Earliness and Crop Morphological Traits Modulate Field Pest Infestation in Green Gram

Breeding has significantly improved drought tolerance in green gram but marked yield losses continue due to damage by insect pests. Important pests of green gram are pod borers, Maruca vitrata (F.), aphids, Aphis gossypii Glover and whiteflies, Trialeurodes vaporariorum (Westwood). Management of these pests has partly been constrained by the limited understanding of crop traits that modulate their infestation. Field experiments were conducted in southeastern Kenya to evaluate a collection of old and new green gram varieties for tolerance to field pests, and to identify traits that confer resistance. The old varieties were KS20 and N26, both released in 1990s whereas the modern counterparts were Biashara, Karembo, and Ndengu-Tosha. Results showed significant differences among the varieties in maturity, leaf area, leaf hair density, leaf moisture content, and pod wall thickness. Earliness significantly reduced pest infestation, whereby KS20 matured early while N26 was late, and the new varieties were intermediate. On average, across the three environments, leaf area (R2 ≥ 0.32) and leaf moisture content (R2 ≥ 0.18) positively correlated with pest infestation while leaf hair density (R2 ≥ 0.30), and pod wall thickness (R2 ≥ 0.54) showed a negative association with pod borer and aphid counts. However, results did not reveal any particular traits that associated with the varieties, which implied that breeding of green gram in Kenya has not selected for tolerance to field pests. Nonetheless, green gram field pest management practices could select for varieties with early maturity, open plant canopy, pubescent leaves, and thicker pod walls.

Reaping the Potential of Wild Cajanus Species through Pre-Breeding for Improving Resistance to Pod Borer, Helicoverpa armigera, in Cultivated Pigeonpea (Cajanus cajan (L.) Millsp.)

Pod borer (Helicoverpa armigera) causes the highest yield losses in pigeonpea, followed by pod fly (Melanagromyza obtusa). High levels of resistance to pod borer are not available in the cultivated genepool. Several accessions of wild Cajanus species with strong resistance, and different resistance mechanisms (antixenosis and antibiosis) to pod borer have been identified. These accessions can be utilized to improve the pod borer resistance of cultivated pigeonpea. Using pod borer resistant Cajanus scarabaeoides and Cajanus acutifolius as pollen donors and popular pigeonpea varieties as recipients, pre-breeding populations were developed following simple- and complex-cross approaches. Preliminary evaluation of four backcross populations consisting of >2300 introgression lines (ILs) under un-sprayed field conditions resulted in identifying 156 ILs with low visual damage rating scores (5.0−6.0) and low pod borer damage (<50%). Precise re-screening of these ILs over different locations and years resulted in the identification of 21 ILs having improved resistance to pod borer. Because these ILs were derived from wild Cajanus species, they may contain different alleles for different resistance components to pod borer. Hence, these ILs are ready-to-use novel and diverse sources of pod borer resistance that can be utilized for improving the pod borer resistance of cultivated pigeonpea.

Comparative Analysis Delineates the Transcriptional Resistance Mechanisms for Pod Borer Resistance in the Pigeonpea Wild Relative Cajanus scarabaeoides (L.) Thouars

Insect pests pose a serious threat to global food production. Pod borer (Helicoverpa armigera (Hübner)) is one of the most destructive pests of leguminous crops. The use of host resistance has been an effective, environmentally friendly and sustainable approach for controlling several agricultural pests. The exploitation of natural variations in crop wild relatives could yield pest-resistant crop varieties. In this study, we used a high-throughput transcriptome profiling approach to investigate the defense mechanisms of susceptible cultivated and tolerant wild pigeonpea genotypes against H. armigera infestation. The wild genotype displayed elevated pest-induced gene expression, including the enhanced induction of phytohormone and calcium/calmodulin signaling, transcription factors, plant volatiles and secondary metabolite genes compared to the cultivated control. The biosynthetic and regulatory processes associated with flavonoids, terpenes and glucosinolate secondary metabolites showed higher accumulations in the wild genotype, suggesting the existence of distinct tolerance mechanisms. This study provides insights into the molecular mechanisms underlying insect resistance in the wild pigeonpea genotype. This information highlights the indispensable role of crop wild relatives as a source of crucial genetic resources that could be important in devising strategies for crop improvement with enhanced pest resistance.

Mitochondrial cytochrome oxidase (COI) sequence-based identification of pod borer of Thylacoptila sp. AA isolated from Senna alata (L.) Roxb. for the first time in India

In the present study, two pod borer larvae were found to damage pods of the host plant Senna alata. As pods (seeds) are commercially important, it prompted the investigators to find out the adult of pod borer larvae by molecular sequencing and reported in this article for the first time from S. alata. Successful amplification was achieved for chloroplast DNA isolated from leaves of host plant S. alata and tissue DNA from unknown larvae and sequenced. The sequences were submitted to NCBI domain, and the taxonomic position of host plant of pod borer larvae was confirmed as S. alata (L.) Roxb. Of the two sequences belonging to pod borer larvae, specimen 1 (S1) matched with already available three sequences of Thylacoptila sp. AA. But, specimen 2 (S2) showed significant variation in its genetic distance in Neighborhood Joining tree and maximum likelihood tree; these factors imply that specimen 2 (S2) is distinct from Thylacoptila sp. AA. Therefore, it is reported that specimen 2 may be represented as Thylacoptila sp. BB where in BB indicates variant from other available sequences. Nonetheless, reporting species of Thylacoptila as insect pest in pods of S. alata is an important contribution to the annals of insect-pest-plant interaction more importantly in medicinally important plant species S. alata.

Effect of Temperature on the Development, Survival, and Fecundity of Heliothis viriplaca (Lepidoptera: Noctuidae)

Heliothis viriplaca (Hüfnagel) (Lepidoptera: Noctuidae) is worldwide an important economic pest of major agricultural crops. The effect of temperature on the biology, in particular the duration of the different developmental stages, of this insect pest was examined. The development, survival, and fecundity of H. viriplaca at five different temperatures (22, 25, 28, 31, and 34°C) were investigated, and experimental population life tables calculated. The results show that the developmental time of larvae, pupae, pre-oviposition, adults, and the entire generation decreases with increasing temperature. Larvae emerged fastest from the eggs at 28°C. The percentage of larvae that reached the seventh instar stage was 32.69, 38.60, 19.67, 3.61, and 20.88% at 22, 25, 28, 31, and 34°C, respectively. The developmental threshold temperature of the eggs, larvae, pupae, pre-oviposition and total pre-oviposition period was 10.53, 12.34, 5.60, 11.47, and 10.01°C, respectively. The effective accumulative temperature was 42.90, 239.68, 309.19, 77.72, and 767.64 degree-days, respectively. The population trend index (I) and the intrinsic rate of increase (r) of H. viriplaca were the highest at 31°C (133.97 and 0.12 d-1, respectively). The results of this study may be useful for predicting the population dynamics of H. viriplaca and thus provides information on this important insect pest that may assist in its management.