Wednesday, May 19, 2021

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. 
• Die-offs are opportunities to seed cheatgrass-invaded areas with native species while the cheatgrass seed bank is relatively low. This gives natives a head start. 
• Two recent federal reports overlook army cutworms (ACW) as a likely cause of die-offs. 
• ACW are the simplest, most direct explanation for these episodic events. The larvae frequently damage crops related to cheatgrass, easily consume seedling plants to the soil, and later migrate after pupating.
• 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. 
• The two federal reports state that fungal pathogens cause cheatgrass die-offs. Fungi have not been clearly linked to die-offs and require a more complex and less direct explanation for these events.

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Cheatgrass (Bromus tectorum) die-offs are bare areas, often covered with gray plant litter, within stands of normal-appearing 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 gone. 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 ACWs consume cheatgrass in early 2003. Entomologists reported 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 ACWs 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 evaluate the effect of September precipitation.

Likely causes 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 army cutworms as a likely cause of die-offs.

By spring, army cutworms are big and easy to spot.
Army cutworms provide the simplest, most direct explanation for cheatgrass die-offs. The conditions that lead to larval outbreaks occur relatively rarely in the west, producing sporadic outbreaks and die-offs. 

Army cutworm behavior matches the damage and spatial patterns seen in cheatgrass die-offs. The larvae consume above-ground parts of plants and are well-known pests of wheat and canola, closely related to cheatgrass and weedy mustards. Army cutworms earn their common name for their habit of moving en masse to find and consume essentially all their preferred plants in an area. 

After ACWs pupate, the adult miller moths fly to high elevations for the summer, where they feed on nectar and are fed upon by bears. The following fall, the moths catch wind currents to migrate back to low elevations. The capriciousness of wind makes it unlikely that outbreaks will appear in the same place more than once. A remote sensing study found that over 80% of sites where die-offs occur in one year do not experience die-offs the following year (Weisberg et al. 2017). 

Adult miller moths emerge in late spring.
Both recent federal reports 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 reminds us that the simplest explanation that fits the evidence is usually the correct one. Army cutworms provide a simpler, more direct explanation for cheatgrass die-offs than do fungal pathogens. 

 None of the five fungi being studied by Meyer et al. and described in their 2016 book chapter have been clearly linked to die-offs. There are also no reports of their studying pathogenic fungi of exotic mustards. Mustards are also missing from die-off areas and are readily consumed by ACWs. 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. Two fungi, Pyrenophora semeniperda and Fusarium spp., kill seeds in the soil; Fusarium spp. can also kill seedlings. 

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 fewer field observations than the ACW explanation. 

Differences in weather conditions when outbreaks occur, local patterns of damage, and local persistence between ACWs and fungi point 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 1 ½ years, broken by unusual late summer rain, to reach outbreak levels. 

Local damage pattern: Cheatgrass die-offs are bare soil. 
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
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 (Fusarium oxysporum). Army cutworms pupate and fly away after creating die-offs; winds usually carry them to different areas for egg laying the following fall.

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 ACWs, 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 described dense cheatgrass

However, the pattern Piemeisel 1951 describes (right) is the opposite of that seen on cheatgrass die-offs. He 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. Die-offs 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.

The most likely cause of cheatgrass die-offs

Army cutworm outbreaks are the most likely cause of cheatgrass die-offs. Outbreaks and die-offs coincide in time and space across low, dry areas in the intermountain west. Army cutworms eat cheatgrass and are capable of finding and consuming essentially all plants in an area.

When we understand army cutworms 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; weather stations in Grand View and Murphy record precipitation. When conditions that lead to army cutworm outbreaks occur though the end of January, it’s time to start looking for larvae and die-offs.

Literature cited

Hammon. 2003. Personal communication.
Klemmedson and Smith. 1964. Cheatgrass (Bromus tectorum). Botanical Review 30:226–262.
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.
Sprague. 1953. Root and crown rots of the grasses. USDA Yearbook of Agriculture 267–272. 

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