To predict cheatgrass die-offs we must understand their cause

Army cutworms created this large die-off near Bruneau, Idaho in 2014.

In brief 

• Exotic cheatgrass fuels rangeland wildfires in the intermountain west.

• Cheatgrass die-offs are large bare patches that appear suddenly in cheatgrass-invaded areas.

• Die-offs are opportunities to reseed invaded areas with native species while there are few cheatgrass seeds to sprout and compete with sown plants. 

• Army cutworms (ACW) consume cheatgrass seedlings to produce die-offs and can also defoliate native shrubs. The larvae hide in plain sight by feeding at night in winter and spring and hiding during the day. Later, they pupate in the soil and fly away. 

• Major ACW outbreaks and die-offs in 2003 and 2014 occurred during drought broken by late summer rain, which germinated cheatgrass for larvae to eat. 

• Two recent federal reports overlook ACW as the most likely cause of die-offs. 

• Both reports state that fungal pathogens cause cheatgrass die-offs. However, fungi have not been linked to die-offs, are rare during drought, and would require a more complex series of events to damage cheatgrass. 

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Cheatgrass (Bromus tectorum) die-offs are bare areas, often covered with gray plant litter, that appear suddenly within stands of normal-looking cheatgrass. Die-offs have distinct boundaries and can cover up to several square miles. Perennial grasses and forbs within die-offs are unaffected, but the exotic annual mustards (Brassicacaea) that often grow with cheatgrass are also missing. Cheatgrass die-offs are sporadic in time and space: widespread die-offs occur relatively rarely, and die-offs only infrequently reappear in the same place.

Die-offs first appeared in low, dry areas of the intermountain west in 2003, during a major army cutworm (ACW) (Euxoa auxiliaris) outbreak. B. Hammon of Colorado State University (personal communication 2003) described conditions leading to the outbreak and die-offs: 

1. a previous year of dry weather created many egg-laying sites, 

2. late summer rain germinated cheatgrass for larvae to eat, 

3. a large flight of ACW (miller) moths in fall laid many eggs, 

4. dry fall and winter weather allowed many larvae to survive and consume cheatgrass seedlings.

Ranchers and at least one researcher watched ACW eat cheatgrass in early 2003. Entomologists saw extensive ACW damage to crops in southwest Colorado and northern New Mexico. I saw a cheatgrass die-off in Nevada on April 17, 2003, but didn’t learn the cause of the bare area until later that year. 

Army cutworm outbreaks in the intermountain west are most likely after a year of dry weather is broken by September rain, followed by a large flight of miller moths, and a second period of dry weather through January.

My 2004 research poster described how ACW outbreaks could create cheatgrass die-offs (Salo and Zielinski 2004). I recognized the appropriate conditions in January 2014 and found ACW in cheatgrass die-offs in late February in Owyhee County, Idaho. Die-offs also occurred in northern Nevada in 2014. In a research paper, I documented larval damage and vegetation recovery (Salo 2018).

A remote sensing study has since confirmed that cheatgrass die-offs are most likely during a dry winter following a previous dry year (Weisberg et al. 2017). The lead author told me their study did not look at the effect of September precipitation.

Army cutworms are the most likely cause of cheatgrass die-offs 

A recent U.S. Geological Survey report (Remington et al. 2021) and an earlier U.S. Department of Agriculture report (Crist et al. 2019) both recognize cheatgrass die-offs as opportunities to reseed cheatgrass-invaded areas with desirable native species, but both overlook ACW as the most likely cause of die-offs.

By spring, army cutworms are big and easy to spot.

Army cutworms are the simplest, most direct cause of these events. Ranchers, who are out on rangelands in winter and at night far more than researchers and federal land managers, are familiar with ACW eating both cheatgrass and crops. Entomologists watch for ACW damage to wheat and canola, closely related to cheatgrass and weedy mustards.

The life histories of ACW and cheatgrass interact to create sporadic and spotty die-offs. To reach outbreak levels, ACW need cheatgrass seedlings for food in winter and early spring. Cheatgrass seeds need significant rain during usually-dry September to germinate in time to feed ACW. The rarity of significant rain at this time means that ACW outbreaks are relatively rare. The larvae earn their common name for their habit of marching en masse to find and consume essentially all their preferred plants--creating bare areas. 

