Over-Summering Ecology of the Wheat Curl Mite (\u3ci\u3eAceria tosichella\u3c/i\u3e Keifer)

The wheat-mite-virus complex is a consistent and significant threat to winter wheat production in the western Great Plains. This complex consists of three viruses (Wheat streak mosaic virus, Triticum mosaic virus, and Wheat mosaic virus that are transmitted by the wheat curl mite (Aceria tosichella Keifer). Yield impacts from this complex are typically associated with the presence of volunteer wheat that emerges prior to harvest as a result of hail occurring during the heading stages of wheat in early summer. Historical literature on pre-harvest germination has been primarily focused on accelerating breeding programs; however, critical gaps in knowledge exist on pre-harvest germination when evaluating risk for the wheat-mite-virus complex. A study was designed to evaluate pre-harvest germination potential of winter wheat by collecting heads at 7-9 day intervals beginning at the water-ripe stage until wheat harvest. In addition, risk categories were established based on the speed of germination because field germination will be limited by moisture availability. A second study was conducted in the field to evaluate the impact of environmental conditions on pre-harvest germination. Results indicate that risk for pre-harvest germination begins at the late milk stage with increasingly greater risk for germination up to harvest. In addition, risk for germination is highly dependent on available moisture following hail events. Historical observations, as well as anecdotal evidence indicate that other hosts besides wheat can support WCM during the over-summering period; however, the risk of these hosts to fall planted wheat is poorly understood. Greenhouse reproductive studies, a field study on mite movement and virus impact, and a weed survey were conducted to evaluate the risk potential of over-summering hosts. Results showed that barnyard grass is a high-risk over-summering host for the wheat-mite-virus complex; however, its frequency is relatively low across the central Great Plains. Green foxtail was comparatively a lower risk host, but it was found in higher frequencies in the weed survey. Foxtail millet, another summer annual, showed significant mite movement under field conditions; however, virus impact was minimal. In addition, greenhouse studies were a good predictor of field potential of all of the over-summering hosts with the exception of foxtail millet. The studies presented in this document provide critical information to better understanding the over-summering ecology and risk of the wheat-mite-virus complex. Advisor: Gary L. Hei

Transmission of \u3ci\u3eTriticum mosaic virus\u3c/i\u3e and its Impact on the Biology of the Wheat Curl Mite \u3ci\u3eAceria tosichella\u3c/i\u3e Keifer (Eriophyidae), and an Evaluation of Management Tactics for the Wheat Curl Mite and the Wheat-Mite-Virus Complex

The wheat-mite-virus complex is an important production constraint to winter wheat production in the Great Plains, and consists of three viruses; wheat streak mosaic (WSMV), wheat mosaic (WMoV) and Triticum mosaic virus (TriMV). Synergistic interactions between these viruses have resulted in increased rates of replication and transmission of viruses, thus increasing the potential impact on wheat yields. The wheat curl mite (WCM), Aceria tosichella Keifer is the only known vector of the viruses within the wheat-mite-virus complex. Currently, three colonies of WCM have been characterized by differential responses to mite resistant genes (biotypes) in wheat and differential transmission of WMoV. A study was designed to determine TriMV transmission for these various wheat curl mite colonies. For each source plant, individual mites were transferred to 10 separate test plants and virus transmission determined via ELISA. Results indicate that TriMV is only transmitted by one of the three wheat curl mite colonies using single mite transfers. An additional study was conducted to determine the impact of TriMV on the biology of the WCM. TriMV infected and uninfected plants were infested with 10 mites from each colony with population counts being taken every seven days. Results indicated that TriMV had a negative impact on the reproductive potential of the WCM. The results demonstrate the importance of the mite source on virus epidemiology. Management tactics to reduce the impact of the wheat-mite-virus complex have focused primarily on the control of volunteer wheat; however, these tactics are not always effective at reducing yield losses. A field study was conducted from 2007-2011 to determine the impact of the combination of resistant variety and planting date on wheat yields under high virus pressure. Results indicated that both management tactics had a significant impact on yield; however, the combination of tactics provided the greatest yield potential under high virus pressure. Advisor: Gary L. Hei

Population Dynamics of the Wheat Curl Mite (Acari: Eriophyidae) During the Heading Stages of Winter Wheat

The wheat curl mite (Aceria tosichella Keifer) is the only known vector of three viruses in wheat—Wheat streak mosaic virus, Wheat mosaic virus, and Triticum mosaic virus. The economic impact of this disease complex is linked to the presence of suitable hosts prior to winter wheat maturing in early summer and the movement of wheat curl mite from wheat to oversummering hosts prior to wheat harvest. Previous research has documented the prevalence and density of mite populations on maturing wheat heads; however, these studies were limited to a few late stages of wheat. A study was conducted to evaluate mite population densities across all stages of head development to determine when wheat curl mites are most abundant and the relative increase in abundance over time. In addition, a study was conducted to evaluate the impact of rainfall on mite populations during wheat heading. A final study was conducted to determine the potential for direct infestation of seedlings germinating from wheat curl mite-infested wheat heads. Results showed a rapid buildup in mite populations from low densities in early heading and peaking at the hard dough stage, with nearly all wheat heads having some mite presence. In addition, high mite populations resulted in direct infestation of germinated seedlings from the early through hard dough stages. Rainfall applications had no observable impact on mite population densities in wheat heads. These results demonstrate the increased potential for mites to infest hosts prior to winter wheat maturing and illustrate the increased risk for these hosts to serve as oversummering hosts

