Progress report for GNE24-333
Project Information
The purpose of this project is to determine how a potential new biological control agent, phoretic mites (Mesostigmata: Macrochelidae), impacts 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. Yet farmers lack effective biological control agents and instead rely on pesticides for fly management. Livestock farmers need environmentally friendly 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. During this two-year project, I will measure the potential sublethal effect of phoretic mites on 1) 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 lab. I also will 3) conduct a survey of endemic macrochelid mites in Maine to identify candidate biocontrols for inundative release. Our results will be shared at agricultural outreach events, through printed insect pest factsheets, at scientific conferences and in scientific publications. Ultimately, research into new biological control agents will benefit livestock and equine farmers and help prevent pesticide resistance, reduce environmental contamination, and improve animal welfare.
1) Can phoretic mites limit the host-seeking activity of stable flies?
I will conduct an observational field study 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. I will capture host-seeking stable flies during varying temperatures throughout the day and will count the proportion that are carrying mites at sub-optimal and optimal temperatures. I hypothesize that stable flies carrying mites will be unable to host-seek during suboptimal temperatures (below 20°C) and therefore we will see a smaller proportion of stable flies with mites during cooler temperatures compared to the optimal warmer temperatures above 20°C. Thus, mites could have the sublethal effect of shortening 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?
I will conduct a laboratory experiment to measure the flight potential of stable flies with and without phoretic mites attached using flight mills (a device that measures flight time and distance). I hypothesize 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. I expect 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?
I will survey and identify macrochelid mites associated with stable flies on livestock farms in New England to create a pool of new potential macrochelid biological control agents. I will collect stable flies and soil samples where stable flies breed from six mixed livestock farms during stable fly activity to collect and identify 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) and this will be he first survey of macrochelid mites in the United States.
The purpose of this project is 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.
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. This may lead to unsuccessful releases, dissatisfaction among farmers, and reduced likelihood to use 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. 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. 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 fairly 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. Researchers in Brazil found and described a new local macrochelid species, mass produced the mite, and released it in filth fly breeding areas to achieve very 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 sublethal impacts of phoretic mites on adult flies.
Effective biological control agents are needed to reduce pesticide use, prevent pesticide resistance, and protect animal health and welfare on livestock facilities. The combination of mites’ lethal and sublethal impacts could represent a multifaceted approach to controlling filth flies using a single biological agent. As part of the proposed research, we will identify macrochelid mite species currently in the Northeast to determine which species are present on livestock farms. This will give us a pool of mite species we can further examine 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, I will collect host-seeking stable flies from on 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 easily switched out between collecting periods. I will set the traps around the outer perimeter of the barns facing towards the animals where stable flies are likely traveling to find a host. I will change the collection cups from each trap every three hours during daylight hours during the following time periods: 7AM-10AM, 10AM-1PM, 1PM-4PM, 4PM-7PM. Our previous work has demonstrated that stable flies are only active during daylight hours and activity is strongly correlated with temperature. In Maine, the optimal flight activity is usually during the warmest portions of the day, though stable flies are active during “sub-optimal” cooler periods in lower numbers. Tempo Disc™ temperature sensors will be set on a post at the same height that stable flies travel and will record temperature and humidity during the same time periods so that we can correlate the number of mites on flies with temperature. Traps will run four days per week from July-October, the time of stable fly activity in the region based on my prior field studies.
Collected flies will be transported back to the lab, frozen at -20°C, and macrochelid mites will be counted to test for a correlation between temperature and proportion of flies with a mite load. Our previous work has shown that roughly 10% of all stable flies carry macrochelid mites, and so we expect to have a zero-inflated data set. Because of this, we will analyze our data using a hurdle model in R (2023.12.0). Predictor variables will be temperature, time of day, and Julian week, and the response variable will be the proportion of flies carrying macrochelid mites.