Adult miller moths emerge in late spring.

After ACW pupate, the adult miller moths fly to high elevations, leaving no fingerprints behind. The moths spend the summer feeding on nectar and being fed upon by bears. 

The following fall, the moths catch wind currents back to low elevations. The capriciousness of wind makes it unlikely that eggs will be laid in the same place more than once. A remote sensing study found that over 80% of die-off sites do not experience die-offs the following year (Weisberg et al. 2017). 

However, both recent federal reports overlook the evidence and state that fungal pathogens cause cheatgrass die-offs. Both cite Meyer et al. 2016’s book chapter, “Community ecology of fungal pathogens on Bromus tectorum.” 

Occam’s Razor shaves away fungal pathogens

Occam’s Razor reminds us that the simplest explanation that fits the evidence is usually the correct one. Army cutworms are the simplest explanation for die-offs—with the most evidence. None of the fungi studied by Meyer et al. and described in their 2016 book chapter have been clearly linked to die-offs. They do not report studying pathogenic fungi of exotic mustards, which are also missing from die-off areas and are readily eaten by ACW.

    

Meyer et al. 2016 state that fungal pathogens “sometimes interact to increase the total impact on B. tectorum stand structure, which can result in stand failure or ‘die-off’,” (page 193). They suggest that “thick litter created by [Rutstroemiaceae] may create conditions conducive to the success of Fusarium seed rot organism the subsequent year,” (page 218). This explanation is more complex, less direct, and supported by less evidence than the ACW explanation. 

Differences between ACW and fungi in weather conditions when outbreaks occur, local patterns of damage, and local persistence point all to the former as the most likely cause of die-offs (Table 1).

Weather: Cheatgrass die-offs occur during dry weather. 

Most pathogenic fungi need wet conditions to grow, spread, and infect plants. Army cutworm outbreaks typically occur during dry weather lasting about 1½ years, broken by unusual late summer rain, to reach outbreak levels. Remote sensing work has also found that die-offs occur during drought (Weisburg et al. 2017). 

Local damage pattern: Cheatgrass die-offs are bare soil. 

Three of the five fungi discussed in Meyer et al. 2016, Ustilago bullata, Tilletia bromi, and a type of Rutstroemiaceae, infect cheatgrass without killing the plants. These organisms prevent the production of normal seeds, but do not destroy plants: they do not create the bare patches seen in cheatgrass die-offs.  

Pathogenic fungi can’t move to seek out host plants. Fungi are moved by wind or water, which typically produce spotty local patterns of fungal diseases. Some fungal diseases, such as late blight of potato, which led to the Irish potato famine, can kill essentially all plants in an area. However, these fungi leave fields of decaying plants, not bare areas. Army cutworms consume plants to bare soil. 

Local persistence: Cheatgrass die-offs usually last only one year. 

The other two fungi discussed in Meyer et al. 2016, Pyrenophora semeniperda and Fusarium spp., kill seeds in the soil; Fusarium spp. can also kill seedlings. P. semeniperda is one of many soil fungi that kill cheatgrass seeds, but the effect of this fungus on cheatgrass stands is negligible (Meyer et al. 2016, page 208). 

Fusarium spp. can be a serious problem in crops, as pathogenic fungi usually persist in an area longer than one year. For example, gardeners rotate tomatoes with other crops and plant resistant varieties to avoid Fusarium wilt (F. oxysporum). Army cutworms, in contrast, leave the scene after creating die-offs, and winds rarely carry moths back to the same spot in later years.

Previous reports of cheatgrass die-offs

Meyer et al. 2016 discuss previous reports of abnormal cheatgrass growth. However, neither appears to have been caused by pathogenic fungi. The first seems to describe an ACW outbreak; the second, a dense stand of cheatgrass. 