Soybean Gall Midge in Nebraska

Soybean gall midge (Resseliella maxima Gagné) was described in 2019 as a new insect species in Nebraska, due to observations of widespread early season injury in eastern Nebraska, eastern South Dakota, western Iowa, and southwest Minnesota soybean fields (Gagné et al., 2019). Since its discovery, soybean gall midge has been causing significant injury and yield losses in soybean in eastern Nebraska. Although only recently identified, soybean gall midge is not likely new to the north- central region of the U.S. In 2011, orange larvae were documented in some isolated fields in northeast Nebraska that had received hail damage during the early half of the growing season. Similar reports were made in 2016 and 2017 in eastcentral Nebraska. Prior to 2018, reports of orange larvae in soybean were confined to the late reproductive stages of a few dead or dying plants. In 2018, several observations were made that raised concerns that soybean gall midge should be designated as a pest of soybean (McMechan et al. 2021). Unlike previous years where damaged plants were found later in the growing season, injured plants were observed in late June and early July. Soybean plants with larval presence exhibit signs of wilting and death, with the greatest frequency of symptomatic plants occurring along field borders adjacent to fields that had been planted to soybean the previous year. In many cases, the presence of dense vegetation (trees, uncut bromegrass, and/or shrubs) along field borders was associated with an increased frequency and intensity of plant injury. As a new species, several knowledge gaps must be bridged in order to development an integrated pest management strategy for soybean gall midge (McMechan et al. 2021) Year- to- year variability in the duration of emergence and injury from soybean gall midge has made it difficult to identify an effective control strategies. In 2018, soybean plants were hand- harvested from a heavily infested field in Saunders County, Nebraska, where a yield loss of 92%, was estimated for a section of the field in the first 100 feet from the field edge (compared to historical yields). Yield losses of 31% and 20% at 200 and 400 feet into the field, respectively, were also recorded (McMechan et al. 2021). Additional yield loss may also occur from early pod shatter from infested plants that mature ahead of the rest of the crop as well as lodging from weakened stems. As of 2020, soybean gall midge had been found in 114 counties in five states, with 39 counties infested in eastern Nebraska (Fig. 1). The distribution of soybean gall midge has increased each year since its discovery, although the extent of field injury is typically far less in newly identified counties

Evaluating Larval Movement and Infestation of Soybean Gall Midge

Soybean gall midge (SGM) Resseliellamaxima Gagnéis a newly identified insect pest of soybean (Gagnéet al 2019). As of August, 2019 a total of 92 counties across five midwestern states (Fig. 1) documented as infested. Heavily injured soybean fields show visible signs of dead or dying plants (Fig. 2a), causing significant economic losses for farmers. Heavily injured soybean plants often die prematurely with several different larval stages (Fig. 2b) still feeding within the plant. Significant concern have been raised about the role of these larvae in infesting adjacent healthy plants. To better understand the behavior and potential for larval infestation a study was designed to determine the ability of different larval instars to move on different surfaces, and their behavior in the soil. An additional study was conducted to determine the larval ability to infest soybean plan

Developing an Injury Severity to Yield Loss Relationship for Soybean Gall Midge (Diptera: Cecidomyiidae)

Soybean gall midge, Resseliella maxima Gagné (Diptera: Cecidomyiidae), is a newly identified pest confirmed on soybean, Glycine max (L.) Merr. (Fabales: Fabaceae). To date, soybean gall midge has been found in Nebraska, Iowa, South Dakota, Minnesota, and Missouri, and has caused severe economic loss to commercial fields since 2018. Much is still unknown about this pest, so research efforts have been focused on biology and management. Larvae feed on the inside of the stem just above the soil line and are difficult to access and time-consuming to sample. In order to accelerate nondestructive sampling efforts, we developed an injury rating system to quantify the severity of plant injury from soybean gall midge larvae. Research plots from 2019 and 2020 in Iowa and Nebraska were evaluated for injury throughout the growing season and yield was measured. Our objective was to describe the relationship between injury severity and yield loss caused from soybean gall midge. A nonlinear regression model was developed to validate our injury rating system and to express the relationship between season long injury severity and yield loss. Results from our analysis indicate the injury rating system we developed correlates well with yield loss caused by larvae and may be an important tool for understanding the economic impact of this emergent pest of soybeans

Wheat Stem Maggot (Diptera: Chloropidae): An Emerging Pest of Cover Crop to Corn Transition Systems

