Alfalfa blotch leafminers, fruit fly-size insects that feed on alfalfa leaves, are attacking more and more Upper Midwestern fields.

When Hay & Forage Grower first reported on the problem two years ago, leafminers had been found in just a few Minnesota and Wisconsin counties. Now they're widespread in the two states and moving into the Dakotas, Iowa, Illinois and Canada.

Growers want to do something, but experts aren't sure what to recommend. Yield loss seems low, even when 30% or more leaves are damaged. In Minnesota, one insecticide is recommended - if you're sure you want to spray.

In the longer run, the same fate is being planned for leafminers as they met in the Northeast 25 years ago. A parasitic wasp introduced from Europe reduced their population so much that the insects today are a rarely noticed curiosity in that region.

Entomologist George Heimpel at the University of Minnesota says he may be ready, by late summer, to release the first parasites in fields. In five years, the epidemic could be over.

Eric Burkness, one of a team of Minnesota entomologists, says it's unclear how much of a problem alfalfa blotch leafminers are.

"Clearly, it has taken off," he says. "Looking at an infested field, it's hard to believe we're not seeing economic losses."

Rob Venette, who works with Burkness and other entomologists in Bill Hutchison's group at the university, says the severe leaf discoloration makes growers concerned and ready to do something.

But a new University of Minnesota brochure says, "At this time, we do not encourage growers to treat. We have not yet been able to consistently show a significant increase in yield or quality by applying insecticides.

"Should a grower feel the need to treat, we recommend Warrior at a rate of 0.03 lb/acre (3.85 fluid ounces) seven to 10 days after cutting."

The insects are synchronized to alfalfa's growth cycle, feeding in the leaves and dropping off to pupate on the soil just about the time you'd take a cutting. They hatch a week or so later, lay eggs in the regrowth, and the cycle goes on.

Insecticide applications should be aimed at egg-laying adults.

Alfalfa blotch leafminers are tiny black flies that make pinhole-size feeding spots on alfalfa leaves. They also lay their eggs in the leaves, where yellow larvae emerge and feed between the upper and lower leaf surfaces. These "mines" turn black and widen as they move outward, ending in a blotch that's comma-shaped.

The insect came to the U.S. in 1968, landing in Massachusetts. Within a few years it had infested fields from Pennsylvania to Quebec and west into Ohio and Michigan. Entomologist Bob Hendrickson, then at the USDA Beneficial Insects Laboratory in Delaware, is credited with tracking down its native enemies in Europe and getting them introduced in the U.S.

"It was one of the most successful biological control projects ever," says Robert Byers at the USDA Pasture Laboratory in Pennsylvania.

Heimpel says he would like to duplicate this "spectacular success" in the Upper Midwest. Land-grant university entomologists are coordinating their efforts, working to reduce the insect to non-pest status.

The wasp parasite is very effective, Heimpel says. It lays an egg in a leafminer larva. It is not known how many eggs one wasp may lay, but enough to overcome high leafminer populations. Later equilibrium develops with low levels of both parasites and leafminers.

A wasp egg does not hatch until the larva quits feeding, leaves its mine and drops to the ground to pupate. Then it hatches and consumes the pupa; instead of a leafminer, a wasp emerges. The wasp is as synchronized to the leafminer cycle as the leafminer is to alfalfa cuttings.

One mystery is how the leafminer escaped this predator. Apparently it shook its pursuer in making the trip from eastern Canada across the Great Lakes. Freed from predation, leafminers built to huge populations and are moving into new territory at about 60 miles per year.