2008-08-30 / Columnists

Mackinac Island Has Long History With Battling Flies

Nature Notes
By Patricia Martin

Recently, at a party, someone came up to me and said, "You've got to write about flies." Now this might be considered unusual party small talk, but for me it seems to be par for the course. I responded by asking, "What is it about flies that you would like me to explain?" He replied that it was wonderful how fewer flies there are on the Island than in past years, and I should write about what's been going on with fly control on Mackinac this summer. So here goes:

Flies have long been an issue on the Island. In the 1930s and 1940s, the flies were so numerous that it was driving people crazy, and they were being blamed for a mysterious illness that seemed to spread among the residents during the summer. Mackinac was chosen as a test site for a new insecticide called DDT, and this chemical was sprayed every summer for years, until the 1960s. This spraying certainly reduced the number of flies and mosquitoes on the Island, along with other beneficial insects, amphibians, reptiles, and birds. This insecticide was so indiscriminate that it almost caused, as Carlson called it, a "Silent Spring." It was, of course, later banned nationwide, and many of the species that were decimated have returned, including the fly population.

About 20 to 30 years ago, some graduate students from Michigan State University did a study on the fly populations on Mackinac, and suggested other methods of fly control. Alternative methods have since been used, including fly traps, fly tape, fly spray on the horses, and biological predators. For years, the city and commercial and private horse owners battled flies, independent of one another, with varying degrees of success.

Late last summer, Dr. Al Sibinic, our veterinarian who had just arrived on Mackinac in the spring to help Mackinac Island Carriage Tours with their horses and other Island animals, noted the fly problem here, and the irritation that it was causing horses and other animals, and he decided to look into what could be done. During the winter, he talked to his cousin, who has a dairy farm. This cousin was dealing with his own fly issues (insecticides that only work for short periods, etc.), and showed Dr. Al an article in one of his dairy magazines about Spalding's fly predators. Dr. Al contacted the Spalding Company in Texas, explained about Mackinac, and the upshot was that the owner of the company agreed to fly to Mackinac to assess the situation.

In the spring, when Dr. Sibinic returned to the Island, a chance conversation with Kelly Bean, the mayor's assistant, led to his discovering that the city was already involved in trying to use fly predators to control flies, particularly around the downtown area. Tom Spalding, along with Dr. Sibinic and several others from Mackinac Island Carriage Tours, toured the Island June 30, looking for hot spots for fly breeding, and came up with the idea of doing an Island-wide attempt to control flies. Dr. Al talked to many of the private and commercial horse owners around the Island and got their support for the project, and then approached the city, who agreed to help coordinate the program. The city agreed to help pay for part of the project, and private and commercial horse owners and others are helping to defray the rest of the cost.

The type of pest control that is the basis of this project is a biological control. This means that you use a natural predator of the flies, along with fly traps and improvement in manure and biowaste management to control the fly breeding, no spraying of harmful insecticides.

This is not the first time that biological controls have been used in our area. As a child, I remember the massive die-off of alewife, a small silvery fish which entered the Great Lakes by the St. Lawrence Seaway and the Whelan channel. These fish had no natural predators in the lakes, and so at the end of their life cycles, they would die, in great abundance. Salmon were then introduced to the Great Lakes. These fish feed heavily on the alewife, don't compete with our native fish species, and provided a good sports fish for the area. It was a win-win situation. Similarly, the plant purple loosestrife was brought into North America as a garden plant. It has escaped into the wild and grows heavily in wetlands, where it chokes out and displaces native aquatic plants like cattails. To control, although not eradicate, these plants in the wetlands, insects from Asia have been introduced that only eat purple loosestrife. They keep the loosestrife populations in control, and don't damage other organisms.

In the case of using fly predators as a biological control, the predators that feed on fly pupa are already here, but not necessarily in the numbers needed to keep fly populations under control. The fly predators are not alien species. Flies need a couple of things to survive and thrive. They need a fly to get to an area, rotting material to live on, and moisture. Cut down on any of these things, and you reduce the number of flies around.

