Let's Revisit the Alfalfa Blotch Leafminer

April 16, 1999
In issue no. 7 (May 8, 1998) of last year's Bulletin, we alerted you to the possibility of discovering a new insect pest in alfalfa fields. We didn't learn much last year, but that doesn't mean that the pest did not spread. With entomologists at the University of Minnesota, University of Wisconsin, and Iowa State University, we are interested in keeping a watchful eye on this potentially threatening pest. Consequently, we will make a concerted effort this year to determine where we might find the alfalfa blotch leafminer in Illinois. In the meantime, the information we presented last year is worth repeating.

The alfalfa blotch leafminer, Agromyza frontella, was first detected in northeastern Minnesota in 1994, although it may have arrived in that state as early as 1991. Since then, leaf mines and punctures (pinholes) caused by alfalfa blotch leafminer have been found in 99 counties in Minnesota, Wisconsin, Iowa, and Illinois (Figure 3, 1997). The only two counties in Illinois in which this insect had been confirmed by the end of 1997 were McHenry and Lake counties in the northeastern corner of the state. However, we received some unconfirmed reports in 1998.

Before invading the upper Midwest, the alfalfa blotch leafminer had been confined to the northeastern United States and Canada since its introduction into North America in 1968. The pest probably was introduced into Minnesota on infested hay from Ontario. It seems to spread faster than its major parasitoids, and severe infestations seem to lag behind the invasion front by approximately 1 year.

The alfalfa blotch leafminer overwinters as a puparium, and adults, which look like very small (1/8-inch) house flies, emerge in the spring. Mated females lay eggs under the lower epidermis of alfalfa leaflets. The females also feed on the alfalfa leaflets by cutting small holes with their ovipositors and lapping up the exposed sap and tissue. This injury leaves conspicuous pinholes on the leaflets. Larvae mine within the leaves and can be seen through leaf surface. The last (third) instar of the larva widens its mine as it feeds, creating a characteristic blotch mine. If you look for this pest, don't confuse it with the common serpentine leafminer that creates a sinuous mine through the leaflet. The serpentine leafminer is not an economic pest.

There is debate about the economic importance of the alfalfa blotch leafminer. Some entomologists claim to have observed significant yield reductions caused by alfalfa blotch leafminers, and others can't obtain improvements in yield or quality of hay by spraying insecticides. Nevertheless, we are very interested in keeping track of this pest's spread into and through Illinois. I and some of the extension educators (IPM and Crop Systems) will be monitoring more diligently this year. We will report anything we find in articles in the Bulletin. We also seek your input. If you believe you have observed the alfalfa blotch leafminer in your area, please give me a call or send me an e-mail message. I will do what I can to verify the observation and report our findings to the entomologists in Minnesota. Thanks in advance for your cooperation.--Kevin Steffey

Author: Kevin Steffey