An alfalfa blotch leafminer?
  ------ You gotta be kidding!

Look in the book for alfalfa blotch leafminer (ABL). The map says this insect resides between the places where the Boston Red Sox and Cleveland Indians play baseball. It's time to redraw the map. The leafminer has now moved west to invade the land of the Milwaukee Brewers (or perhaps they're just jumping on the Green Bay Packer bandwagon). For whatever reason, the leafminer appears to be fairly widespread in our area. Some fields in Fond du Lac County that had not been cut by mid-June were showing significant ABL feeding.

The ABL adult form is a small black fly, slightly larger than a fruit fly, that emerges in the spring from an overwintering pupae on the ground. Once emerged, the fly cuts a tiny flap on the underside of the alfalfa leaflet and deposits an egg under the flap. The female adult can do this as many as 150 to 200 times on different leaflets. When the larva hatches from the egg, it begins to eat or "mine" its way between the upper and lower surfaces of the leaflet. A mine tunnel is formed as feeding progresses. The progression is usually from the leaflet base toward the tip. The mature larvae leave their mines and drop to the ground to pupate.

As a result of larval feeding, a large, white blotch is left on the alfalfa leaf. It somewhat resembles a large "comma." The larvae pupate on the ground and emerge again for a second generation of egg laying. This typically occurs about mid-July. Adult leafminers can cause pinhole damage to leaves as they feed and lay eggs. The cycle repeats itself for three generations during the course of a growing season.

The good news in this whole story is that economic damage is rare. Although some fields may show up to 25 percent of leaflets with ABL blotches, controlled field studies have shown even high densities of leafminers did not significantly reduce the yield or forage quality of alfalfa. Some studies suggest that economic losses begin to occur as mining activity reaches 50 mines per 100 leaflets. Early harvest can significantly reduce ABL populations in subsequent cuttings. In the northeast U.S., parasites have been used successfully to reduce overall ABL populations and this can hopefully be accomplished in our part of the world as well. In the meantime, producers and agronomists are encouraged to scout for and be able to identify damage from this insect pest.