The wheat stem maggot (Meromyza americana Fitch) (WSM) is a minor pest of wheat, rye, and other grasses. In 2017, growers in Nebraska reported dead center whorls and excessive tillering in early-season cornfields that followed wheat or rye terminated after planting corn. A survey was conducted to evaluate the risk factors for this insect in cover crop to corn transition systems. In each field, management practices and the percentage of injured plants were recorded. Symptomatic corn plants were collected from each field and dissected to determine larval and plant characteristics. In a few cases, small patches of a field were planted to a cover crop to manage soil erosion, and injured plants were only found where the cover crop was present. From these observations, the hypothesis is that terminating a cover crop after planting corn allowed the WSM larva to move from the dying cover crop to corn to complete its development. Cornfields infested with WSM had a frequency of injured corn plants from 0 to 60% with yield losses estimated at 30 bushels/acre. This paper provides the first detailed documentation of WSM injury in corn and addresses important management practices that may have influenced this uncommon situation

Developing the framework for a risk map for mite vectored viruses in wheat resulting from pre-harvest hail damage

There is a strong economic incentive to reduce mite-vectored virus outbreaks. Most outbreaks in the central High Plains of the United States occur in the presence of volunteer wheat that emerges before harvest as a result of hail storms. This study provides a conceptual framework for developing a risk map for wheat diseases caused by mite-vectored viruses based on pre-harvest hail events. Traditional methods that use NDVI were found to be unsuitable due to low chlorophyll content in wheat at harvest. Site-level hyperspectral reflectance from mechanically hailed wheat showed increased canopy albedo. Therefore, any increase in NIR combined with large increases in red reflectance near harvest can be used to assign some level of risk. The regional model presented in this study utilized Landsat TM/ETMþ data and MODIS imagery to help gap-fill missing data. NOAA hail maps that estimate hail size were used to refine the area most likely at risk. The date range for each year was shifted to account for annual variations in crop phenology based on USDA Agriculture statistics for percent harvest of wheat. Between 2003 and 2013, there was a moderate trend (R2 ¼ 0.72) between the county-level insurance claims for Cheyenne County, Nebraska and the area determined to be at risk by the model (excluding the NOAA hail size product due to limited availability) when years with low hail claims (\u3c400 ha) were excluded. These results demonstrate the potential of an operational risk map for mite-vectored viruses due to pre-season hail events

Impact of Timing and Method of Virus Inoculation on the Severity of Wheat Streak Mosaic Disease

Wheat streak mosaic virus (WSMV), transmitted by the wheat curl mite Aceria tosichella, frequently causes significant yield loss in winter wheat throughout the Great Plains of the United States. A field study was conducted in the 2013–14 and 2014–15 growing seasons to compare the impact of timing of WSMV inoculation (early fall, late fall, or early spring) and method of inoculation (mite or mechanical) on susceptibility of winter wheat cultivars Mace (resistant) and Overland (susceptible). Relative chlorophyll content, WSMV incidence, and yield components were determined. The greatest WSMV infection occurred for Overland, with the early fall inoculations resulting in the highest WSMV infection rate (up to 97%) and the greatest yield reductions relative to the control (up to 94%). In contrast, inoculation of Mace resulted in low WSMV incidence (1 to 28.3%). The findings from this study indicate that both method of inoculation and wheat cultivar influenced severity of wheat streak mosaic; however, timing of inoculation also had a dramatic influence on disease. In addition, mite inoculation provided much more consistent infection rates and is considered a more realistic method of inoculation to measure disease impact on wheat cultivars

Developing the framework for a risk map for mite vectored viruses in wheat resulting from pre-harvest hail damage

There is a strong economic incentive to reduce mite-vectored virus outbreaks. Most outbreaks in the central High Plains of the United States occur in the presence of volunteer wheat that emerges before harvest as a result of hail storms. This study provides a conceptual framework for developing a risk map for wheat diseases caused by mite-vectored viruses based on pre-harvest hail events. Traditional methods that use NDVI were found to be unsuitable due to low chlorophyll content in wheat at harvest. Site-level hyperspectral reflectance from mechanically hailed wheat showed increased canopy albedo. Therefore, any increase in NIR combined with large increases in red reflectance near harvest can be used to assign some level of risk. The regional model presented in this study utilized Landsat TM/ETMþ data and MODIS imagery to help gap-fill missing data. NOAA hail maps that estimate hail size were used to refine the area most likely at risk. The date range for each year was shifted to account for annual variations in crop phenology based on USDA Agriculture statistics for percent harvest of wheat. Between 2003 and 2013, there was a moderate trend (R2 ¼ 0.72) between the county-level insurance claims for Cheyenne County, Nebraska and the area determined to be at risk by the model (excluding the NOAA hail size product due to limited availability) when years with low hail claims (\u3c400 ha) were excluded. These results demonstrate the potential of an operational risk map for mite-vectored viruses due to pre-season hail events