The hard-shelled mollusks were first seen in Lake St. Clair in 1988, said Dr. Tom Coon of Michigan State University’s Department of Fisheries and Wildlife. They apparently came from the Caspian Sea in the former Soviet Union, carried in the bilge water of ocean-going ships. They affect the aquatic food chain by filtering surface water, making it clearer. Some fishermen claim this is causing walleye to dive deeper into murkier waters. Ohio scientists have observed the zebra mussels feeding on and routing native Lake Erie mussels.

Since its first appearance, the zebra mussel has spread through all five Great Lakes, moved into the Illinois River through the Chicago River basin, up the Mississippi River as far as St. Paul and down to the Tennessee River. They have also been seen in the Kentucky and Ohio rivers.

The mussel has also been seen in two inland lakes in Cass County in Southwestern Michigan. It has also migrated upstream in two rivers fed by Saginaw Bay, the Saginaw and Kawkawlin rivers. In New York state, they have been discovered in the Erie Canal between Lockport and Palmyra.

The mussel spreads by attaching itself to any hard surface, especially any surface that has been in the water such as boat hulls or insides of intake plants of power plants and water supply systems. In bodies of water, they can quickly colonize any rock they find. The mussels can even "piggyback" and attach to themselves. These aggregate mollusks can reach sizes of up to four inches in diameter, as compared to an individual mollusk, which is from one-half to three-quarters of an inch in diameter.

The problem the zebra mussel poses to agriculture is that it has the potential to clog irrigation systems. "So far, we haven’t seen any negative effects, but the potential for problems is pretty great," said Dr. Coon.

Zebra mussel larvae and adults can attach themselves to the inside of pipes and block the flow of water, something that has been observed in eastern Europe. Municipalities along the shores of the Great Lakes are already monitoring intakes, which are at risk from the mussel.

"You can have all sorts of zebra mussel growth in the line at the source of the water," said Extension Sea Grant Agent Chuck Pistis. "Five years of growth can plug up big mains, so you can imagine what it could do to a three-inch PVC line."

The zebra mussel has nuisance potential for operators of overhead spray rigs. Water at high pressure can rinse them away. The biggest potential for problem is with subirrigation systems.

"In the Saginaw Bay area, some growers use subirrigation where they flood their drainage tiles to irrigate. If zebra mussels infest that system, that would be probably the worst case for a grower because that means replacing drain tile to the tune of $400-500 an acre," said Dr. Coon.

Saginaw bay growers haven’t confirmed any problems at this point, but are keeping a sharp eye out for the mussel, said Bay County Extension Agent Mike Staton.

"The growers at risk are the ones who subirrigate and pump bay water backwards into tile lines and raise the water table," said Mr. Staton. "There is a concern that the mussel could live in that habitat, reproduce and clog up the tiles. And we are dependent on bay water for our agriculture."

Irrigation systems at lesser risk are those which are frequently moved and which have one-inch or larger nozzles. The mussel has a higher potential to wreak havoc on systems with permanent underground installation, especially if the systems have nozzle apertures of one-half inch or less.

Even if the mussels get into an underground irrigation system, a number of conditions must exist for the mussels to survive, said Mr. Pistis. They must have a food supply and a certain oxygen level to survive. Their favorite foods include algae, which needs light to grow, light which is not present in underground systems.

Operators of irrigation systems do have some options in controlling the mussel. High temperatures are the key. Mussels are killed on contact with water that is 140 degrees or higher and don’t live long in air temperatures above 90.

In New York, the recommendation is to flush the irrigation system with hot water. Chlorination may work in a system that is hard to flush. Zebra mussels cannot tolerate chlorine at 0.25 to 0.50 parts per billion. Chlorine may require testing at the site to determine the proper concentration. It may also have negative effects on non-target species. Chlorination is a viable option for overhead systems, possibly with a permit, but probably not allowable for subirrigation systems due to the potential to contaminate ground-water.

Overhead irrigators can also make sure they are using non-contaminated water, said Dr. Coon. They can use the same type of filtration systems the Israelis developed for drip irrigation. Systems must also be monitored for presence of zebra mussel larvae. When these are found, irrigation should not take place until they are removed.

Another method that was tried in the Ukraine in the 1960’s, said Dr. Coon, was pumping ammonium nitrate through the irrigation system. The Ukrainians were able to deliver fertilizer and kill the mussels at the same time. Potassium compounds have also been effective in at least inhibiting the mussels’ respiration.

The best thing for growers to do is to keep an eye out for the mussels and be ready if problems arise.

”If you have an opportunity to inspect all your systems, do it,” said Mr. Pistis.

Reprinted from the Great Lakes Vegetable Growers News, June 1993, Page 19