Report 1: Cheatgrass winterkill in southwest Idaho in 1960 

Meyer et al. 2016 cite winterkill of cheatgrass observations by Piemeisel 1938; the source is actually Klemmedson and Smith 1964. Klemmedson’s original photos and descriptions of the event are archived at the Rocky Mountain Research Station (below). 

Klemmedson documented the 1960 cheatgrass die-off

Klemmedson describes an event in 1960 near Glenns Ferry, Idaho strikingly similar to the 2003 and 2014 die-offs: large, litter-covered bare areas that end abruptly normal-appearing cheatgrass; unaffected perennial Sandberg bluegrass (Poa secunda); and a summer cover of Russian thistle (Salsola kali). I have suggested that this event, and a similar one in 1949 in Payette County, Idaho, were caused by ACW outbreaks (Salo 2017, slides 26, 27). 

Glenns Ferry, Idaho recorded conditions before the 1960 die-off strikingly similar to those before the 2003 and 2014 ACW outbreaks and cheatgrass die-offs: a previous year of dry weather, heavy September rain, and a dry fall and early winter from October through January (Table 2).

Klemmedson and Smith 1964 suggest that desiccation or pink snow mold caused the 1960 event and cite Sprague’s 1953 description of the mold. According to Sprague, Microdochium nivale = Fusarium nivale) attacks grasses “in late winter, either under the snow or during raw winter weather.” The attacked leaves turn into “pink or straw-colored mats, which dry to paper films,” (page 271). 

However, snow and raw winter weather would have been unlikely during the dry winter of 1959–1960. In addition, Klemmedson’s photos and descriptions show the litter that often covers cheatgrass die-offs, not the papery films of pink snow mold. The weather conditions, photos, and descriptions all point to ACW, rather than pink snow mold, as the cause of the 1960 die-off. 

Report 2: Cyclic succession on abandoned cropland in southern Idaho in 1941 

Meyer et al. 2016 cite Piemeisel’s 1951 report of “degenerate” cheatgrass stands “in which seed production was prevented and stand loss ensued,” (page 195). Meyer et al. 2106 continue, “He credited this effect to increasing intraspecific competition, but it seems plausible that plant pathogens…could have played a role. This process is very similar to the ‘die-off’ or stand failure in B. tectorum monocultures documented in recent years.” 

Piemeisel 1951 describes dense cheatgrass stands

However, the pattern Piemeisel describes (right), and that Meyer et al. say is similar to cheatgrass die-offs, is the opposite of that seen on cheatgrass die-offs. 

Piemeisel reports islands of cheatgrass, “as small as a few feet in diameter…in parts of a field in 1941 where downy chess [=cheatgrass] was beginning to establish,” (page 56). The “degenerate” stand at the center was “a disk composed of a very dense, short growth of immature plants…with barely emerging heads.” Plants in the outer portions of the islands were progressively more robust as the plant density decreased. 

Cheatgrass die-offs, on the other hand, are large bare areas cut out of normal-appearing cheatgrass stands—the inverse of Piemeisel’s islands. He certainly seems to describe intraspecific competition in cheatgrass, not a die-off.

Army cutworms are the most likely cause of cheatgrass die-offs

Researchers and ranchers have watched the larvae consumer cheatgrass, mustards, and the leaves of native shrubs (Salo 2018). The life cycles of cheatgrass and ACW, driven by weather, interact to produce periodic larval outbreaks that create die-offs sporadically across low, dry areas in the intermountain west.

When we understand ACW enough to predict their outbreaks, we’ll know when and where to look for die-offs. My “trapline” in Owyhee County, Idaho monitors fall miller moth flights; nearby weather stations in Grand View and Murphy record precipitation. When conditions that lead to ACW outbreaks occur though the end of January, it’s time to start looking for larvae and die-offs. Reseeding die-offs with desirable native species will let the sown plants get started while there are few cheatgrass seeds in the soil to sprout and compete with them.

Literature cited

Crist et al. 2019. Science Framework for Conservation and Restoration of the Sagebrush Biome: Linking the Department of the Interior’s Integrated Rangeland Fire Management Strategy to Long-term Strategic Conservation Actions. Part 2. Management applications. USDA RMRS-GTR-389. 237 p.

Hammon. 2003. Personal communication.

