzel), although each farm appeared to have either one or the other species. On one farm there was also aBradysia species – one that is normally associated with glasshouse crops – and, since then, we have found yet another Bradysia species infesting mushrooms in the UK. The flies sent from the United States we identified as L. ingenua but the flies from Brazil were a species not previously recorded in that country.

It would now be interesting, and potentially very useful for us all, to build up a picture of the sciarid species with which we are all plagued. In order to do this I would like to receive preserved sciarid fly specimens from growers across the world. If growers are interested in sending flies to us at HRI, we need about 50 fresh specimens – it is no use collecting the dead flies from traps etc(!) – which have been put into a small vial or tube containing 70% alcohol. An identification label written in pencil (alcohol removes ink very effectively), placed in the container, completes the job.

As I have explained, we need mainly the male flies which are smaller than the females and have noticeably longer antennae. Along with the sample we need as much information about the growing conditions as possible – where they came from, the compost type or growing medium, the area of the country, any climatic conditions that might be relevant, and such like. The more information we have, the better we can build up the world-wide picture. So don’t be shy, mail me those pesky sciarids!

References

Menzel, F.; Mohrig, W. & Groth, I. (1990): Beiträge zur Insektenfauna der DDR: Diptera – Sciaridae. – Beitr. Ent. – Berlin, 40(2): 301-400.

Amorim, D.S. (1992): A Catalogue of the family Sciaridae (Diptera) of the Americas South of the United States. – Rev. Bras. Ent. – São Paulo, 36(1): 55-77.