Jasti Sri Vishnu Murthy

Biology and management of cassava mealybug Phenacoccus manihoti Matile -Ferrero (Hemiptera : Pseudococcidae) - Vellanikkara Department of Agricultural Entomology, College of Agriculture 2025 - 201,xxxiiip.

Ph. D

Cassava is one of the most important crops that ensures food security and supports the livelihoods of millions of farmers across tropical and subtropical regions. One of the major threats to the productivity and sustainability of cassava is the cassava mealybug, Phenacoccus manihoti Matile-Ferrero (Hemiptera: Pseudococcidae), a highly destructive, sap-sucking insect capable of causing substantial yield losses under severe infestations. In India, this invasive pest was first reported in 2020 at the Kerala Agricultural University, Vellanikkara campus, Thrissur and currently poses a significant threat to cassava cultivation in India. However, research on its biology, distribution, and management within the Indian context remains limited. The present study aimed to address this gap by investigating the biology, population dynamics, and genetic diversity of P. manihoti, characterizing its endosymbionts, and developing effective management strategies. A preliminary survey was conducted across Kerala on cassava to record the population density of P. manihoti, associated mealybug complex, symptoms, alternative hosts, natural enemies, and ant species associated with P. manihoti. The invasive pest P. manihoti exhibited a sparse, patchy distribution, with epicentres of infestation identified in the districts of Thrissur, Kannur, Kottayam, and Alappuzha. This scattered distribution reflects its limited natural dispersal capacity. During the survey, five mealybug species associated with cassava were identified using both morphological and molecular methods. Damage symptoms on cassava caused by P. manihoti include leaf curling, bunchy top, shortened internodes, and stunted growth. Nine alternative hosts belonging to seven different plant families were identified. Natural enemies associated with P. manihoti colonies included seven species of coccinellid beetles, one lepidopteran predator (Spalgis epius), and five species of spiders. During the study, three entomopathogenic fungal isolates were obtained from P. manihoti cadavers. Additionally, seven ant species were recorded in association with P. manihoti, suggesting potential mutualistic interactions. A survey was conducted at two selected locations, Madakkathara (Thrissur, 2023) and Krishnapuram (Alappuzha, 2024), to study the population dynamics of P. manihoti. In Madakkathara, maximum temperature showed a strong positive correlation with mealybug abundance (r = 0.806, p < 0.01), while minimum relative humidity showed a moderate negative effect (r = -0.478, p < 0.05). In Krishnapuram, maximum and minimum relative humidity, as well as the number of rainy days, exhibited strong negative correlations with mealybug density (r = -0.915 to -0.675, p < 0.01), indicating suppression under humid conditions. Biology studies of P. manihoti reared on the M4 cassava variety revealed that the egg, first, second, and third instar nymphs lasted 6.8, 6.4, 4.6, and 5.3 days, respectively, with the adult stage lasting 20.9 days. The complete life cycle averaged 44.2 days, and with an average of 259 eggs were produced per female. Morphometric measurements indicated progressive increase across stages. The genetic diversity of P. manihoti was analyzed based on mitochondrial cytochrome oxidase I (mtCOI) and the nuclear internal transcribed spacer 1 (ITS1) gene sequences across fourteen populations of Kerala. DNA sequence polymorphism analysis of the mtCOI sequences revealed one segregating site and two haplotypes, with an overall haplotype diversity of 0.143 and a nucleotide diversity of 0.00029, indicating a genetically homogeneous population with minimal variation. In contrast, analysis of the ITS1 region showed moderate genetic variation, with nine haplotypes and 17 segregating sites were identified, resulting in a haplotype diversity of 0.879. Metagenomic DNA from two P. manihoti samples, Thrissur (CMBT) and Kannur (CMBK), was analyzed to profile the gut-associated microbiota. Using Illumina MiSeq sequencing targeting the V3–V4 