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Monitoring
the Spread of
Alfalfa Blotch Leafminer
in Illinois
Robert Wiedenmann & Jonathan Lundgren
Alfalfa blotch leafminer (Agromyza frontella) is a small fly,
accidentally introduced from Europe, which is a pest of alfalfa. The leafminer
larva feeds in the alfalfa leaf, making a diagnostic comma-shaped mine,
which can result in near-total consumption of leaflets. The leafminer
was originally introduced into the Eastern US in the 1960s, and it was
successfully brought under biological control after several parasitic
wasps were brought from France and released in the early 1970s. Those
parasites have kept the leafminer under complete control throughout most
of the east ever since. Two of those wasps, the braconid, Dacnusa
dryas, and the eulophid, Chrysocharis liriomyzae, were thought
to be the most important control agents, responsible for the control.
However, the leafminer also found a home in Ontario, where it remained
at low levels (not at pest levels) for a number of years until it was
accidentally brought into the upper Midwest in the early 1990s. Once reaching
Minnesota and Wisconsin, the populations exploded (up to 100% of plants
in some fields were damaged – many were totally gray by defoliation)
and the leafminer gradually moved south, reaching Illinois by 1997.
In the upper Midwest, the sharp population increase has been followed by declines after a few years. The eulophid parasite (C. liriomyzae) apparently followed the pest into Wisconsin and Minnesota, and the braconid (D. dryas) was introduced from a site where it was established in Ontario. Several researchers consider C. liriomyzae to be at least partly responsible for the decreases seen in Wisconsin and Minnesota.
Our program in the Center for Economic Entomology has been investigating this invasion, along with UI extension educators and faculty to document the presence and spread of the leafminer throughout Illinois, and help develop recommendations for growers. Because of other potential insect pests in alfalfa, this leafminer is not a good candidate for control with insecticides. Alfalfa weevil and pea aphid are currently kept under biological control by other species of parasitic wasps. Spraying insecticides against the leafminer would likely kill many of these parasites, thus would cause worse pest problems. Therefore, it is crucial to inform growers of this consequence of inappropriate use of insecticides, develop alternative solutions and document the presence and effects of the biological control agents.
Jon Lundgren Searching for ABL in
Alfalfa
(R. Wiedenmann)
We coordinated sampling in 2001 throughout the northern half of Illinois. Samples were taken in early May to find the emerging overwintered adult flies, and again in late July, to find larvae from the second generation. The May samples showed leafminer adults in 13 counties, mostly in the north, but as far south as Knox County. As many as 95% of alfalfa stems in Stephenson County showed feeding damage, though defoliation per plant was minimal. Samples from July showed leafminer larvae and mining in 7 counties, as far south as Tazewell and Ford Counties. However, feeding damage was minimal in all areas.
One of the pending questions is whether the low population levels seen in 2001 indicate the pest was just arriving in most areas, or possibly was being attacked by the parasitic wasps. Sampling in 2002 will aim to detect presence (and levels) of parasitism by C. liriomyzae and D. dryas. It is possible that the parasites followed the pest here from Wisconsin. Another scenario is that C. liriomyzae was already present here, utilizing a native leafminer species. If the parasite is already present, its parasitism might keep the invasive leafminer from becoming a serious pest in Illinois. If so, it is important for us to understand this situation, and convey the information to growers, so they do not inadvertently diminish the natural level of control.