Hessian fly (Diptera: Cecidomyiidae) virulence in Louisiana: assessment of field populations from 2023 to efficacy of 27 Hessian fly resistance genes in wheat

Abstract

The Hessian fly, Mayetiola destructor (Say), is one of the most important insect pest plaguing wheat (Triticum aestivum, L) producers across the United States and around the world. Genetic resistance is the stalwart for control of Hessian fly. However, new genotypes (biotypes) arise in deployment of wheat containing resistance genes, so field populations must be evaluated periodically to provide information on the efficacy of those deployed genes. Louisiana (LA), with its diverse agricultural landscape, is not exempt from the challenges posed by this destructive pest. We previously documented the resistance response of wheat lines harboring Hessian fly resistance (H) genes against field populations collected in 2008 from across the southeastern United States, including Iberville Parish, LA. In the spring of 2023, we reevaluated the resistance response of 27 H genes from the field populations collected from Iberville Parish, LA, and compared the results with those observed in 2008. Sixteen H genes showed comparable resistance to the field populations from both years. While 3 of the H genes, H11, H23, and H24, showed a significant decrease in resistance, 2 genes, H16 and H31, had marked increase in resistance. Furthermore, 6 additional H genes were evaluated in 2023, with 4 showing >70% resistance. Our results clearly identify a total of 20 H genes that are moderate to highly effective against the 2023 Hessian fly population from Iberville Parish, LA. The resistance response documented in this study offers valuable information to wheat breeders in the region for effective management of this insect pest.

Introduction

The Hessian fly (Hf), Mayetiola destructor (Say) (Diptera: Cecidomyiidae), is a small, dipteran insect known for its devastating effects on wheat crops. Native to Europe, it was introduced to North America in the late 18th century, likely through contaminated straw bedding used by Hessian soldiers during the American Revolutionary War (Pauly 2002, Smiley et al. 2004, Watson 2005, Alvey 2009, Morton 2013). Since then, it has become a formidable adversary to wheat farmers. To date, the most successful control strategy to avoid wheat yield loss is genetic resistance in wheat, Triticum aestivum L. (Ratcliffe and Hatchett 1997). Resistance in wheat is controlled by single resistance (R) genes that are completely or partially dominant (Gallun 1977, Harris et al. 2003). Insect virulence is controlled by recessive genes at single loci and work on a gene-for-gene basis with resistance (Hatchett and Gallun 1970, Formusoh et al. 1996).

Planting wheat cultivars harboring Hf resistance (H) genes has continued to provide the most economical and environmentally sound strategy of control (Cambron et al. 2010). To date, 37 H genes (designated H1–H36 and Hdic) have been identified with several deployed for commercial production (Liu et al. 2005, Zhao et al. 2020, Prather et al. 2022). However, Hf populations are able to overcome wheat resistance when deployed as single H gene cultivars. Selection pressure placed on fly populations as a result has created new insect genotypes within 6–8 yr of the release of the cultivar (Anderson and Harris 2006). These new genotypes of Hf challenge the effectiveness of formerly resistant wheat, particularly in the southeastern United States where wheat is widely grown for forage and bedding (Buntin and Raymer 1989a, 1989b, Morton et al. 2011).

The objective of the current study was to assay the response of 27 H genes to a population of fly from Louisiana (Iberville Parish) collected in the spring of 2023 and compare it to a population of fly collected in 2008 from the same Parish. This snapshot of evolution will identify the genes that are efficacious for the protection of wheat in this part of Louisiana as well as identify H genes that are no longer effective or show waning effectiveness.

Materials and Methods

Field collected wheat samples infested with Hf were received in May 2023, from a wheat field near Baton Rouge, LA, in Iberville Parrish. The samples were placed in 2 gallon plastic boxes and misted with water regularly to allow for adult emergence in the greenhouse. Adults were then transferred to 4-inch pots containing ‘Newton’ wheat, which has no Hf resistance genes. Plants were maintained in a growth chamber at 18 °C with a photoperiod of 16:8 (L:D) h after infestation for 14–16 days, to allow for larval pupation.

