University of Illinois

No. 7/May 8, 1998

Look for a New Insect Pest in Alfalfa This Year

Entomologists in Minnesota and Wisconsin have been watching the rapid spread of an alfalfa insect pest that is new to the upper Midwest. 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. The only two counties in Illinois in which this insect has been confirmed are McHenry and Lake counties in the northeastern corner of the state. However, I was aware of an unconfirmed report of alfalfa blotch leafminers as far south as Champaign County in 1997.

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. Entomologists in Minnesota suspect that the pest was introduced into their state on infested hay from Ontario. The pest seems to spread faster than its major parasitoids, and severe infestations seem to lag behind the invasion front by about one 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 the 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.

Debate about the economic importance of the alfalfa blotch leafminer rages on. 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. 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 (, Extension Entomology, (217)333-6652