Late Winter Rains and Army Cutworms

Southwest Idaho's Winter of Ice Fog ended when snow fell in early February. The ridge of high pressure that had smothered us under a season-long inversion broke up. This allowed a procession of rainstorms to wash in from the Pacific.

The Treasure Valley smelled of damp, warm soil. Ranchers, farmers, and water managers cheered the promise of ample irrigation water and plentiful grass. An artist used every shade from Absinthe to Wintergreen to paint the Boise Green Belt in living, photosynthesizing color.

The rains couldn't save this year’s crop of cheatgrass in the dry areas along the Snake River south of Boise. Last fall, a prodigious storm germinated a flush of the winter annual grass, along with its annual mustard cousins. Sadly for the plants, their good luck didn't last. Happily for me, their subsequent misfortune confirmed an accusation I made 11 years earlier.

Clouds of miller moths returned from their summer in the mountains shortly after the rain storm. The moths laid eggs that hatched into army cutworms a month or so later. The larvae soon got down to business eating the tiny green cheatgrass and mustard plants.

The dry winter that followed was ideal for the cutworms, which are thought to develop fungal diseases in damp weather. But the cheatgrass and mustards struggled in the dry weather. The annual plants died from drought or were consumed by army cutworms. Perennial grasses, with their deeper roots, survived on the hills above areas where the annuals had died.

Hungry army cutworms roamed the bare areas looking for food...

...or hid under cowpies, during the day...

...where hungry centipedes stalked.

After the larvae consumed the annual plants, they went arboreal and climbed sagebrush...

...and fourwinged saltbush and kept eating.

Army cutworms also climbed the hills to munch on perennial Sandberg bluegrass, which seemed able to outgrow the larvae's feeding.

When they ran out of plants to eat, the cutworms dined on their fallen comrades.

This spring I caught army cutworms in the act of consuming cheatgrass and creating cheatgrass die-offs. Eleven years had passed since a rancher told me army cutworms were responsible for die-offs I saw near Winnemucca, NV in 2003, and an entomologist later described to me the conditions that allowed the larvae to explode.