region of the 16S rRNA gene, a diverse bacterial community was identified. A total of 192 genera were detected in the CMBK sample, while 169 genera were recorded in the CMBT sample. Taxonomic classification based on the Kraken SILVA database revealed Candidatus Tremblaya as the dominant genus, accounting for 97.48 and 99.06 per cent of the bacterial community in CMBK and CMBT, respectively. This study assessed the functional response of Anagyrus lopezi to second and third-instar P. manihoti nymphs at varying host densities. Logistic regression analysis revealed a negative linear and a positive quadratic coefficient of host density (No) on the proportion parasitized (Na/No), which confirmed a Type II functional response. Further, Holling’s disc equation revealed a higher searching rate (a = 0.033 h⁻¹) and shorter handling time (Th = 1.17 h) for third instars when compared to second instars (a = 0.025 h⁻¹; Th = 1.72 h), respectively. Varying densities of A. lopezi (1–8 individuals) were tested against 25 and 100 P. manihoti nymphs. At the higher host density (100 nymphs), per capita searching efficiency declined significantly (from 0.006043 to 0.002454), indicating mutual interference (m = 0.44316). At the lower host density (25 nymphs), searching efficiency remained relatively stable (m = 0.15655), reflecting reduced interference due to limited host availability. The functional response of A. lopezi to varying host densities, along with the effect of different densities of A. lopezi on P. manihoti, significantly influenced the reproductive strategies and foraging efficiency of A. lopezi. The efficacy of various treatments against P. manihoti, viz., entomopathogenic fungi (Purpureocillium lilacinum, Lecanicillium lecanii @ 1×108 spores ml-1), horticultural mineral oil (HMO @ 2%), botanicals (neem oil and karanj oil @ 2%), insecticidal soap combined with eco-neem plus (@ 2%) were evaluated along with the biological control agent, Anagyrus lopezi De Santis (Hymenoptera: Encyrtidae) and an untreated control. Under polyhouse conditions, A. lopezi achieved the highest suppression (69.36%), followed by insecticidal soap combined with eco-neem plus (55.85%), neem oil (50.34%), karanj oil (49.57%), P. lilacinum (46.35%), L. lecanii (42.35%), and HMO (40.88%). A subsequent field trial with two applications at 15 day intervals showed insecticidal soap + eco-neem plus as the most effective (94.33% reduction), followed by karanj oil (92.68%), neem oil (92.26%), HMO (82.13%), P. lilacinum (75.46%), A. lopezi (75.39%), and L. lecanii (62.06%). The integration of fast-acting botanical treatments with the sustained efficacy of biological control agents, notably A. lopezi and P. lilacinum, provides a balanced and ecologically sustainable management strategy. This study provided comprehensive insights into the distribution, biology, endosymbionts, genetic diversity, and management of P. manihoti. Field surveys across Kerala revealed a patchy distribution of P. manihoti with localized infestations and association with multiple mealybug species, natural enemies, and mutualistic ants. Population dynamics showed that high humidity and rainfall play a significant role in suppressing infestations. Genetic analysis of P. manihoti populations across Kerala revealed low diversity in the mtCOI gene but high variability in the ITS1 region, indicating differential evolutionary patterns within these markers. Metagenomic profiling confirmed Ca. Tremblaya as the dominant endosymbiont in P. manihoti, highlighting its key role in the host’s microbiome. Field trials demonstrated that insecticidal soap with eco-neem plus, neem oil, and karanj oil were effective for short term suppression. However, the incorporation of A. lopezi as a principal component of the biological control strategy in this study underscores its critical role in the sustainable management of P. manihoti. Together, these findings laid a strong foundation for developing integrated, eco-friendly, and sustainable management strategies for P. manihoti in India.


Agricultural Entomology
Cassava
Phenacoccus manihoti Matile -Ferrero
Hemiptera : Pseudococcidae
Biology and management

632.6 / JAS/BI Ph.D
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