Final report for GNE24-333
Project Information
The purpose of this project was to determine how a potential new biological control agent, phoretic mites (Mesostigmata: Macrochelidae), impact flight and host-seeking behavior of stable flies (Diptera: Muscidae) on livestock farms, as well as to survey endemic macrochelid mites in the region. Stable flies are a global pest that create economic and animal welfare issues and cost U.S. cattle farmers billions of dollars each year due to reduced milk yields and feed conversion rates (Taylor et al. 2012). Yet farmers lack effective biological control agents and instead rely on pesticides for fly management which has led to pesticide resistance (Cook 2020). Livestock farmers need additional stable fly management options and macrochelid mites have shown potential as a biological control agent of pest flies. Macrochelid mites feed on the eggs and larvae of stable and house flies, and then use phoresy for dispersal, attaching themselves to adult flies for transportation to new feedings areas (Azevedo et al. 2015). During this two-year project, the following nonconsumptive effects of phoretic mites were measured: 1) mite impact on flight activity of host-seeking stable flies in the field at different temperature ranges and 2) flight endurance of stable flies carrying mites in the laboratory. Additionally, 3) a survey of endemic macrochelid mites in Maine was conducted to collect candidate biocontrol agents for inundative release. Results showed that flies carrying mites were less likely to host seek at cooler sub-optimal temperatures (below 25°C). Additional laboratory trials revealed that attached mites lowered total flight duration, and there was a negative correlation between flight time and number of attached mites. On-farm mite collections resulted in several hundred collected mites that can be identified and tested for future biological control commercialization. The results were shared at agricultural outreach events and at scientific conferences. Ultimately, research into new biological control agents will benefit livestock and equine farmers and help prevent pesticide resistance, increase farmer profitability, and improve animal welfare.
1) Can phoretic mites limit the host-seeking activity of stable flies?
An observational field study was conducted of host-seeking stable flies on livestock farms across different temperature ranges to test the hypothesis that mites can reduce host-seeking flies during sub-optimal flight temperatures. Host-seeking stable flies were captured during varying temperatures throughout the day and the proportion of flies that carried mites at temperatures below 25°C and temperatures above 25°C were quantified. It was hypothesized that stable flies carrying mites will be unable to host-seek during suboptimal temperatures and therefore there will be a smaller proportion of stable flies with mites during cooler temperatures compared to warmer temperatures. Thus, mites could shorten the duration each day that livestock is under fly pressure, improving animal welfare and livestock productivity.
2) Do phoretic mites limit the flight endurance of stable flies?
A laboratory experiment was conducted to measure the flight potential of stable flies with and without phoretic mites attached to tethered flies in the laboratory. The hypothesis for this experiment was that flies without mites will have longer flight periods compared to flies with mites, and that flight endurance will decrease with increasing numbers of attached mites. It was expected that this relationship between mite burden and flight endurance will be a linear relationship and will identify the mite load threshold at which flight endurance is significantly impacted. If mites can limit flight endurance, stable flies may not be able to fly as far or as long to seek out a blood meal which may help relieve the total amount of pest pressure on livestock.
3) What are the common endemic macrochelid mite species in Maine?
Macrochelid mites associated with stable flies were collected on livestock farms in Maine to create a pool of new potential macrochelid biological control agents. Stable flies and soil samples were collected where stable flies breed from twelve mixed livestock farms during stable fly activity to collect and phoretic mites in the region. Surveys of endemic stable fly predatory mites have only taken place in Brazil, Columbia, and the United Kingdom (McGarry & Baker 1997, Azeveda et al. 2017, Zapata-Usuga et al. 2022) but this was the first survey of macrochelid mites in the United States.
The purpose of this project was to measure how phoretic mites impact 1) the activity periods and 2) flight endurance of stable flies, and 3) survey endemic phoretic mites present on livestock farms in the Northeast as a next step toward identifying candidates for biological control. Stable flies' frequent, painful bites, and ability to build up in very dense numbers on livestock farms cause significant animal welfare issues and economic impacts. Stable flies cost cattle farmers in the United States roughly 2.2 billion dollars annually by reducing milk yield, growth rates, and feed conversion rates (Taylor et al. 2012). They are also a major pest on equine farms, though no research has quantified their impact on horses (Machtinger et al. 2013).
