NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665

March 25, 1999

Milkweed Little Threat to Crops, Boon to Butterflies

When a particular weed species starts showing up in greater abundance than is considered normal, it's not unreasonable for concerned weed watchers to conclude that extra steps should be taken to control the weed. Not unreasonable, but also not always necessary or desirable, according to a weed researcher at North Dakota State University.

Common milkweed has been much more noticeable in North Dakota farm fields as well as along roadsides and in other marginal areas in recent years. This has lead to some sentiment for placing milkweed on the state's prohibited noxious weed list, meaning that producers would be required to control it.

Rod Lym of the NDSU plant sciences department doesn't agree with this feeling, for a variety of reasons. For one, common milkweed is a plant native to the area, not an alien invader like leafy spurge.

Milkweed does show up in crop fields, says Lym, but research does not indicate any yield loss from milkweed competition. Also, the fact that milkweed seems to be more abundant is not an ominous sign, but a result of the wet weather cycle of recent years.

In any event, Lym says, there really isn't an effective herbicide available to control common milkweed, leaving producers with few tools to use if they were required to control it, especially in noncrop areas. Mowing might be an alternative, he says, but that would accomplish little but prevent the plants from going to seed.

Once the area goes back into a drier weather cycle, the milkweed population will most likely drop off as well. Different environmental conditions favor different weeds, Lym points out. A return to very dry conditions would tend to favor kochia, for example.

Besides being largely a nonfactor in crop production, milkweed is actually a desirable plant in one interesting aspect, Lym says. It is the main food source for the monarch butterfly.

During the dry years of the late 1980s and beyond, there was great concern about the declining population of monarch butterflies. Starting about 1993, the wet cycle in the northern United States and Canada started, and the milkweed population started to increase. This past winter observers of the monarchs' wintering area in Mexico reported record numbers of butterflies.

The monarch butterflies of North America make an amazing migration each year, from their overwintering roosts in the mountains of Mexico as far as 3,000 miles north in the spring. The offspring of these butterflies are the larvae that feed on milkweed in this area and become the adults that migrate to Mexico in the fall, and their offspring return the next spring. How the homing systems of these migrating butterflies—generations removed from those that have actually made the trip—work is an unsolved mystery.

Lym says there is an obvious correlation between the milkweed and monarch populations, and an indication that a plant out of place may not always be a weed. There is certainly nothing wrong with producers controlling milkweed on cropland, he says, but it would be a mistake to place it on a list that would require control in noncrop areas.

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Source: Rod Lym (701) 231-8996

Editor: Gary Moran (701) 231-7865