January 2025 Status: The field portion of this project is completed. I set up five Nzi traps along the perimeter of the University of Maine mixed livestock farm during August-October in 2023 and 2024. In 2023 we collected 204 trap samples for a total of 4222 stable flies, of which 745 had any type of mite, and 316 carried macrochelid mites. Of those flies with mites, they averaged just under 4 mites each, with a high of 55 mites per fly. I am still processing and counting mites on the flies collected from this past summer.
Objective 2. Impact of macrochelid mites on host flight endurance
To measure the impact of phoretic mites on stable fly flight endurance, stable flies with and without mites will be attached to a flight mill. To obtain the flies for this experiment, late instar stable fly larvae will be acquired from the USDA stable fly laboratory colony in Florida and held until pupation. The pupae will be checked daily for fly emergence and all flies used will be less than 24 hours old. This correlates to the age when most stable flies acquire phoretic mites. In a procedure that I already have developed, newly emerged adult stable flies will be 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 will be plugged with a piece of cotton to restrict the fly’s movement and make it easier for the mites to attach. Mite loads will range from 1-30 mites, which corresponds to what we see occurring in the wild. Stable flies will be anesthetized with CO2 and attached to a tether on their dorsal abdomen using a small drop of glue. Both mite-infested and control flies will be handled in the same manner, though no mites will be added to the vials of control flies. Once a fly is tethered, it will be attached to a flight mill to measure how long it can fly before exhaustion. If a fly stops moving, a single puff of air will be used to encourage them to fly. If they do not begin flying again after this, we will consider them exhausted and remove them from the study. A linear regression will be 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 using the package lme4 (Bates et al. 2014).
This will be completed in the summer of 2025.
Objective 3: Survey of endemic macrochelid mites on livestock farms
We will capture stable flies to search for mites using two techniques. First, adult stable flies will be captured weekly from six dairy and/or equine farms during the stable fly seasonal activity period (late summer to early fall). The flies will be captured alive using Nzi traps by placing a single trap at each farm for 12 hours each week during the day and then collecting the flies and freezing them. Captured flies will be examined for mites under magnification, and all mites on flies will be removed and slide mounted (Anderson 1954) for identification to species. Second, soil from identified fly breeding areas on each farm will also be collected and set up in Berlese funnels to capture additional predatory mites. Each farm will be 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 are found, two quarts of soil from that area will be collected and brought back to the lab. Samples will be taken monthly from each farm from June-October. The soil samples will be 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 will run in the Berlese funnels for one week. Mites will be removed from the samples and those belonging to the Macrochelidae family will be slide mounted using Hoyer’s medium and identified to species.
Part of the field work portion of this project has been completed. Flies were hand collected from six equine farms and frozen. All flies are currently being examined for mites. The soil samples will be taken during the summer of 2025.
Education & Outreach Activities and Participation Summary
Participation Summary:
I will engage with three entities to reach the widest audience possible: farmers, extension specialists, and the scientific community. I plan to partner with Cooperative Extension specialists that work directly with livestock and equine farmers and managers. I will use their reach and platforms to distribute our results and overall stable fly management information. Specifically, I plan to create and update livestock pest fact sheets that will be posted to their website, printed for distribution at outreach events, and handed out as a resource at farm inspections and visits. Currently, there is very little pest management information available through extension agencies in the region and so this effort will have a significant impact on the education and management of livestock pests. I will also attend multiple farmer-focused events to talk about our research including the Maine Ag Trade Show, The Big E, and the Maine Equine Extravaganza. I will stay in touch with farmers we partner with for our research sites to convey our results back to them. To reach a scientific audience, I plan to present my results at the 2025 Entomological Society of America annual meeting. I will publish the sublethal impacts and macrochelid mite survey results in a peer-reviewed journal. My goal for this work is to develop more effective and environmentally friendly stable fly control options.
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. In addition, most equine farms are women-owned and operated, and so this historically underserved group is particularly disadvantaged in the region. Like most agricultural sectors, equine farms were once male dominated, but over the past few decades this trend has shifted and now the majority are owned by women. This woman-dominated sector will benefit from increased education, resources, and ultimately additional control options for these biting flies.
January 2025 Update: 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 100 times.