The most common stable fly management practice is pesticide application to animals and premises, which has led to pesticide resistance (Cilek & Green 1994, de Barros et al. 2019, Reissert-Oppermann et al. 2019). Although there are commercially available parasitoid biological control agents, many studies have shown these parasitoids have little to no impact on filth fly (i.e., stable, house, and other flies that breed in decomposition) populations (Miller et al. 1993, Andress & Campbell 1994). One explanation for this failure may be the short flight ranges of parasitoids, which are sometimes less than six meters depending on the species and conditions and may not be sufficient to reach fly breeding areas (Pawson & Petersen 1988, Lysyk 1995, Petersen & Cawthra 1995, Tobin & Pitts 1999, Machtinger et al. 2015). Stable fly breeding areas are often ephemeral and shift with management and weather conditions, which means farmers need to know where flies are currently breeding each time they release commercial parasitoids (Machtinger et al. 2016). This may lead to unsuccessful releases, dissatisfaction among farmers, and reduced likelihood of using biological control agents in the future. Despite current biological control limitations, there is significant interest in new biological control agents among farmers. Over 80% of equine owners in a survey on pest control practices reported they are interested in seeing more research conducted on stable and house fly biological control agents (Machtinger et al. 2013).
Mites in the Macrochelidae family are a promising biological control agent of filth flies because they impact multiple fly life stages. They feed on fly eggs and early instar larvae and are also phoretic on adult flies. Macrocheles muacaedomesticae and M. sabadius are found associated with filth flies worldwide while other species in the Macrocheles genus have more localized distributions (Azevedo et al. 2015). A single mite can consume up to 13 eggs and 24 larvae per day and has a quick life cycle that takes only days to complete (Hunter & Rosario 1988). Macrochelid mites disperse via phoresy when they sense their habitats are declining in value, such as when fly breeding areas begin to dry out and fewer flies are developing in these areas (Benoit et al. 2025). This allows them to move with their hosts to new breeding areas and gives them an advantage over the limited flight ranges of parasitoid wasps. Macrochelid mites are large, so phoresy may offer a non-lethal mechanism of fly control by reducing the flight ability of host-seeking flies and thereby limiting time spent feeding on livestock. While mites associated with stable flies have been studied in Europe, Brazil, and Colombia, no such work exists in the United States (McGarry & Baker 1997, Azeveda et al. 2017, Zapata-Usuga et al. 2022). Researchers in Brazil mass released macrochelid mites in filth fly breeding areas to achieve high levels of stable and house fly control. This study represented the first inundative biological control field release of macrochelid mites to control filth flies and achieved up to 90% and 73% reductions in house and stable flies respectively (Azevedo 2022). To date, no research in the United States has surveyed the phoretic mites on stable flies and globally no one has examined nonconsumptive impacts of phoretic mites on adult flies.
Effective biological control agents are needed to increase farmer profitability, prevent pesticide resistance, and protect animal health and welfare on livestock facilities. The combination of mites’ predation and nonconsumptive impacts represents a multifaceted approach to controlling filth flies using a single biological agent. As part of the proposed research, macrochelid mite species were surveyed across livestock farms in Maine. This has created a pool of mite species that can be further examined for biological control potential, which is an important step in identifying new biological control agents.
Research
Objective 1: Impact of macrochelid mites on stable fly host-seeking ability
To measure how phoretic mites impact stable fly host-seeking activity at varying temperatures, host-seeking stable flies were collected from the University of Maine’s mixed livestock farm using Nzi traps. These traps have high stable fly capture rates and function similarly to malaise traps, except they are also visually attractive to flies through their color and shape (Mihok 2002). After the flies land on the trap, they are funneled upwards and trapped in a collecting cup which is switched out between collecting periods. Traps were set around the outer perimeter of the barns facing towards the animals where stable flies were likely traveling to find a host. Stable flies are active during daylight hours and do not host-seek after dark (Schofield & Brady 1996) and therefore stable flies were collected only during daylight hours. Collection cups from each trap were changed every three hours during daylight hours during the following time periods: 7AM-10AM, 10AM-1PM, 1PM-4PM, 4PM-7PM. Previous work has shown that the optimal temperature for stable fly activity is around 28-32°C with activity dropping off above and below that temperature. However, stable flies are active during sub-optimal temperatures (below 32°C), though in lower numbers (Semakula et al. 1989). Tempo Disc™ temperature sensors were attached on a post at the same height that stable flies travel and recorded temperature and humidity during the same time periods to correlate the number of mites on flies with temperature. Traps were run four days per week from August-October 2023 and 2024, the time of stable fly activity in the region based on our laboratory's prior field studies.