Klemmedson and Smith. 1964. Cheatgrass (Bromus tectorum). Botanical Review 30:226–262.

Meyer et al. 2016. Community ecology of fungal pathogens on Bromus tectorum. In Germino et al. editors, Exotic Brome-Grasses in Arid and Semiarid Ecosystems of the Western US.

Piemeisel. 1938. Changes in Weedy Plant Cover on Cleared Sagebrush Land and Their Probable Causes. USDA Technical Bulletin No. 654. 

Piemeisel. 1951. Causes affecting change and rate of change in a vegetation of annuals in Idaho. Ecology 32: 53–72.  

Remington et al. 2021. Sagebrush Conservation Strategy—Challenges to Sagebrush Conservation. U.S. Geological Survey Open-File Report 2020–1125. 327 p.

Salo and Zielinksi. 2004. Cheatgrass dieoffs: of drought, cutworms, and bears? (poster). Society for Range Management Annual Meeting, Jan. 24–30, Salt Lake City, UT.

Salo. 2017. Army cutworms (Euxoa auxiliaris) consume winter annual plants and shrub foliage. Society for Range Management Annual Meeting, Jan. 29–Feb. 2, 2017, St. George, UT.

Salo. 2018. Army cutworm outbreak produced cheatgrass “die-offs” and defoliated shrubs in southwest Idaho in 2014. Rangelands 40(4):99–105.

Sprague. 1953. Root and crown rots of the grasses. USDA Yearbook of Agriculture 267–272. 

Weisberg et al. 2017. Development of remote sensing indicators for mapping episodic die-off of an invasive annual grass (Bromus tectorum) from the Landsat archive. Ecological Indicators 79:173–181

Pollinators could benefit from the pandemic

My recent piece for Big Sky Journal describes how landscaping with native plants can help our declining insect pollinators.

I started my research by talking with my friend Diane Jones, owner of Draggin' Wing High Desert Nursery in Boise, ID. Jones remembered that 2020 at first looked like a tough year in the plant business. Organizations cancelled their spring sales, where Jones usually sells plants early in the season. 

But, as people spent more time at home, they noticed their yards could use some attention. As they saved time and money on commuting and socializing, people started on yard projects. They went to  Draggin’ Wing for plants, ideas, and advice. “People kept coming and coming,” Jones remembered. 

Looking back over her almost 20 years in business, Jones has seen a steady increase in the use of native plants in landscaping. Concern over the precipitous drop in our pollinators is driving much of the interest in natives, Jones said. Much of the pandemic yard work involved creating pollinator-friendly yards.

Monarch butterflies (Danaus plexippus) are, sadly, our best-known pollinator in peril. Monarchs rely solely on milkweeds for raising their young. As the plants’ roadside and wildland habitats are converted to other uses, and adjacent crops are sprayed with herbicides, milkweeds—and monarchs—are disappearing.

Native milkweeds:  Asclepias tuberosa, A. speciosa, A. fascicularis (© Draggin' Wing High Desert Nursery)

We have fewer monarch butterflies west of the Rockies and they seem to be declining faster than those in the east. While eastern monarchs overwinter in Mexico, their western siblings head to groves on the southern California coast. Where over a million orange-and-black butterflies covered the trees as recently as 1997, fewer than 30,000 were found three years ago. This past winter, western monarchs numbered fewer than 2,000. Researchers feared the worst. 

However, while we were adapting to working from our living rooms and remembering to unmute on Zoom, monarch butterflies were adapting to living in San Francisco and laying eggs. At least some of the missing monarchs appear to have stopped in the Bay Area to create a pandemic baby boom instead of continuing on to their winter homes. (The predicted human baby boom turned out to be a baby bust.)

David James, at Washington State University, has tagged and tracked western monarch butterflies since 2012. In a recent academic paper, James says that the new stay-home-and-reproduce behavior is likely due to warmer temperatures in northern California and the availability of non-native African milkweeds, which are widely planted in the area. 

