Ocimum basilicum is closely related to Ocimum canum and has great medicinal properties in traditional medicine. Each home in India was supposed to have this plant in their courtyard and worship Ocimum spp. They had insect repellent properties. I remember to have planted lots of Ocimum plants near water tap im my garden in Jaipur India which have multiplied to several plants . I dont know if it could be correlated with Ocimum or not but one thing is certain I never had dengu living in India for last 25 years or the period from which we had these plants raised in our garden. One thig is certain mosquito dont like smell of tulsi or Ocimum spp.