Collected flies were transported back to the lab, frozen at -20°C, and attached macrochelid mites were counted to test for a correlation between temperature and mite loads. In 2023, 204 trap samples were collected for a total of 4222 stable flies, of which 745 had any type of mite, and 316 carried macrochelid mites. In 2024, 251 trap samples were collected for a total of 6810 stable flies of which 1699 carried any type of mite and 452 carried macrochelid mites. Of those flies with macrochelid mites, they averaged just under 4 mites each, with a high of 55 mites per fly. Preliminary analyses revealed that fewer stable flies captured during temperatures below 25°C were carrying phoretic mites compared to stable flies at temperatures above 25°C (t=-2.8, df=111, p=0.006) (Fig. 1). This suggests that phoretic mites may help reduce host seeking activity during cooler periods of the day if mites are present in high numbers.
Objective 2. Impact of macrochelid mites on host flight endurance
To measure the impact of phoretic mites on stable fly flight endurance, the flight duration of stable flies was measured with and without mites. Due to the government shutdown at the time of this experiment, wild caught flies were used instead of flies from the USDA’s rearing facility. Wild captured stable flies were held in a cage with a 1:1 honey:water solution prior to the start of the study for up to 48 hours. Flies were contained in a 1.5 mL vial with macrochelid mites from our laboratory colony for 1 hour to allow mites to attach to the flies. Extra space at the end of the vial was plugged with a piece of cotton to restrict the fly’s movement and make it easier for the mites to attach. Mite loads ranged from 1-19 mites, as well as unmited flies that served as controls. Mites did not attach to flies at higher numbers than 19 per fly in this experiment, though it was attempted. Stable flies were anesthetized with CO2 and attached to a thread tether on their dorsal abdomen using a small drop of rubber cement glue. Both mite-infested and control flies were handled in the same manner, though no mites were added to the vials of control flies. Once a fly was tethered, it was attached to a central rod inside a 61 x 61 cm white flight box to measure how long it flew before exhaustion. Whenever a fly stopped moving, it was allowed to land briefly before removing the landing structure to encourage them to fly. If they did not begin flying again after two consecutive attempts, they were considered the exhausted and removed them from the study. A linear regression was used to determine the correlation between mite load and flight endurance with mite load as the predictor variable and flight duration as the response variable. Flight duration significantly declined as mite loads increased (F=23.87, df= 1,24, P <0.0001; R2=.48) (Fig. 2). This implies that mites can reduce the overall time spent host-seeking, and limit the distance flies can travel to find a host.
Objective 3: Survey of endemic macrochelid mites on livestock farms
Mites associated with stable flies on farms were captured using two techniques. First, adult stable flies were captured weekly from 12 dairy and/or equine farms during the stable fly seasonal activity period (late summer to early fall). The flies were captured alive by hand into individual vials and frozen. Captured flies were examined for mites under magnification and counted. Second, soil from identified fly breeding areas on each farm was collected and set up in Berlese funnels to capture additional predatory mites. Each farm was inspected for developing fly habitat by digging through common breeding areas such as edges of manure piles and discarded hay. Once stable fly larvae and pupae were found, two quarts of soil from that area were collected and brought back to the lab. Soil samples were set up in Berlese funnels with an incandescent light bulb over the top of the funnel to dry out the samples, and a vial of alcohol under the funnel to catch any specimens. Each sample was set up in the Berlese funnels for one week. Soil samples were taken monthly from each farm from June-October 2025. Additional molecular research to identify collected mites is ongoing, as this research continues beyond the scope of this Northeast SARE funded project. Stable fly associated mites will be identified using Sanger sequencing and then slide mounted to maintain voucher specimens.