Twenty-seven wheat lines containing known H genes were seeded in 2 replicates in flats as previously described (Chen et al. 2009, Cambron et al. 2010). Fifteen seeds per line were seeded in 2 flats per replicate. These lines carried the following H genes: H9, H10, H11, H12, H13, H14, H15, H16, H17, H18, H19, H20, H21, H22, H23, H24, H25, H26, H28, H31, H32, H33, and H34 (Table 1). Each flat also included wheat lines harboring H3, H5, H6, and H7H8 genes as differential checks as previous work has shown no resistance in these 4 genes to known Hf populations in the southeastern United States, similar to the methodology used in previous work (Cambron et al. 2010). All wheat lines are maintained and increased at the USDA-ARS Crop Protection and Pest Control Research Unit in West Lafayette, IN. The 4 flats were held in a growth chamber at 18 °C with a photoperiod of 16:8 (L:D) h. When the seedlings reached the 1.5 leaf stage, with the second leaf starting to emerge (approximately 9 days after planting), individual flats were caged and 150 gravid females from the field sample were aspirated from the plastic boxes and released under the netting. Fly material was removed after allowing for oviposition for 3 days. Data were recorded on day 14 postinfestation and recorded as the number of resistant and susceptible plants (Table 1). Plants were rated as resistant (R) if they were not stunted, exhibited a normal growth, and when dissected contained dead red first-instar larvae as shown in Nemacheck et al. (2023). Plants were deemed susceptible (S) if they contained living larvae and exhibited stunting and the darker green color seen with infestation as shown in Nemacheck et al. (2023). Plants with no dead larvae were recorded as escapes and discarded from analysis. The percentage of resistant plants was calculated by dividing the resistant plants by the total number of plants tested carrying the H gene in question. The level of efficacy for the H genes to protect wheat was rated as described by Chen et al. (2009). With this system, H genes are defined as effective if they confer ≥80% resistance to a field population. H genes conferring 50%–80% resistance are defined as moderately effective, and H genes conferring <50% resistance are defined as ineffective.

A chi-square analysis was done to compare the H gene resistance/susceptibility profiles and those with a P ≤ 0.05 were considered to be genes whose profile in this population differs significantly between the collections from 2008 and 2023. Hierarchical Cluster analysis was performed using JMP Ver. 15 (SAS Institute Inc., Cary, NC, USA), to determine whether the H genes would cluster according to the efficacy of Hf resistance they provide against the field population used in the current study. The cluster data are represented as a dendrogram.

Results

The resistance response of 27 wheat resistance (H) genes was evaluated against the Hf field populations collected from Iberville Parish, LA, in 2023 and compared with the resistance response observed in 2008 (Cambron et al. 2010) (Table 1). Our results revealed 16 genes (H3, H5, H6, H7H8, H9, H10, H12, H13, H14, H17, H18, H21, H22, H25, H26, and H32) showing comparable phenotypic profile against Hf field populations collected from the LA region in both years. Of these, except for H3, H5, H6, H7H8, H10, and H14, all other genes exhibited effective resistance, having >80% plants that showed resistance to the field populations (Table 1). The 4 H genes used as differential checks (H3, H5, H6, H7H8) had a similar response to the LA field populations collected from both years (Table 1). The genes H10 and H14 showed moderately effective resistance with slightly over 50% in both years (Table 1). Three of the genes H11, H23, and H24 showed a dramatic decrease in resistance in the 2023 screening when compared with evaluations done in 2008 and significantly decreased from 44% to 7% (P = 0.004), 73% to 41% (P = 0.04), and 100% to 57% (P = 0.001), respectively. Interestingly, there was a significant increase in resistance observed in the lines with genes H16 and H31 from 35% to 91% (P = 0.00001) and 69% to 100% (P = 0.004), respectively (Table 1). The current study (2023) also included 6 additional H genes (H15, H19, H20, H28, H33, and H34) that were not included in the previous (2008) study. Of these 6 genes, H15 and H34 showed resistance of 57% and 17%, respectively, H19 showed resistance > 70% and H20, H28, and H33 showed 100% effective resistance (Table 1). Cluster analysis revealed 3 main groups based on the effectiveness of H gene resistance to Hf population (Fig. 1). These groups were categorized as (i) Ineffective: genes showing <40% Hf resistance; (ii) Effective-genes showing >80% Hf resistance; and (iii) Moderately effective: genes showing 40%–80% Hf resistance (Fig. 1).

