Interrelationship between selected sugarcane characteristics and reaction to pink mealybug infestation

Thirty four sugarcane varieties/clones with different Saccharum parentage and a susceptible check were evaluated in the screenhouse and in the field. Based on the rating scale utilizing percentage of infested tillers, 14 were highly susceptible, ten susceptible, four intermediate, four resistant and three highly resistant. The resistance of three varieties/clones in the field was partly due to the morphological characters of the sugarcane plant that exposed the mealybug colonies to the deleterious effects of the environment. Morphologically, the characteristics of the sugarcane plant responsible for its resistance to pink mealybug were the few leafsheaths retained on the stalk (self trashing), wide spaces between the stalk and the leafsheaths (loosely wrapping leafsheaths), and the thick and hard rind with high fiber content. The number of leafsheaths retained on the stalk was highly and positively correlated with mealybug infestation. However, the space between the stalk and the leafsheath and the rind hardness were highly but negatively correlated with survival and fecundity of the mealybugs. The settling behavior tests indicated that the nymphs could not differentiate a resistant from a susceptible sugarcane not until after contact and initial feeding. Biochemically, the resistant sugarcane had higher sucrose content but lower percent wax on the rind than the susceptible variety. Eight amino acids (aspartic acid, serine, threonine, glutamic acid, glycine, alanine, valine and lysine) were found common to the susceptible and resistant sugarcanes, however, tyrosine, leucine, isoleucine and histidine were found only in some of the resistant sugarcanes. The significant correlations were obtained between the total percentage of amino acid composition and the mealybug infestation. Sucrose content, however, was negatively correlated with the growth index and the mealybug infestation. However, wax on the rind was highly and positively correlated with the mealybug infestation