Record summer heat tough on monarch butterflies

Some of the Mid-Columbia’s most colorful wildlife did not fare well in this summer’s record-breaking heat.

At one monarch butterfly breeding site near Vantage, temperatures were above 100 degrees for 16 consecutive days. Only half as many monarchs as usual were at the site, used for the past three years to study the butterflies, said David James, an associate professor of entomology for Washington State University based in Prosser.

The orange and black butterflies breed on milkweeds in Eastern and Central Washington, from the Tri-Cities north to Canada, he said. Optimal temperatures for caterpillar development and survival are around 80 to 85 degrees.

Prolonged periods of hotter temperatures this year in the state caused mortality in immature stages, leaving fewer butterflies to emerge from chrysalises. The summer just past was the hottest on record in Washington.

Of the monarchs that survived at the Vantage site, many likely left looking for nectar as flowers dried up in the heat and the drought, James said.

The start of the summer was promising, with more migrant monarchs than normal entering Washington after overwintering in areas south, James said.

It was a different story three months later. As a consequence of the heat, relatively few headed south as part of the annual fall migration, he said.

However, other parts of the Northwest did not have the extreme heat of Central and Eastern Washington, which will help overall population numbers for the year. More monarch butterflies than normal were reported in September in places like Southern Oregon and Idaho, James said.

He’ll be watching the return in the spring to see if the hot summer has a long-term impact. He does not anticipate a significant decline in numbers, but without the extreme heat it might have been the best population of monarchs seen in the Northwest for many years, he said.

A more important driver of the number of returning butterflies may be the weather along the California coast. If overwintering survival is poor there because of El Niño storms, then the number returning to the Northwest may be low, he said.

A new generation of monarch butterflies returns to the Northwest in June, usually stopping in the same places. James said they likely follow the Columbia River to the site near Vantage.

He has been studying the migration of Northwest monarch butterflies for several years. The monarch butterfly population in the western United States historically has been smaller than in the East, and less is known about their migration path starting from the West.

Long before this hot summer, migrant butterflies were growing scarce. U.S. Fish and Wildlife is conducting a one-year status review of the monarch to determine if protective status is justified under the “threatened” classification of the Endangered Species Act.

“In less than 20 years, their population size in North American has plummeted by roughly 90 percent. That’s an extreme, if not alarming, drop,” James said last winter as the federal status review began.

Part of the drop is because of severe weather in the United States and deforestation in Mexico, James said. But the biggest problem for monarchs is the loss of milkweed wildflowers.

Milkweed is the only plant on which female monarchs lay their eggs and it is the only food source for the emergent caterpillars. Herbicides are being sprayed on crops, including Midwest corn and soybeans, that kill milkweed, James said.

Milkweed still is found along roadways in Washington, but California, a major breeding ground for monarchs, and Oregon have a recent history of routinely spraying most roadside vegetation, including milkweed.

To learn more about the biology of monarch butterfly migration from the Northwest, James and volunteers tag wild monarchs and monarchs raised specifically for James’ research. Some have been raised by inmates at the Washington State Penitentiary in Walla Walla using eggs supplied by James.

At this time of year Mid-Columbia monarchs are traveling south either to California or possibly to Arizona and then on to Mexico.

Anyone who finds one of the butterflies with a sticker attached to its wing is asked to notify James. He recommends snapping a photo with a mobile phone and emailing it to him at the address on the sticker. That gives him good data without disturbing the butterfly, which he can identify by the serial number on the sticker.

This story was originally published October 10, 2015 at 5:01 PM with the headline "Record summer heat tough on monarch butterflies."