Results
Objective 1. Impact of macrochelid mites on stable fly host-seeking ability
In 2023, 204 trap samples were collected for a total of 4222 stable flies, of which 745 had any type of mite, and 316 carried macrochelid mites. In 2024, 251 trap samples were collected for a total of 6810 stable flies of which 1699 carried any type of mite and 452 carried macrochelid mites. Of those flies with macrochelid mites, they averaged just under 4 mites each, with a high of 55 mites per fly. Preliminary analyses revealed that fewer stable flies captured during temperatures below 25°C were carrying phoretic mites compared to stable flies at temperatures above 25°C (t=-2.8, df=111, p=0.006) (Fig. 1). This suggests that phoretic mites may help reduce host seeking activity during cooler periods of the day if present in high numbers.
Objective 2. Impact of macrochelid mites on host flight endurance
Control flies with no mite loads averaged 47 minutes of flight, while flies with mites averaged 26 minutes. Flies carrying smaller mite loads (under 10 mites) averaged 39 minutes of flight, while those with a larger mite load (10-19 mites) averaged 8.5 minutes of flight. Flight duration significantly declined as mite loads increased (F=23.87, df= 1,24, P <0.0001; R2=.48) (Fig. 2). This implies that mites can reduce the overall time spent host-seeking and limit the distance flies can travel to find a host.
Objective 3: Survey of endemic macrochelid mites on livestock farms
Macrochelid mites were found on all farms in this study which indicates that they are a common mite family in the region. Identification of the mites is beyond the scope of this grant, but the collected pool of mites will be identified during subsequent research activities.
Discussion
The project explored the role of naturally occurring predatory mites (family Macrochelidae) as biological control agents of stable flies. Experimental results demonstrated that phoretic mites reduce stable fly activity through non-consumptive effects. Specifically, stable flies carrying mites were less likely to host-seek at sub-optimal temperatures and flight duration decreased as mite load increased. These findings suggest that in addition to their previously documented ability to reduce stable fly populations through predation (Azevedo et al. 2022), macrochelid mites also suppress flies’ host-seeking abilities.
If developed into a commercial biological control strategy, these mites could provide an alternative to chemical insecticides. Reduced pesticide use would decrease environmental contamination and slow the development of insecticide resistance, a widespread issue in pest management on livestock farms.
This research investigated the potential of macrochelid mites as biological control agents of stable flies by examining their nonconsumptive effects on fly behavior and performance, as well surveying mites associated with flies and fly breeding habitats on farms.
In objective 1, a two-year field study assessed the impact of phoretic macrochelid mites on stable fly host-seeking behavior across a range of daytime temperatures. Several thousand flies were collected during fly activity periods and individually examined for mite presence and abundance. Flies carrying mites were less likely to engage in host-seeking at cooler temperatures below 25°C, suggesting that mites may reduce the duration of daily fly activity. This reduction in host-seeking behavior could ultimately decrease the time livestock are exposed to biting flies.
In objective 2, a laboratory experiment quantified the effect of mite load on stable fly flight endurance. Flight duration was measured for flies carrying between 0 and 19 mites. Results showed a clear negative relationship between mite load and flight endurance, indicating that mites shorten the time flies can actively host-seek. Reduced flight capacity may limit dispersal between farms and surrounding communities, as well as shorten total time spent host-seeking, further decreasing fly activity.
In objective 3, a statewide survey was conducted to document the presence and diversity of macrochelid mites on livestock farms. Mites were collected from stable flies and breeding substrates at 12 farms across Maine from June through October. Preliminary findings indicate that macrochelid mites are commonly present in these systems, suggesting strong potential for establishment if used in augmentative biological control programs.
Together, these findings demonstrate that macrochelid mites can negatively affect stable fly behavior and activity, and are widespread on livestock facilities. This supports their potential as effective, locally adapted biological control agents to reduce stable fly populations, reduce host-seeking activity and improve livestock welfare and productivity.
Education & outreach activities and participation summary
Participation summary:
I participated in an outreach event called Equine Extravaganza in June 2024. I set up displays of different types of fly traps and used specimens to teach people how to identify common fly pests. I spoke with several dozen farm owners on how best to manage pest flies on their farms.
I was an invited speaker for the University of Maine Cooperative Extension Summer Equine Speaker Series in September 2024. I gave an hour long webinar on equine pests in the region as part of their Pasture and System Management for Healthier Horses. After a live presentation, the recorded webinar was uploaded to YouTube where it was viewed over 160 times.
