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Bug swarms in Missoula warn of winter weather switch

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Those little blue bugs that filled the Missoula air like snow all September will soon get replaced by real October snow.

This fall produced a particularly big swarm of leafcurl aphids in neighborhoods with green ash trees, according to Missoula County Extension Service horticulturalist Sandy Perrin.

The millimeter-long bits of blue fluff have flustered a lot of evening deck parties, but don’t do much agricultural damage.

“Their whole mission is to mate and lay eggs and die,” Perrin said. “It happens in a short period of time — maybe a week or two. They don’t harm anything other than fly up your nose.”

Leafcurl aphids lay those eggs in the crevasses of tree bark, looking for fruit trees as well as the green ash. In the spring, they hatch just as new leaves are budding out, and then feed on the sap. That makes the leaves curl up and results in a lot of sticky bits of “honeydew” on anything under the trees, but doesn’t usually hurt the host tree.

The aphids retreat underground for much of the hot summer, and re-emerge in fall just when Missoulians hope to celebrate the last warm evenings.

The home invasion of other bugs, such as black flies, box-elder bugs and beetles, foretells the shift from pleasant to problematic weather this weekend. Daytime temperatures dipped about 20 degrees between Tuesday and Wednesday, from the low 80s to the mid-60s. A more significant shift should hit western Montana on Monday.

“There’s an 80% chance of at least some snow Monday into Tuesday, with a small probability of multiple inches,” National Weather Service meteorologist Brian Conlan said on Wednesday. “It should be quite cold by Tuesday."

That means low temperatures in the Bitterroot Valley in the mid-20s, while highs will settle in the mid-40s. Some uncertainty remains about the path of the cold front, which could swing through Missoula and Butte or drop farther south, Conlan said. Northwest Montana should stay warmer in either case, with highs in the upper 40s to 50 degrees.

That means this weekend is the deadline for yardwork, garden harvesting, unhooking hoses, blowing out irrigation lines and other pre-winter chores. However, Perrin recommends keeping some hoses handy for a last watering of stressed trees and shrubs.

“Once we lose the leaves, give your shade and conifer trees one good, deep soaking before we go into winter,” Perrin said.

2021 was an odd water year across western Montana, according to National Weather Service log books. While the winter months delivered a mostly routine amount of moisture, spring and summer dried up.

“In June we got just slightly more than half our normal amounts for rain,” Conlan said. “And in July we got a quarter of what would be normal. It was the summer that really did us in.”

Butte got particularly parched. It usually receives 12.76 inches of precipitation in the water year from Oct. 1 to Sept. 30. This just-finished water year ranked second-driest in the past 125 years with 6.73 inches. Those records date back to 1895.

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