Be on the lookout for these alfalfa insects

Mike Rankin
Crops and Soils Agent
University of Wisconsin - Extension

Alfalfa weevil feeding

        Although not at economic threshold levels, many alfalfa fields had at least some feeding from alfalfa weevil during the initial spring growth.  The wet weather that delayed harvest has also allowed for more time for alfalfa weevil adults to lay eggs.  As is the case nearly every year, growers and consultants are reminded to monitor alfalfa regrowth until plants are 10-12 inches tall.

If the second crop is not re-growing in the expected time frame, check carefully in the stubble for larvae and/or new adults. Only 4-5 larvae per plant can hold back growth of the second crop.

Yellowing caused by severe infestation of potato leafhopper

Feeding from ABL larvae (2001)

Pinhole damage from adult ABL (2001)

       We haven't really heard from this little critter for several years but I noticed a lot of feeding in several fields as first cutting was being made.  The Alfalfa blotch leafminer (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 that 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.