This isn’t the first time James has seen monarchs change their migration and breeding habits. Four decades ago, he saw a similar shift among monarchs in Australia, where the insects were introduced. James is hopeful that western monarchs will adapt and thrive, but cautions that we don’t yet understand all the challenges they will face. 

If you didn't finish your pandemic yard work, you can still help pollinators by planting native milkweeds for our iconic monarch butterflies—and the more than 100 other insects that use them. Draggin’ Wing High Desert Nursery carries the three kinds of milkweed pictured here.

How did wheat take over the world?

I was smitten by the wheat fields of Montana in high school. My dad and younger brother and I were headed to Glacier National Park when the rolling fields stretched from the horizons to capture my heart. I was sure that some day I'd live that far from town and drive tractors on fields that big. But I didn't wonder why there was so much wheat in Montana or how the crop I wanted so badly to raise affected the ecology of the area. 

Montana State University professor Catherine Zabinsky has thought about those things and wrote a book about them. My recent review  of Amber Waves: The Extraordinary Biography of Wheat, from Wild Grass to World Megacrop is in the current Issues in Science and Technology. 

I was living 35 miles from town when I wrote the review, but I wasn't on a Montana wheat farm.

Yellowstone cutthroat comeback

Many stories about troublesome exotic species are tragedies. Some of the endings have been written: an exotic fungus wiped out the American Chestnut and brown tree snakes have driven more than half of Guam’s birds and lizards to extinction. Other stories are long-running dramas: water hyacinth clogs waterways in the southeast U.S. and Mediterranean fruit flies damage fruits, nuts, and vegetables around the world.

My latest article, The Cutthroat Comeback in Big Sky Journal, is an inspiring fish tale of gaining ground against a troublesome exotic species. Lake trout are large and efficient predators that eat native Yellowstone cutthroat trout. Yellowstone National Park is using population modelling and commercial gillnetters to reduce lake trout numbers in Yellowstone Lake, while researchers develop effective methods for controlling lake trout eggs. With the help of their friends, the native cutthroat trout are reclaiming their high, cold home waters.

April is the best time to spot cheatgrass die-offs

Cheatgrass die-offs might sound alarming, but few people in the West are sorry to see cheatgrass gone. This exotic annual grass spreads into sagebrush grasslands where native shrubs and grasses are missing or weakened. When fire comes, dried cheatgrass burns readily and spreads fire among native plants, killing or damaging them. But when cheatgrass dies out in an area, seeds of longer-lived native plants can get started without competing with this invasive exotic grass.

Cheatgrass die-offs are easy to spot in April. For bonus points, you might also find the army cutworms that create them by eating each grass shoot as it comes up. The bare die-offs stand out against the surrounding cheatgrass, which is having a growth spurt in the warm temperatures and plentiful moisture of spring.

Die-offs can be almost completely bare, where native shrubs were killed by fire and cheatgrass moved in. These are an alarming sight.

Die-offs in areas with native shrubs (sagebrush and saltbushes) are a little harder to spot. You'll see shrubs without any herbaceous plants growing underneath them. The shrubs might be missing a good portion of their leaves, as they are here.

To find cheatgrass die-offs look for:

1. Bare areas where cheatgrass grew in previous years, with or without shrubs.

2. Sharp borders to bare areas, changing abruptly from bare soil to normal-looking cheatgrass.

3. Native shrubs, if present, missing most of their leaves: army cutworms also climb shrubs and eat leaves.

4. Gray-green larvae hiding during the day under cowpies and plant litter or in the soil.

If you find cheatgrass die-offs, seize the opportunity to seed native plants and revegetate these areas.

Light on the Devils: Coming of Age on the Klamath

Light on the Devils pulled me in and made me want to know more about the people and places it described.

After reading Louise Wagenknecht's book, I searched aerial photos to track her family's moves from Hilt, California, downriver to Happy Camp, and then to Seiad Valley during the 1960s. I wondered where her high school classmates committed suicide, died in vehicles rolling off mountain roads, gave birth between junior and senior years, and dropped out to work in the timber.

