Documenting a decline in monarch butterfly populations

Every August, Washington Department of Fish and Wildlife (WDFW) biologists based in the arid, eastern portion of our state head out in their regions to dig through patches of milkweed, a perennial flowering plant known for the milky substance it seeps when damaged.

With so many cool plants and animals in this state, why do they spend time on something that many people view as a common weed? The answer is- it’s not because of the milkweed itself, but what it attracts.

Milkweed is the only plant that “hosts” monarch butterflies; the only plant that monarch females will lay eggs on and larvae eat. That’s because the plant provides all the nourishment caterpillars need to turn into butterflies. In addition, studies of pollinator abundance and diversity have found habitats with milkweed support a greater number of pollinator species and individuals. There are many species of milkweed throughout North America, and in eastern Washington two are native: showy milkweed (Asclepias speciosa), the most common, and narrow-leaved milkweed (Asclepias fascicularis).

With their large orange and black wings, monarchs are probably the most recognized butterfly in North America. They are known for their annual long-distance fall migration to overwintering sites, with their “grandchildren” making their way back to northern breeding habitats in the spring.

While monarchs east of the Rocky Mountains make their way to Mexico each year, those in our area primarily head to the California coast, usually in early October. Remarkably, monarchs return to the same groves of trees as the generations of their “families” before them, although it’s still a bit of a mystery as to how they know where to go each winter.

While the sight of thousands of butterflies on the move is amazing, fewer and fewer monarchs are making the annual trip. In the 1990s, hundreds of millions travelled from the U.S. and Canada to forested groves in California. It’s estimated that only a fraction of the population remains now- some data even suggesting that the overwintering population has been reduced as much as 99%.

This is because monarchs, and many other butterflies and moths, are losing habitat to a variety of factors. Groves of trees where they overwinter in California and Mexico are being destroyed by development, wildfire, and drought. Chemical insecticides and weed treatments are killing the butterflies and the plants they rely on for sustenance. Milkweed is often viewed as a weed and mowed or treated with herbicide. And overall warmer temperatures and decreased soil moisture mean a decline in milkweed plants.

In an effort to monitor the health of monarch and milkweed populations, biologists each year take to the field to conduct population surveys. This involves looking for butterflies, signs of them, or milkweed plants. WDFW partners with the Xerces Society on this- a nonprofit organization devoted to protecting wildlife through the conservation of invertebrates and their habitats.

While counting butterflies seems like a dream job for some, it is more complicated than it sounds. It happens twice a summer- once in early August and once in late August- and starts with watching the air above milkweed patches to see if monarchs can be observed flying above them. Next, the biologists painstakingly search the leaves of each plant for caterpillars and eggs. While late-stage caterpillars are large and can be pretty obvious, young larvae can be tiny and hard to spot, and eggs are truly a challenge to find. The eggs are tiny white globes stuck to the underside of the leaves or sometimes the stems. Yellow, black, and white caterpillars hatch three to five days after the egg is laid. Sometimes surveys don’t turn up caterpillars themselves, but evidence that they have been there, like chewed leaves.

The last step in the monarch survey process is to count how many milkweed plants are in the area, to compare to previous years.

Unfortunately, too often lately there are many fewer plants than the preceding year. And in recent years, there is little evidence of eggs or caterpillars either. Recent surveys around Lincoln and Grant counties didn’t turn up many encouraging signs this year.

The hope and plan is that this trend can be reversed through conservation efforts. And you don’t have to be a biologist to help. There are several things that anyone living within the monarch’s eastern Washington range can do to help try to save our monarch populations. The most immediate way is to plant native milkweed in your yard or property. As long as you use the plant native to the area you are planting it, new milkweed patches can give monarchs a place to reproduce.

You can also plant native late-summer flowering plants to provide nectar for monarch butterflies and other pollinators. Nectar provides monarchs with energy to migrate, breed, and overwinter. You can find examples of good native plants and flowers on the Xerces Society website.

Avoid using insecticides and herbicides that could kill butterflies, caterpillars, or the plants that host them. If you know thats monarchs roost on or near your property, please consult with a biologist before cutting or trimming trees. And when you are out in nature, look for and take photos of monarchs and milkweed and report them on the Western Monarch Milkweed Mapper.




The Washington Department of Fish and Wildlife is dedicated to preserving, protecting and perpetuating the state’s fish and wildlife resources.

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The Washington Department of Fish and Wildlife

The Washington Department of Fish and Wildlife

The Washington Department of Fish and Wildlife is dedicated to preserving, protecting and perpetuating the state’s fish and wildlife resources.

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