In vitro insecticidal effect of commercial fatty acids, ß-sitosterol and rutin against the sugarcane aphid, Melanaphis sacchari Zehntner (Hemiptera: Aphididae)

The sugarcane aphid, Melanaphis sacchari Zehntner (Hemiptera: Aphididae), is the main pest of sorghum, Sorghum bicolor L. Moench (Poaceae), in Mexico. To control this insect, farmers currently use synthetic chemical insecticides, which are toxic to humans and biodiversity. However, natural products are a promising potential source of safer alternative means to control different agricultural pests. The main objective of this study was to evaluate the insecticidal effect of contact by fumigation of pure molecules of four commercial fatty acids (palmitic, stearic, pentadecanoic and linoleic acids), the phytosterol ß -sitosterol, and the flavonoid rutin. The results showed that fatty acids were the most effective against M. sacchari ; the highest mortality rate (85%) was produced by linoleic acid and the LC 50 was 1,181 ppm, followed by stearic and palmitic acids with mortality percentages of 74 and 63%, respectively, at a concentration of 2,500 ppm at 72 h. The positive control, imidacloprid, had 100% mortality in 24 h and the tween 20 negative control exhibited 4% mortality in 72 h. Our results show that commercial fatty acids are effective against adults of M. sacchari , and can be considered an environmentally friendly alternative to the frequent use of synthetic chemical insecticides.

Genome-Wide Association Mapping of Resistance to the Sugarcane Aphid in Sorghum bicolor

Since 2013, the sugarcane aphid (SCA), Melanaphis sacchari (Zehntner), has been a serious pest that hampers all types of sorghum production in the U.S. Our understanding of sugarcane aphid resistance in sorghum is limited to knowledge about a few genetic regions on chromosome SBI-06. In this study, a subset of the Sorghum Association Panel (SAP) was used along with some additional lines to identify genetic and genomic regions that confer sugarcane aphid resistance. SAP lines were grown in the field and visually evaluated for SCA resistance during the growing seasons of 2019 and 2020 in Tifton, GA. In 2020, the SAP accessions were also evaluated for SCA resistance in the field using drone-based high throughput phenotyping (HTP) and visual scoring under greenhouse conditions. Plant height and flowering time were also recorded in the field to confirm that our methods were sufficient for identifying known quantitative trait loci (QTL). This study combined phenotypic data from field-based visual ratings, reflectance data, and greenhouse evaluations to identify genome-wide associated (GWAS) marker-trait associations (MTA) using genotyping-by-sequencing (GBS) data. Several MTAs were identified for sugarcane aphid-related traits across the genome, with a few common markers that were consistently identified on SBI-08 and SBI-10 for aphid count and plant damage as well as for reflectance indices-based traits on SBI-02, SBI-03, and SBI-05. Candidate genes encoding leucine-rich repeats (LRR), Avr proteins, lipoxygenases (LOXs), calmodulins (CAM) dependent protein kinase, WRKY transcription factors, flavonoid biosynthesis genes, and 12-oxo-phytodienoic acid reductase are identified near SNPs that had significant associations with different SCA traits. In this study, plant height and flowering time-related genes were also identified. The total phenotypic variation explained by significant SNPs across SCA-scored traits, plant height, and flowering time ranged from 0 to 74%, while the heritability value ranged from 4 to 74%. These results supported the existing literature, and also revealed several new loci. Markers identified in this study will support marker-assisted breeding for sugarcane aphid resistance.

Juice chemical properties of 24 sorghum cultivars under varying levels of sugarcane aphid (Melanaphis sacchari) infestation

Sugarcane aphids [Melanaphis sacchari (Zehntner)] have become a significant pest of grain, forage, and sweet sorghum [Sorghum bicolor (L.) Moench] in the USA in recent years. However, the effects of sugarcane aphid damage on sweet sorghum juice quality have not been well studied. A three-year (2015–2017) field study was conducted at Tifton, GA to assess planting date effects (April, May, or June planting) and cultivar responses (24 cultivars) to sugarcane aphids in sorghum. Aphid damage ratings were measured in all three years and cumulative aphid days were measured in 2016 and 2017. Cumulative aphid days (ln scale) and damage ratings (relative marginal effect) were correlated in five of the six plantings. Stem juice was collected at maturity from seven plantings for chemical analyses, which included HPLC, fluorescence excitation-emission spectrophotometry with parallel factor analysis (EEM/PARAFAC), and cyclic voltammetry (CV). Aphid damage ratings and cumulative aphid days were negatively correlated with sugar-related traits, particularly brix and total sugars. In four plantings, significant negative correlations (r ≤ −0.493) between trans-aconitic acid concentration and aphid damage were observed. Fluorescence and electrochemical properties related to the presence of polyphenols also showed correlations with aphid damage, particularly in the resistant landrace No. 5 Gambela. These secondary metabolites may play a role in sugarcane aphid resistance or tolerance. Stability analysis revealed that the more tolerant cultivars were able to maintain high concentrations of total sugars and trans-aconitic acid across environments.

Comparative Transcriptome Analysis Reveals Genetic Mechanisms of Sugarcane Aphid Resistance in Grain Sorghum

The sugarcane aphid, Melanaphis sacchari (Zehntner) (Hemiptera: Aphididae) (SCA), has become a major pest of grain sorghum since its appearance in the USA. Several grain sorghum parental lines are moderately resistant to the SCA. However, the molecular and genetic mechanisms underlying this resistance are poorly understood, which has constrained breeding for improved resistance. RNA-Seq was used to conduct transcriptomics analysis on a moderately resistant genotype (TAM428) and a susceptible genotype (Tx2737) to elucidate the molecular mechanisms underlying resistance. Differential expression analysis revealed differences in transcriptomic profile between the two genotypes at multiple time points after infestation by SCA. Six gene clusters had differential expression during SCA infestation. Gene ontology enrichment and cluster analysis of genes differentially expressed after SCA infestation revealed consistent upregulation of genes controlling protein and lipid binding, cellular catabolic processes, transcription initiation, and autophagy in the resistant genotype. Genes regulating responses to external stimuli and stress, cell communication, and transferase activities, were all upregulated in later stages of infestation. On the other hand, expression of genes controlling cell cycle and nuclear division were reduced after SCA infestation in the resistant genotype. These results indicate that different classes of genes, including stress response genes and transcription factors, are responsible for countering the physiological effects of SCA infestation in resistant sorghum plants.