Diversity study of Beauveria bassiana species for finding the most virulent strain to manage Bemisia tabaci in cotton.

Beauveria bassiana (Bal.-Criv.) is an important entomopathogenic fungus being used for the management of various agricultural pests worldwide. However, all strains of B. bassiana may not be effective against whitefly, Bemisia tabaci, or other pests, and strains show diversity in their growth, sporulation, virulence features, and overall bioefficacy. Thus, to select the most effective strain, a comprehensive way needs to be devised. We studied the diversity among the 102 strains of B. bassiana isolated from 19 insect species based on their physiological features, virulence, and molecular phylogeny, to identify promising ones for the management of B. tabaci. Strains showed diversity in mycelial growth, conidial production, and their virulence against B. tabaci nymphs. The highest nymphal mortality (2nd and 3rd instar) was recorded with MTCC-4511 (95.1%), MTCC-6289 (93.8%), and MTCC-4565 (89.9%) at a concentration of 1 × 106 conidia ml-1 under polyhouse conditions. The highest bioefficacy index (BI) was in MTCC-4511 (78.3%), MTCC-4565 (68.2%), and MTCC-4543 (62.1%). MTCC-4511, MTCC-4565, and MTCC-4543 clustered with positive loading of eigenvalues for the first two principal components and the cluster analysis also corresponded well with PCA (principal component analysis) (nymphal mortality and BI). The molecular phylogeny could not draw any distinct relationship between physiological features, the virulence of B. bassiana strains with the host and location. The BI, PCA, and square Euclidean distance cluster were found the most useful tools for selecting potential entomopathogenic strains. The selected strains could be utilized for the management of the B. tabaci nymphal population in the field through the development of effective formulations. KEY POINTS: • 102 B. bassiana strains showed diversity in growth and virulence against B. tabaci. • Bioefficacy index, PCA, and SED group are efficient tools for selecting potential strains. • MTCC-4511, 4565, and 4543 chosen as the most virulent strains to kill whitefly nymphs.

Evaluation of bioefficacy potential of entomopathogenic fungi against the whitefly (Bemisia tabaci Genn.) on cotton under polyhouse and field conditions.

The whitefly, Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae), is becoming a serious problem on Bt cotton. It causes enormous crop loss through its direct feeding and as a vector of cotton leaf curl virus. Chemical-dependent management is harming the environment and increased insecticide resistance is often observed in the fields. Identification of most virulent strains of entomopathogenic fungi (EPF) is essential to serve as an important component of an IPM program for management of B. tabaci. Compared to B. tabaci adults, the nymphal stage is reported to be more susceptible to entomopathogens, and targeting nymphs also helps vector management. We evaluated the bioefficacy of EPF and chemical pesticides against nymphs of B. tabaci on Bt cotton under polyhouse and field conditions. The bioefficacy index (BI) was considered as a mechanism to select the most effective EPF strains for field evaluation. The highest nymphal mortality under polyhouse conditions was recorded for Metarhizium anisopliae NA-01299 (86.7%), Beauveria bassiana MT-4511 (85.1%), Cordyceps javanica IT-10498 (81.1%), IT-10499 (81%), and B. bassiana NA-0409 (78.2%) relative to other EPF strains, spiromesifen (69.6%), buprofezin (62.2%) and pyriproxyfen (52.7%) at 7-days-post-spray treatment (DAS). However, among all the EPF, the highest BI was recorded in C. javanica IT-10499 (77%), IT-10495 (75.4%), Fusarium verticillioides IT-10493 (74.6%), and B. bassiana MT-4511 (73.1%). The pooled data of two-year field trials (2017-18 & 2018-19) revealed that the highest nymphal mortality was recorded for MT-4511 (85%), IT-10499 (83.2%), and pyriproxyfen 10% EC (78.6%) at 7-DAS. The BI-based selection of EPF proved to be a useful predictor of field efficacy. A sequential spray of the selected EPF would be a vital approach for resilient and sustainable integrated management of the B. tabaci nymphal population under field conditions.

Population genetic structure of the cotton bollworm Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) in India as inferred from EPIC-PCR DNA markers.

Helicoverpa armigera is an important pest of cotton and other agricultural crops in the Old World. Its wide host range, high mobility and fecundity, and the ability to adapt and develop resistance against all common groups of insecticides used for its management have exacerbated its pest status. An understanding of the population genetic structure in H. armigera under Indian agricultural conditions will help ascertain gene flow patterns across different agricultural zones. This study inferred the population genetic structure of Indian H. armigera using five Exon-Primed Intron-Crossing (EPIC)-PCR markers. Nested alternative EPIC markers detected moderate null allele frequencies (4.3% to 9.4%) in loci used to infer population genetic structure but the apparently genome-wide heterozygote deficit suggests in-breeding or a Wahlund effect rather than a null allele effect. Population genetic analysis of the 26 populations suggested significant genetic differentiation within India but especially in cotton-feeding populations in the 2006-07 cropping season. In contrast, overall pair-wise F(ST) estimates from populations feeding on food crops indicated no significant population substructure irrespective of cropping seasons. A Baysian cluster analysis was used to assign the genetic make-up of individuals to likely membership of population clusters. Some evidence was found for four major clusters with individuals in two populations from cotton in one year (from two populations in northern India) showing especially high homogeneity. Taken as a whole, this study found evidence of population substructure at host crop, temporal and spatial levels in Indian H. armigera, without, however, a clear biological rationale for these structures being evident.

In-season changes in resistance to insecticides in Helicoverpa armigera (Lepidoptera: Noctuidae) in India.

Discriminating doses of fenvalerate, cypermethrin, quinalphos, and endosulfan were determined with an insecticide-susceptible Helicoverpa armigera (Hübner) strain. In-season changes in insecticide resistance were monitored with discriminating dose assays at weekly intervals throughout the cropping season for 6 yr from 1993 to 1999 in central India. Resistance to pyrethroids was high throughout all seasons. Resistance to 0.75 microg of quinalphos was consistent, with seasonal averages ranging from 23 to 27% survival over the 6 yr. Resistance to 10.0 microg of endosulfan was moderately high at an average of 40-47% survival during 1993-1994 and in 1997-1998. It was lower in 1996-1997 at 27%, and in 1998-1999 at 33%. The weekly monitoring data for all seasons were pooled and the consolidated 6-yr seasonal average profile indicated that resistance to quinalphos and endosulfan was low during September at 21 and 27% survival, respectively, but increased to 28 and 37% by the end of November. Resistance levels to organophosphates and endosulfan increased during the season, depending on the use of these compounds. At almost all monitoring sites, the within-season changes in quinalphos resistance for all seasons through the study period followed a trend similar to that of endosulfan. The results suggest the possibility of cross-resistance between these compounds. Based on this study and the existing information on cotton pest management, we have developed a "window strategy" for cotton pest management with specific emphasis on the management of insecticides for effective control of H. armigera. This strategy has contributed to improved control at reduced costs in extensive trials.