PPC120 | TECHNICAL

In this article, Killgerm’s Avril Turner explains the importance of species identification in fly monitoring and control programmes.

As per the shifting seasons, we always see an uplift in flying insect numbers following spring and into the summer due to better climatic conditions for all flies. During trend analysis, a seasonal trend is the only one we would reasonably expect to see. 

In any fly data appraisal, this is accepted and probably the only trend we wouldn’t do anything about. If the seasonal trend builds year on year as a total count, then we would need to find the reason why. 

The crux of any monitoring and control programme absolutely must begin with the correct species identification.Whatever small fly species that is peaking, being reported, noticed, creating a general nuisance or any other public health concern. 

We have seen a large number of small type flies causing more issues this year. The climate has been even more perfect for many of them. Of course, always check the wing venation, and if you don’t know, send a sample off for identification by an entomologist.

An online CPD quiz based on this feature is now available on the BPCA website. BPCA Registered members and affiliates can take a CPD quiz at any time bpca.org.uk/find-cpd or sign up at bpca.org.uk/affiliate 

Fruit flies

Those irksome, small (approx. 2.5-4.0mm), weak flyers, with the distinctive red eye, buff to yellow and black striped abdomen, and a sneaky habit of infesting whatever is fermenting. 

Confusingly Drosophila spp. most commonly, Drosophila melanogaster has many names from the common fruit fly, vinegar fly or beer fly usually associated with the particular environment where they are causing issues.

There are also other species of fruit flies. One of the more challenging is the dark-eyed fruit fly, Drosophila repleta. These individuals have a dark red eye (who could have guessed?) but overall very similar to D. melanogaster. 

Behaviours do differ, though. As opposed to hovering weakly around the breeding source, D. melanogaster will alight on higher surfaces as adults, whilst their eggs and larvae stay within the feeding medium. 

The challenge arises when the breeding source is overlooked, resulting in repeated fly treatments without solving the issue. The advice is simple: always check the drains, as there is likely one you’ve missed.

Fruit fly monitoring
Fruit flies are highly attracted to UV light, making fly units a good option; fruit flies can become bleached by UV and end up appearing a ghostly white shade so watch out for body composition and as always, characteristic wing veins for identification.

Yellow is also an attractive colour to fruit flies, therefore yellow boards in demi-diamond type traps can be used, utilising the small well at the base of the demi-diamond for added attraction with a fruit fly lure for example cider vinegar or the specialised AF fruit fly trap and lure.

Fruit fly control
Start by locating breeding sources or even potential sources. Any drink residues, build-up of damp or liquid debris, anything rotting or fermenting. Key sources are fruits or sweeter substances. Decaying fruit or vegetables, even dairy, can be subject to fruit fly infestations. 

Checking behind fitments, drip trays, bottle bins (a favourite place!). Cleaning of these areas can cut off the breeding source, leaving the adults, which can be treated with space treatments such as fog, mist or ultra low volume (ULV). Certain products can also be directly applied to an alighting surface, providing direct contact dosing.

Fruit flies as adults do not have a particularly long life-span. Under ideal conditions, an average of 40-50 days can be expected. They are among the fastest in the flying insect world, completing their full metamorphosis from egg to adult in around 7 days at 28°C.Consider this incredibly fast breeding and living cycle when inspecting and treating.

In severe cases, insecticides and strict hygiene must work together for lasting control.

Drain flies

This general description covers several types of small flies. 

Let’s take a look at some of the more common species that lurk below the surface. Which, by the way, are even worse flyers than the fruit flies. Phorid flies, whose other names include humpbacked flies, scuttle flies or coffin flies. These are all the same Phoridae family. 

Confusingly, these flies tend to be a similar size to the fruit fly, but overall much darker in appearance. Not confusingly, the various common names are typically based on their visual characteristics and behaviours. 

They are poor at flying; therefore run or ‘scuttle’ across the alighting surface. They also have a pronounced humped back (a convex mesonotum). 

One of the most common Phorid flies is Megaselia scalaris, and the coffin fly reference comes from their skill to locate a dead body (6ft under), lay their eggs, and continue to proliferate the species. Another feature is their legs, noticeably longer and tending to hang down.

