Diversity, abundance, and parasitoid-phytophagous insect interactions in a selected pesticide-free rice ecosystem in Homagama, Sri Lanka

Yield loss associated with pest attack is a major concern for Sri Lankan rice farmers, and developing a sustainable pest management approach is still in need. This study assessed phytophagous rice insects and parasitoid diversity, and their interactions across vegetative, reproductive, and ripening stages in a pesticide-free rice field, located in the wet zone of Sri Lanka. Biweekly sampling was conducted during April to September, 2024. Flying adults were observed for 10 minutes in five quadrats, and non-flying insects and other life stages were handpicked, reared, and identified upon emergence. A total of 32 phytophagous insect species were identified, with the highest diversity during the reproductive stage. Eighteen parasitoid species (10 Hymenopterans and 8 Dipterans) were documented, parasitizing nine rice insect species. Parasitoid diversity was initially low during the vegetative stage, peaked at 40 days after planting, declined sharply, increased moderately during the reproductive stage, and remained lower during the ripening stage. An egg parasitoid species, eight larval, two larval-pupal, four pupal, two adult parasitoid species, and one hyperparasitoid (Brachymeria lasus) were reported. This is the first study in Sri Lanka to report the association of parasitoids, Brachymeria albotibialis, B. excarinata, Carcelia sp., Ceracia sp., Cotesia baoris, Palexorista solensis, Spathidexia aurantiaca, and Thecocarcelia oculata with phytophagous rice insects. Telenomus triptus showed the highest parasitism level (85%) on Scotinophara coarctata, displaying a significant relationship with host abundance across the rice plant growth phases. Possible bodyguard manipulation was observed in Spathidexia aurantiaca and Cotesia baoris. Overall, the study highlights diverse phytophagous rice insect-parasitoid interactions and underscores the strong potential of parasitoids in the suppression of pest populations in rice ecosystems.