Discussion

Currently, H genes in wheat are deployed as single gene releases in cultivars in the wheat-producing areas within the United States to combat the menace of Hf infestation. While this is the most economical means of controlling the insect, the result and challenge is the rapid rise in virulent genotypes of Hf that overcome the H gene-mediated resistance upon deployment. Since the effectiveness of H genes breaks down within years of deployment (Ratcliffe and Hatchett 1997), along with the fear of development of highly virulent Hf genotypes, it is crucial to regularly evaluate the field populations of Hf to determine the efficacy of resistance with the available H genes.

Over the years, several studies have been undertaken to document the changes in H gene resistance response to Hf populations collected from various regions within the United States and around the world (Ratcliffe et al. 2000, Bouhssini et al. 2009, Chen et al. 2009, Cambron et al. 2010, Garcés-Carrera et al. 2014, Shukle et al. 2016). Ratcliffe et al. (2000) analyzed the biotype composition of Hf populations from the southeastern, midwestern, and northwestern United States and their virulence to resistance genes in wheat. Biotype L was identified to represent a significant percentage of most Hf field populations and the wheat cultivar ‘INW9811’, harboring H13 gene, was highly effective against most Hf populations tested from the eastern and northwestern United States. In a study by Bouhssini et al. (2009), out of the 28 H genes tested, only wheat lines ‘Cando’ and ‘KS93WGRC26’ harboring the resistance genes H25 and H26, respectively, showed effective (100%) resistance against Hf population collected from Syria. The developed Hf virulence diversity matching the genetic diversity in wheat host is attributed to the co-existence of wheat and Hf in the fertile crescent region (Bouhssini et al. 2009). Of the 21 H genes tested by Chen et al. (2009) against 6 Hf populations, collected from different regions in the United States (TX, OK, and KS), only 5 genes including H13, H21, H24, H26, and Hdic, conferred a high level (>80%) of resistance to all field collections. The remaining 16 H genes (H3, H4, H5, H6, H7H8, H9, H10, H11, H12, H16, H17, H18, H22, H23, H25, H31) exhibited variability in their resistance level depending upon which Hf population they were tested against (Chen et al. 2009). In another study, similar variation in Hf virulence was observed among 5 regional populations collected from 3 counties in TX and 2 counties in OK (Garcés-Carrera et al. 2014). This difference was likely owing to the difference in the biotype composition of the 5 Hf field populations. Despite this observed variability in virulence, some resistance genes including H12, H13, H17, H18, H22, H25, H26, and Hdic, conferred resistance against all the Hf populations collected from 5 counties (Garcés-Carrera et al. 2014). Several genes (H12, H18, H24, H25, H26, and H13) showed effective resistance against Hf collections from various regions within southeastern United States (NC, SC, GA, AL, and LA) in 2008 and 2014 (Cambron et al. 2010, Shukle et al. 2016). These studies clearly indicate that while some H genes show robust resistance to various Hf field populations, others show variability in resistance levels owing to the changes in Hf virulence profile.