Owl midge, Psychodidae family, Psychoda spp. Other common names include: filter fly, moth fly, drain fly. 

They appear to be ‘fluffy’ because they are coated in tiny setae and scales. They also tend to hold their wings out and above their body, similar to a pitched roof. 

They can range in size, usually they are tiny at 2 to 5mm but larger species can be found, all with a visually similar appearance. 

Potentially found in any damp environment, although usually even more drain-related than the other species we have looked at so far. The larvae must have a liquid or semi-liquid food source to swim around in to enable feeding. They have a siphon for oxygen exchange at the surface meniscus in the liquid. 

Very large numbers of this pest can occur, creating huge dense clouds, especially seen when the drainage system is opened. Owl midges are a typical secondary issue, due to an initial problem with drains, drainage, organic buildup, slime formation or biofilm accumulation. All due to a faulty system that’s not free running or free draining, hence another strong association with sewage.

Inspection for drain flies should include searches for anything that is decaying, particularly organic matter present in drains, sewage, rubbish disposals, and generally damp, rotten areas and items. 

Drains in my experience, are usually key to drain flies (who knew!), but seriously, we may want to carry out in-depth drain inspections using cameras to look for any decaying matter within the drain itself, any breaches in the system, any dead ends, any accumulations of organic decaying debris.

Drain fly monitoring
These small flies are very easy to see when they are scuttling or weakly trying to fly around; they just don’t move very quickly.

Standard monitoring with fly units is possible; they may be found in fly units, window traps and again, simple demi-diamond type yellow sticky pads may give good results. 

Drain fly control
Location and removal of the breeding source should always be the first action when treating drain type flies. 

Following source identification, we can usually carry out knockdown treatments in the area, surface treatments or space treatments such as aerosols, fogs, mists or ULV. This can limit adult numbers, but won’t solve the issue. 

There are also specialised drain treatments using enzymes or a microbial action (such as PX Viribus) to break down the decaying organic matter which work incredibly well. They do need to be used on a regular basis, just a one-off treatment will not cut it. 

Remember to always read the label, carry out a Control of Substances Hazardous to Health (COSHH) assessment, and follow a risk assessment, safe work procedure/method statement for any insecticides used. 

The enzyme-based treatments are a little different but should still be risk assessed. Other remedial works by a plumber or drainage specialist may be required to prevent the problem from coming back.

Fungus gnats

Fungus gnat, Sciaridae family, other common names include: sciarid flies. 

These tiny flies (2 to 8mm) are a nuisance to humans but are not known to be disease vectors or cause any harm to humans. Typically, an issue in environments with house plants, many more offices and other types of premises have more plants due to the aesthetic and environmentally enriching attributes the greenery brings. 

However, if overwatered, fungus starts to inhabit the layer just below the top soil. This is where the fungus gnat larvae thrive: feeding on the fungal growth due to the damp environment in the compost.

They can also thrive amongst mushrooms, decaying plant matter, and industrial plant growing environments such as herbs, and plants destined for garden centres and can seriously affect crops. They can damage the root structure, stunting the growth and affecting the overall health of the plant. 

Development is rapid, fuelled by the warmth and humidity in their preferred indoor environment. 

Fungus gnat monitoring 
Akin to the other species of small flies, fungus gnats are generally attracted to yellow, and yellow boards can be used. Many of the amateur use products are yellow sticky boards featuring different shapes, used in the pot of the plant you want to monitor.

On an industrial scale, large yellow fly boards can be used. Fungus gnats are attracted to UV light, so fly units may be utilised. 

Fungus gnat control
Let the top layer of soil dry between waterings to help control fungus gnats. Their presence often signals overwatering, which can lead to fungal growth and root decay.

Besides drying the soil, surface sprays can be used−avoiding leaves. In severe cases, apply synthetic pyrethroids or pyrethrins as a knockdown, following label directions. Few insecticides are approved for plants, most falling under horticultural rather than public health rules.

To sum up: first, identify the species−this guides inspection and reveals attractants and breeding sites. Once located, remove these sources by cleaning drains, fixing system breaches, or drying topsoil.

Alongside this, treat to reduce adult numbers, target alighting surfaces, and use space sprays if needed. Combine these steps with customer cooperation to prevent recurrence, following integrated pest management principles.