Project Report: The use of Predatory Flies to biologically control pest flies at Glynllifon College Pig Unit


Glynllifon is a land-based campus situated on the Glynllifon Estate near Caernarfon. Recent investments at the farm include a state-of-the-art pig unit that consists of a herd of 50 hybrid Rattlerow WhiteLand sows. Although there are two boars on site, most gilts and sows are served using AI. The modern facility is insulated and has an automated ventilating system so that temperatures in each room are controlled to keep pigs warm in winter and cool during the summer months. The flooring is slatted, so that slurry is collected in tanks and used for first-cut silage to reduce fertiliser costs. The unit has been subdivided into 10 enclosed rooms including 2 farrowing rooms, 3 nursery/grower rooms, 4 fattening rooms and 1 dry sow area, each with a separate slurry tank. 

Flies have recently been an issue in the unit, especially in the finishing rooms. The presence of flies can not only cause a nuisance to both staff and the pigs, but flies can also carry diseases; they can spread over 100 different pathogens, such as salmonella spp and transmit more than 65 human and animal intestinal diseases, eye disease and can infect wounds and skin. Flies can also have a significant effect on productivity due to a reduction in feed intake, resulting in serious economic losses. 

The house fly is the most common species affecting the pigs at Glynllifon. They can complete their lifecycle in as little as seven days in the warmer summer months. Due to only 15% of the total fly population being adults at any one time, killing adult flies alone will not control the problem effectively. 

With pressure mounting on chemical use on farm and resistance being promoted due to the same adulticides continuously being used, this project looks at the use of predatory flies as a biological control to break the pest fly lifecycle and ultimately control fly numbers in the pig unit. 


Biological fly control on pig units

Biological fly control is an alternative solution to chemicals which involves the use of beneficial insects to provide an effective and environmentally friendly strategy against flies in and around livestock farms. There are a number of different insects available which target different stages of a fly’s lifecycle, that can be used as part of an integrated pest management system. The Biolfy (Hydrotea aenescens) is the most commonly used beneficial for slatted pig units. The Biofly is a predatory fly which targets pest fly larvae in wet muck areas, such as slurry pits below slats. The larvae of Biofly is a carnivorous predator which feeds on the larvae of various pest fly species. The Biofly does not cause a nuisance to farm staff or animals as the adult will fly just a few inches above the slurry (beneath the slats) and will begin laying eggs soon after being released. The eggs develop into larvae which will actively consume pest fly larvae in the slurry, breaking the life cycle of the fly. The fly larvae predator develops in the slurry without the need to predate on fly larvae, so can sustain its self even when fly larvae levels are low.

The main aim of the project was to try and reduce the number of house flies in the pig unit to improve productivity and animal health and welfare by introducing and evaluating the effectiveness of using fly larvae predators as a biological fly control. 


Materials and Methods

Before the first release of Biofly, some pre-trial data was collected. Four x 1ft strips of fly tape were hung up in all 4 finisher rooms and 3 nursery rooms. The tapes were placed in the same location in each room and the flies counted and recorded every Monday and Thursday, with fresh tapes hung up after each count. A total of 3 counts were completed before the first release of Biofly which occurred 17th August 2022. Temperatures of all rooms were recorded to ensure there were no significant variations which could affect the trial data.

Following the first release of Biofly, the same monitoring method with the fly tape was used until the final count which took place on 4th October 2022.

The Biofly release protocol can be seen below (Figure 1). One sachet of Biofly contains approximately 9000 pupae. A tube of Biomite was used in each treatment room alongside the Biofly for the first release due to the high fly challenge within the unit to assist with reducing the pest fly population. Biomite is an egg/ juvenile larvae predator, therefore targets a different stage of the fly life cycle compared with Biofly.

(Figure 1)



As can be seen from table 1, the average percentage change in fly numbers overall in all the finisher and nursery rooms (control and treatment rooms) reduced by 50%. The fly population in both the treatment and control nursery rooms reduced by 55% and 40% respectively. This shows that there has likely been contamination of Biofly in the control rooms, causing the overall reduction rather than only a visible reduction in the treatment rooms. There was an 81% reduction in flies recorded on tapes in the control finisher rooms and a 57% increase in fly numbers on tapes in the finisher treatment rooms. The finisher treatment rooms had a very low fly challenge reported initially, with an average of only 17 flies being recorded per treatment room, in comparison to 60 flies recorded in the control finisher rooms, therefore likely the reason behind the high percentage increase. The total cost for biological fly control per pig place when taking into account all nursery and finisher rooms at Glynllifon College, amounted to £0.95. In normal circumstances the first releases of beneficial insects would occur around May, before the flies became a problem. Due to the late start at Glynllifon, Biomite was introduced to knock down the initial high population of flies which were already there, therefore increasing the treatment cost. 

Throughout the trial it was reported that there were black marks on the fly tape which appeared to have once had a fly on, which had managed to escape therefore this needs to be taken into account as may have caused false readings. The age of the pigs in each room were also varied, as well as slurry levels in the pits, which will all have an effect on fly numbers, due to the diet the pigs will be receiving and ammonia levels in the room. Higher sugar diets will attract more flies as well as higher levels of ammonia. 


Average number of flies and percentage change in fly numbers on tape pre- and post- Biofly release


Pre- Biofly


Percentage change (%)

All Finisher and Nursery rooms combined




All Finisher rooms




All Nursery rooms




Control nursery room




Treatment nursery rooms




Control finisher rooms




Treatment finisher rooms




Table 1: Percentage change in number of flies pre and post- Biofly release in nursery and finisher rooms


(Figure 2)


(Figure 3)


Figures 2 and 3 show the number of flies counted on the fly tape for each room over the trial period. It is clear that there has been an overall drop in fly numbers for both the finisher rooms and nursery rooms following the release of Biofly which has been marked on the graph with a green line. 



To conclude, there has been a significant decrease in the overall fly population across the Glynllifon pig unit. It is unclear as to whether the Biofly was sufficiently contained within each room, but the data suggests that the Biofly has reduced the pest fly population throughout the whole of the building rather than just the treatment rooms. Another study could be completed comparing rooms with the same ages of pigs as well as same slurry levels for a more comparative trial.