The life cycle of the fly, particularly the housefly and the stable fly, which are our two main problems, begins with the female laying eggs (each female can lay up to 900 eggs) on rotting organic material. The purpose of flies, in general, is to break down organic material. The eggs hatch within hours into larvae. Over the next five days or more, they feed on organic matter, until they form a cocoon. Within the cocoon, they undergo a metamorphism, which changes them into adult flies. It is when they're pupa in the cocoon, that the predators attack. The predators are actually parasitoids, which mean that they're somewhere between a predator and a parasite. They are tiny Hymenopteran insects of the Pteromalide family (the bee, wasp, ant family). They don't sting, nor do they bite. They spend their whole lives on the surface of the manure or organic material. The tiny female predator searches for a fly's pupa and drills a hole in the pupa case, inserts her ovipositor, and deposits one to a dozen eggs in the pupa. She will then proceed to ingest fluids from the developing pest fly and go on to lay more eggs (up to 100) in other fly pupa. As the eggs she deposited hatch and grow, they eat the larva within the pupa. They then mature and begin the cycle all over again. This is a cycle that goes on, whether we add more predators or not, but by adding more predators, we reduce the number of pest flies that ever get a chance to mature. The four species of predators that are provided by Mr. Spalding's company include: Spalangis cameroni, Muscidifuras zaraptor, Trichomalopsis, and Muscidifuraz raptorellus, none of which are harmful to humans, other animals, or plants.

In looking into this program, I discovered a couple of things that I didn't know. The flies that grow and live on manure are houseflies, which are 3/16 to 5/16 inch in length and are black with four vertical stripes on the thorax. These are the flies that are the most common around the barns and in the houses. They've been implicated in the spread of a number of diseases caused by bacteria and protozoa. These are the flies that lay eggs on garbage, manure, spilled feed, decaying fruit, compost piles, rotting plant debris, and landfills. It usually takes from eight days to two weeks for a housefly to develop into an adult. They then can live from two to 3.5 weeks.

The second fly that seems to cause problems on Mackinac are the stable flies, or biting stable flies, which resemble the houseflies, but have dark spots or checkerboard markings on their abdomen and a needle-like proboscis, which they use to bite with. These flies feed on the blood of warmblooded animals, such as people, horses, and dogs. The female biting stable fly lays its eggs not on horse manure, but on urine-soaked straw, compost piles, wet bay hales, and grass clippings. So it's not just horse manure that breeds flies.

It's the biting of these flies that causes great distress to animals, and can spread bloodborne pathogens from one animal to another.

Besides increasing the amount of fly predators in a potential breeding area, there are other ways to cut down on pest fly problems. One is to remove, concentrate, or dry manure or other rotting organic matter. Just turning a compost pile in at least seven days will interrupt the pest fly's life cycle. The composting center, run by the Department of Public Works, which composts manure, yard waste, and other biodegradable materials, turns them at least that often, and the temperature generated by the breakdown of the organic material is generally sufficient for killing or preventing the growth of fly larvae. Attracting fly traps, placed away from buildings a few feet off the ground and in the sun, will draw houseflies away from horses and people. Biting stable flies require a specific type of trap designed for stable flies, which attracts them by visual stimuli, rather than scent. These traps are hung near where the flies might reproduce.

I have no empirical proof that the increased uses of fly predators, which have been spread in select locations since the end of June, have decreased the fly population on Mackinac, but there seems to be a lot of anecdotal evidence to that effect. More and more people have been commenting to me about the low number of flies in evidence around the horses and about the town. I know personally that I have greatly cut down on the use of chemical fly spray on my horse, for which I am grateful, as he has enough skin problems without adding hazardous chemicals to the mix, and I have not dumped as many fly traps as last summer. This year was something of an experiment in the widespread use of fly predators, and we may see even better results when we start them next year, before the fly populations begin to hatch out. If anyone is interested in more information, or would like to make a contribution to the project, contact Kelly Bean at the City of Mackinac Island, or Dr. Al Sibinic.

Trish Martin is a year-around resident of Mackinac Island, has earned a master's degree in botany from Central Michigan University, and owns Bogan Lane Inn.

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