Bio-physical bases of antixenotic mechanism of resistance in bitter-gourd (Momordica charantia L., Cucurbitacae) against melon fruit fly, Bactrocera cucurbitae (Coquillett) (Diptera: Tephritidae).

Plants genotypes possess different phenotypic and/or biochemical properties, which resultantly induce in them different mechanisms of resistance. These mechanisms enable the plants to avoid, tolerate or recover from the effects of insect pest attacks. The results of the present studies revealed that there were significant variation in tested bitter-gourd genotypes for percentage fruit-infestation and larval-density per fruit. Col-II and Faisalabad-Long were ranked resistant genotypes and identified as resistance source for melon fruit fly, Bactrocera cucurbitae. The larval density per fruit had a significant positive correlation (r = 0.992) with percentage fruit infestation. The fruit-length, fruitdiameter, number of longitudinal ribs/fruit and number of small ridges/cm 2 , which were significantly lowest in resistant and highest in susceptible genotypes, had a significant positive correlation with the percent fruit infestation and larval-density per fruit. However, fruit toughness, height of small ridges, height of longitudinal ribs and pericarp thickness, which were significantly highest in resistant and lowest in susceptible genotypes, had a significant negative correlation with the percent fruit infestation and larval-density per fruit. Step-wise multiple regression analysis indicated that the tested morphological traits explained 100% of the total variation in fruit infestation and larval-density per fruit. However, the fruit-length, fruit-diameter, fruit-toughness and number of longitudinal ribs showed 95.49% of the total variation in fruit fly infestation and 99.67% of the total variation in the larval-density per fruit. The maximum variation, in fruit infestation and larval-density per fruit, was explained by fruit toughness (63.4 and 49.2%, respectively) followed by fruit-diameter (23.22 and 22.34%, respectively) and number of longitudinal ribs (8.23 and 11.57%, respectively). These can be used as marker traits to induce resistance against melon fruit fly in bitter gourd; whereas, rest of the morphological fruit-traits explained less than 2% variation in the fruit infestation and less than 1% variation in the larval-density per fruit.