I presented my research to scientific audiences during the following events:
-2024 School of Biology and Ecology Graduate Student Lightning Talk
-2025 Entomological Society of America annual meeting
-2025 University of Maine Student Symposium
Veterinary entomology research is very limited in the Northeast as there are very few researchers working in this field in Maine or anywhere in the region. This means farmers have access to little region-specific research that may directly improve their livelihoods and animals’ welfare.
Project Outcomes
Project Impacts on Agricultural Sustainability
This project has contributed to agricultural sustainability in Maine by improving knowledge of pest fly ecology and investigating alternative management strategies that may reduce reliance on chemical controls.
Social Benefits
The project engaged a broad network of farmers and community members. Over 300 farmers were directly contacted through outreach efforts, and more than 70 farmers volunteered to host research activities on their farms, from which 12 sites were selected across Maine. These interactions increased awareness of pest fly biology and management practices. Farmers received individualized feedback during site visits, including identification of key fly breeding habitats on their farms and recommendations for management. Additionally, farmers were shown how to distinguish between pest fly species such as stable flies and house flies, which differ in their ecology and control strategies. This knowledge enables farmers to make more informed management decisions by targeting specific species. Interest in continued participation was high, with many farmers expressing willingness to participate in future research. This indicates strong interest in learning and utilizing new pest fly control methods.
Environmental Benefits
The project explored the role of naturally occurring predatory mites (family Macrochelidae) as biological control agents of stable flies. Over 200 macrochelid mites were collected from adult flies, with several times that number recovered from manure samples across study sites. Experimental results demonstrated that phoretic mites reduce stable fly activity through non-consumptive effects. Specifically:
- Stable flies carrying mites were less likely to host-seek at cool temperatures below 25°C
- Stable fly flight duration decreased as mite load increased
These findings suggest that macrochelid mites may suppress stable fly populations both through direct predation and by suppressing flies’ host-seeking abilities. If developed into a commercial biological control strategy, these mites could provide an alternative to chemical insecticides. Reduced pesticide use would decrease environmental contamination and slow the development of insecticide resistance, a widespread issue in pest management on livestock farms.
Economic Benefits
Stable flies are a significant economic pest in livestock systems. By identifying biological control options and improving farmer knowledge of fly breeding habitats, this project provides tools that may reduce pest pressure and associated economic losses. Farm-specific recommendations given during site visits helped farmers to target management efforts more effectively, potentially reducing pesticide applications and saving money from ineffective management actions. While direct economic impacts were not quantified in this study, reductions in fly activity and improved management efficiency can contribute to long-term cost savings for farmers. This project suggests that biological control using predatory mites has potential to enhance sustainable pest management through predation as well as through non-consumptive effects.
Knowledge Change
While interacting with numerous farm owners, I realized that many lack basic knowledge on sustainable pest fly management. Many farmers are unable to recognize which pest species are on their farms and the most effective management strategies. As a result, many farmers rely on daily pesticide use unaware of how it contributes to pesticide resistance, while others buy traps that are ineffective for their target pests. For example, one farm owner routinely used fly bait that is registered for house flies to control biting flies, which do not feed on bait. She was unaware that house and stable flies are different species that require different management strategies.
Future Research
Mites collected during this project will be identified in collaboration with the Maine Center for Environmental Genetics using Sanger sequencing and compared across molecular databases such as BOLD and GenBank. This will help guide future work on selecting and testing different mite species as novel biological control agents.
New biological control agents are only useful if farmers are willing to invest in them. I am interested in a survey to measure which tools and at what costs farmers are willing to invest to control pest flies. To better understand how farm owners make decisions on pest management practices, I intend to survey farm owners to assess:
- Knowledge of common pest species
- Current management practices
- Willingness to adopt alternative control strategies
- Willingness to pay for pest management
Additionally, future work also involves collaboration with an insect physiologist, Dr. Lydia Fyie, to investigate the mechanism with which mites impact flight duration by measuring the physiological impact of phoretic mites on stable flies. The flight experiment will be replicated using lab-reared flies and will then the flies will be analyzed for changes in lipids and carbohydrates to better understand the energetic costs these mites have on flies.
Future research will continue to work with macrochelid mites with the long-term goal of commercializing these mites for livestock farmers in the United States. This includes optimizing rearing methods, evaluating mite release rates on farms, and determining how they can best fit into existing sustainable pest management techniques such as compatibility with current biological control agents and insect growth regulators.