Doctoral student Ryan Watchorn has been busy this summer investigating the use of fathead minnows (Pimephales promelas) to control mosquito larvae in ponds in the Powder River Basin of Wyoming. More specifically, Watchorn would like to target the larvae of the species Culex tarsalis, which can carry the West Nile virus.
Sage grouse populations in the Powder River Basin could be devastated by an outbreak of West Nile virus since the birds have no defense against it. When ponds were constructed in the late 90s and early in the decade of 2000 for coalbed methane activities, a lot of mosquito habitat was created. Many of these new ponds occurred in semi-arid areas with little mosquito habitat. Two outbreaks of the West Nile virus devastated certain sage grouse populations in the basin.
In order to control the mosquito populations, companies treated the ponds with larvacide briquettes. The effort was fairly expensive and required repeated applications over the growing season.
The Northeast Wyoming Sage Grouse Working Group decided that a biological control method might be more economical and longer lasting, so they approved a grant for Watchorn’s advisor, Brad Fedy of the University of Waterloo (Ontario), to investigate using fathead minnows as a control measure.
Watchorn has three major objectives in his research:
1. Does the presence of fathead minnows influence mosquito larvae density in the selected reservoirs?
2. What pond and water quality characteristics support survivability, reproductive success and overwinter survivability?
3. When will the use of fathead minnows be an economically sustainable alternative?
According to Watchorn, fathead minnows have been shown to eat mosquito larvae, but there are a host of variables that must be studied. He is studying 15 ponds — six control ponds with no fathead minnows and nine treatment ponds that have been planted with fathead minnows, adults and fry at a rate of 2,500 per acre. The ponds range in size from less than a quarter acre to 3.29 acres.
Watchorn noted that having fathead minnow fry in the ponds provides a shallow-water predator. Adult fathead minnows stay in the deeper water while the fry will move into water that is only 2 to 3 inches deep where the mosquito larvae are most prevalent. In other words, for control of the larvae, fathead fry should be present.
Fathead minnows will reproduce two to three times each summer if the right conditions exist. Watchorn said the minnows need a solid substrate in order to spawn. He has found that the minnows will deposit their sticky eggs on rocks if they are present. Most of the plains ponds lack rocks, so Watchorn has supplied a cheap spawning site — a skid pallet. He anchors the pallet by putting a spike in the center of the pallet and attaching a rope to it and an anchor. This arrangement allows the pallet to drift.
Watchorn said that the pallets not only provide a spawning site for the minnows but also give them shade and a place to congregate.
One variable that is being addressed regards a pond drying up at the end of the summer. Is it more economical to introduce fathead minnows or are the larvacide briquettes more practical? Watchorn said that if the fathead minnows only live for two to three months, they still might be more economical.
Not only is Watchorn monitoring the larvae density by doing a 350 milliliter dipper-cup-sampling of the ponds, but he is also sampling the adult mosquito populations by placing night traps around the pond.
Sampling the larvae can be hit or miss. For instance, Watchorn has found that the larvae tend to be on the windward side of a pond, so if he only sampled the leeward side, the larvae might be underestimated and vice versa. Hence, the samples are arranged randomly. The adults are frozen and sent to the University of Montana for identification. Having the identification done in the United States avoids a lot of customs problems.
The water samples are analyzed for pH, alkalinity, hardness, nitrogen, dissolved oxygen, nitrites, nitrates, calcium, manganese, sodium, iron and zinc, among other attributes. The ponds are mapped with a GPS. Watchorn also measures the depth of the ponds and plugs in the coordinates.
While the two year study is only half done, it looks as though Watchorn is well on his way to answering the three questions he set out to answer. Next summer’s research will provide some definitive answers. Stay tuned; a small minnow might be a major player in the control of mosquitoes and West Nile virus in the Powder River Basin.