PhD

A study was undertaken at the Acarology laboratory, Department of Agricultural Entomology, College of Horticulture, Vellanikkara during 2013-2016, to investigate the biology, incidence, crop phenology relationship and natural enemies of the two spotted spider mite, Tetranychus urticae Koch infesting cucumber in polyhouse as well as to study the biology, efficacy and prey stage preference of the predator, Neoseiulus ongispinosus (Evans) on T. urticae and standardize the optimum predator: prey ratio of N. longispinosus for biological control of T. urticae in polyhouse. Purposive surveys, conducted in fifteen polyhouses in four districts of Kerala viz., Thrissur, Palakkad, Wayanad and Thiruvananthapuram, revealed the occurrence of four species of tetranychid mites, namely, Tetranychus truncatus Ehara, T. urticae Koch, T. okinawanus Ehara, Eutetranychus orientalis (Klein) and one species of tarsonemid mite, Polyphagotarsonemus latus (Banks) on cucumber. The occurrence of T. truncatus and T. okinawanus is a new report on cucumber in polyhouse. Hence, DNA barcoding was carried out to confirm the species identity of T. truncatus (TOCRF001-15) and T. okinawanus (TOIR001-15). Studies on the relationship of crop stage and mite incidence on cucumber revealed that the population of spider mites was significantly higher during the late fruiting stage of the crop, followed by early vegetative stage. Relatively lower population was recorded at flowering stage and early fruiting stage.Four species of insect predators and six species of mite predators were recorded in association with spider mites on cucumber. The insect predators were Stethorus pauperculus (Weise), Oligota sp., Scolothrips sp. and an unidentified species of Cecidomyiidae. The predatory mite fauna included Agistemus garrulus Chaudhari, Amblyseius paraaerialis (Muma), Cunaxa sp., Cheyletus sp., Neoseiulus longispinosus (Evans) and Tydeus gossabaensis Gupta.Neoseiulus longispinosus was found to be the predominant species of predatory mite on spider mites infesting cucumber. Tetranychus urticae recorded a developmental period of 6.75 days in male and 7.15 days in female. Adult male lived for 8.95 days while mated and unmated female lived for 11.59 days and 13.04 days, respectively. Mated and unmated females recorded a fecundity of 47.91 and 36.08 eggs, respectively. The sex ratio was female biased (1:4.6) in T. urticae. Total developmental period of N. longispinosus was 3.91 and 4.27 days for male and female, respectively. Adult male lived for 19.66 days and the female, for 22.75 days. On an average, a single female laid 31.33 eggs and the progeny consisted of both males and females in the ratio 1:3.31. Parthenogenesis was not observed in N. longispinosus.
The adult of N. longispinosus recorded significantly higher predation compared to the nymph. Both nymph and adult, showed preference towards egg compared to active stages of the prey. The time needed to eliminate the available prey population was found to increase with increase in prey density.
Studies conducted to identify the optimum predator: prey ratio required for field release of N. longispinosus against T. urticae on cucumber in the laboratory showed that, at ratios of 1:5 and 1:10, the prey population was completely eliminated by tenth day. The prey population recorded in the ratios, 1:20, 1:25 and 1:33 were on par with this. In the polyhouse, the predator: prey ratios of 1:20 and 1:25 were found to significantly reduce the population of T. urticae on cucumber. The present study has revealed the potential of the predatory mite, N. longispinosus as a biocontrol agent of the spider mites. The short life cycle, longer life span of adults, female biased sex ratio and preference for egg stages and above all, the density dependant nature of the prey predator relationship, brought out in the present study could provide a platform for viable biocontrol strategy based on N. longispinosus for management of spider mites under protected cultivation in Kerala.