Studies on mechanisms of resistance in different host plants against cotton mealybug phenacoccus solenopsis tinsely (Hemiptera: Pseudococcidae).

Abstract

Plants species possess different morphological and biochemical properties, which resultantly induce in them different mechanisms of resistance. Present studies were carried out to investigate the mechanism of resistance in different host plants against Phenacoccus solenopsis. The results revealed a significant variation in tested plant species for percentage infestation and population of P. solenopsis. On the basis of field data, Digeria arvensis, Launea nudicaulis and Conyza bonariensis plants were ranked as incidental host plant species of P. solenopsis as they supported mealybug for < 3 months. The resuls of physico-morphic based antixenosis study revealed that attractiveness of first, second and third instars of P. solenopsis had positive correlation with trichome density (r= 0.56, 0.65, 0.41), trichome length (r= 0.26, 0.30, 0.33), leaf area (r=0.38, 0.44, 0.26) and leaf thickness (0.50, 0.57, 0.50). The resuls of biochemical based antixenosis also demonstrated association between attractiveness of first, second and third instar nymphs of P. solenopsis with phosphorus (r=0.05, 0.27 and 0.03), potassium (r= 0.03, 0.27, 0.03), nitrogen (r= 0.12, 0.07, 0.12), sodium (r= 0.21, 0.47, 0.27), total soluble sugar (r= 0.01, -0.12, 0.06), reducing sugar (r= 0.10, 0.1, 0.06), crude protein (r= 0.12, 0.07, 0.12) and chlorophyll (r= 0.29, 0.36,0.12) contents, respectively. The results of biochemical based antibiosis revealed that phosphorus, potassium and sodium had positive association with nymphal mortality, nymphal durations, pre-oviposition and oviposition periods of the female but had negative association with crawlers density. Nitrogen, total soluble sugar, chlorophyll and crude protein had positive association with crawler density but negative with nymphal duration, nymphal mortality and reproductive periods. Sodium had negative association with crawler density. Coefficient of determination values (R2) exhibited that phosphorus explained 27.5, 29.3, 49.3, 27.78, 31.6, 33.9, 45.2, 52.9 and 68.8%; potassium demonstrated 21.7, 30.8, 11.3, 21.3, 26.4, 24.1, 14.6, 7.5 and 18.07%; nitrogen attributed 8.2, 9.6, 9.1, 2.5, 4.9, 4.4, 6.5, 0.15 and 17.38%; crude protein contributed 8.2, 9.6, 9.1, 2.5, 4.9, 4.4, 6.5, 0.1 and 17.3%; total soluble sugar explained 27.8, 7.9, 8.6, 26.0, 12.8, 17.34, 8.8, 20.4 and 25.4%; reducing sugar described 0.01, 0.3, 3.18, 0.1, 1.2, 0.2, 1.68, 1.37 and 1.48%; sodium demonstrated 31.4, 22.2, 39.2, 25.2, 37.2, 38.0, 52.3, 30.4, 33.6% and chlorophyll attribiuted 12.0, 12.0, 4.4, 7.9, 5.96, 10.0, 0.3, 2.3 and 1.8% of total variation in preoviposition-period, oviposition-period, crawlers/ovisac, mortality of 1st, 2nd and 3rd instars and nymphal duration of 1st, 2nd and 3rd instars, respectively. The results of biochemical based tolerance mechanisms exhibited that nitrogen, crude protein, sodium, total soluble sugar and chlorophyll were negatively correlated with mealybug density, but phosphorus, reducing sugar and potassium were positively correlated. The tolerance level was affected at higher density (100 CMB/plant). As a tolerance mechanism, plants exhibited reduction in nitrogen, crude protein sodium, total soluble sugar and chlorophyll contents while demonstrated an increase in phosphorous, reducing sugar and potassium contents when infested with different densties of P. solenopsis.The pairwise Mahalanobis distances among three clusters established for 25 plant species revealed that members of cluster-2 demonstrated the maximum diversity against cluster-1 for infestation and population incidence of P. solenopsis (D2=96.0) and for antixenosis and morphological traits (D2=208.9). Cluster-2 established maximum diversity against cluster-3 for antixenosis and biochemical traits (D2=25.3). Cluster-1 expressed the maximum diversity against cluster-3 for antibiosis and biochemical traits (D2=55.2) and against cluster-2 for tolerance and biochemical traits (D2=18). The results of Principal components (PCs) explained that 82% of the total variability in infestation and population of P. solenopsis and 55.2% of the total variability in antibiosis and biochemical traits of the selected plant species were explained by PC1. The results also revealed that 73% of the total variability in antixenosis and morphological traits and 99% of the total variability for tolerance and biochemical traits of the selected plant species was explained by PC1 and PC2 commutatively. On the basis of antixenosis, antibiosis and tolerance mechanism of resistance, it was observed that D. arvensis plant exhibited resistance due to provision of different morphological and biochemical traits against P. solenopsis.