In a previous study carried out in 2008 (Cambron et al. 2010), we evaluated the efficacy of 21 H genes to fly populations collected from Iberville Parish, LA. In the current study, we reevaluated the response of the 21 plus additional 6 H genes to the Hf field populations from the same Parish in LA, collected in 2023, to determine changes in their resistance efficacy over the last 15 years. Our study clearly showed 14 H genes with comparable percentage of resistance from both years (Table 1), indicating stability in the efficacy of these genes and no change in the virulence profile of the Hf populations over the years. The 27 H genes clustered into 3 groups based on their level of effective resistance against the Hf population tested (Fig. 1). Ten genes had more than 80% resistance indicating these cultivars can continue to be deployed for effective control of the insect pest in this region (Table 1). Surprisingly, despite being widely deployed in the southeastern United States since 2007, H13 with 100% resistance still provided effective protection (Table 1). Although H18 still provides strong resistance (96%), it is a temperature-sensitive gene and loses its effectiveness above 20 °C (Mass et al. 1987). Incorporation of H18, into a winter wheat would ideally provide strong protection against Hf in the fall, but potentially be ineffective in the warmer spring weather in LA. Two of the H genes, H23 and H24, exhibited a dramatic decrease in percent resistance since the original study and, therefore, can no longer be considered ideal for use in management strategies. H23, a relatively new resistance gene deployed, showed the greatest decrease in efficacy going from moderately effective (73%) to ineffective at 41% (Table 1). Similarly, H24 dropped from 100% effective in 2008 to only moderately effective with 57% resistance in 2023. This decrease in efficacy for both H23 and H24 is surprising given no current knowledge of the widespread deployment of these genes. H11 that showed only 44% resistance in 2008 further dropped to 7%, thus confirming that this gene is not suitable to provide resistance against the Iberville Parish field population. One of the other resistant genes, H21, also showed a 12% decrease in resistance over what was observed in 2008 but was still >80% effective maintaining its stature as a good candidate for deployment. Interestingly, 2 genes, H16 and H31, exhibited a marked increase in resistance from 35 and 69% to 93% and 100%, respectively. These genes could serve as ideal candidates for deployment due to the observed change from ineffective (H16) and moderately effective (H31) to now highly effective.

In addition to the 21 H genes, 6 other H genes were evaluated in the current study (2023) that were not included in the previous (2008) study. Of these, while H15 (57%) and H19 (72%) showed moderately effective resistance and H34 is ineffective with mere 17% resistance, the genes H20, H28, and H33 showed 100% resistance (Table 1). These 3 genes can now be introduced into breeding programs as a source for effective resistance against the Hf.

The Hf remains a persistent threat to wheat production in LA, necessitating a combination of strategies for its management and control. By employing resistant wheat varieties, practicing crop rotation, and the use of parasitoids, breeders can continue to help mitigate the impact of this destructive insect pest. Our study has clearly identified a total of 20 H genes that are moderate to highly effective against the 2023 Hf populations from Iberville Parish, LA. Of these, 14 genes were common between the 2008 and 2023 Hf populations. While this study is a brief snapshot of one area in the southeastern United States, it does allow for one to note the changes in the virulence profile of Hf to wheat cultivars containing commercially available resistance genes, over time. While new and novel sources of resistance are being routinely identified to add to the repertoire of already available H genes (Nemacheck et al. 2023, Subramanyam and Nemacheck 2023), it is extremely important to periodically evaluate the phenotypic profile of the available H genes to Hf field collections to monitor the increase of virulence in the insect population throughout the country. It has been proposed by Gould (1986) that pyramiding highly effective H genes would provide more durability. Another possibility is to combine the highly effective H genes with moderately effective ones. These data provide valuable information to breeders for H genes that are efficacious in this region and can be useful in implementing effective breeding programs and developing elite wheat cultivars.

Acknowledgments

We thank Steve Harrison, School of Plant, Environmental and Soil Sciences, Louisiana State University for the Hessian fly field collection. This work is supported by USDA Project Plan Number: 5020-22000-019-000D. Mention of a commercial or a proprietary product does not constitute endorsement or recommendation for its use by the USDA.

Author Contributions

Brandon Schemerhorn (Conceptualization [Equal], Formal analysis [Lead], Methodology [Lead], Writing—original draft [Lead], Writing—review & editing [Equal]), Sue Cambron (Data curation [Lead], Methodology [Equal], Writing—review & editing [Equal]), and Subhashree Subramanyam (Conceptualization [Equal], Formal analysis [Supporting], Writing—review & editing [Equal])

References