Then I searched topographic maps for the Three Devils peaks of the book's title. I traced the path of the Klamath River through the forests of northern California, where Wagenknecht's stepfather laid out clear cuts in old growth timber. I looked for the roads and bridges built to "get out the cut"—infrastructure the cut didn't pay for. I planned a road trip to see how the clear cuts were filling in and search for remaining old growth trees.

Between the crescendos of tragedy—floods, fires, and logger deaths—Wagenknecht records the steady bass beat of disappearing old growth timber and degraded watersheds and wildlife habitat. At the height of the post-World War II timber boom, a neighbor remembers California condors and Wagenknecht's family moves into a house with a large supply of kindling: veneer peeled from old growth pine.

But I didn't need a guide to follow Wagenknecht's journey from science geek kid who loved animals, through her dismay and disbelief when her stepfather tells her women can't be veterinarians and his co-worker tells her women can't fight fires—after she had just fought one, and on to her decision to attend college. She saw the contradictions in growing up female and told the world, and herself, that she wanted to be a science teacher—a nice, acceptable job for a high school valedictorian.

At home, Wagenknecht's stepfather decreed she would spend time doing her hair and nails, plus three agonizing hours a week holding up the walls and reading album covers at high school dances. But she, the oldest child, was also his hunting and fishing companion. She was the one who helped him fell trees for firewood and then split, load, and unload the fuel. She made a good hand and he made a good teacher. Despite their conflicts at home, they reached détente in the woods.

This book describes Wagenknecht's path toward a career with the Forest Service—which included fighting fires—and her détente in the woods with society.

Hunting the ancient agave

Agaves spend much of their lives as toothy, spikey plants we avoid while hiking. Most agaves bloom only once, throwing their heart and carbohydrates into raising an exorbitant stalk of fragrant flowers that ripen into dry fruits stacked with flat, black seeds. We're captivated by the transformation from spikey to florescent.

Wendy Hodgson makes agave seeds fly in Cascabel.

However, for over 8,000 years, people have harvested agaves just before flowering, usurping the carbohydrates in the stem for human food and beverage. The leaves also provide fibers for rope, baskets, bags and sandals.

Although people have used agaves continuously in Mexico, the plants' cultivation and culture faded from what is now Arizona by the 18th century. Residents of the Middle San Pedro Valley have periodically wondered if we could grow agave in our area. After all, Sobaipuri Native American agave fields dot our community, and wild agaves grow in the valley.

When this question came up again last winter, we learned that survivors of pre-Columbian cultivated agave had recently been found in our area. Wendy Hodgson and her colleague Andrew Salywon, both with the Desert Botanical Garden, named the new species Agave sanpedroensis. Wendy and Andrew even suggested that this low-water-use crop could be revived to diversify agriculture and stimulate new industries in the U.S. Southwest.

Agave sanpedroensis.

We contacted Wendy, who was as eager as we were to connect. In January 2019, she told a packed Cascabel Community Center about the eight (and counting) species of formerly cultivated agaves she and her colleagues have found. In February, dozens of people braved blustery weather on a follow-up field trip. Wendy explained how she recognizes the different species of agaves and outlined how we can help her survey the plants in our valley.

Wendy and Andrew work with archaeologists, who have documented extensive agave fields at many sites in central and southern Arizona. Rows of rocks along contour lines slowed runoff across the fields and rock mulch piles reduced evaporation. Near the fields, archaeologists find knives used for removing leaves and roasting pits where agave stems, or "heads," were baked to sweetness.

The plants we see today are genetically identical to those the Sobaipuri and other groups developed to suit their needs. Unlike their wild relatives, these cultivated agaves produce little or no seed. Instead, the plants reproduce vegetatively, usually by pupping, to produce offspring identical to the parent. Cultivated agaves are also sweeter than their wild relatives and their leaves are easier to cut.

After surviving drought, insects, and rodents for centuries--or millennia--climate change and development threaten these rare plants. As cultivated crops, they fall into a protection gap: the Endangered Species Act protects only wild species and the Archaeological Resources Protection Act protects only inanimate artifacts.

Wendy fears that these recently discovered, living storehouses of information on people, culture, and plants could be lost. She is helping residents of the Middle San Pedro Valley learn about the ancient agaves in their area and work to protect them.