3 years ago

Hopper parasitoids do not significantly benefit from non-crop habitats in rice production landscapes

Agricultural intensification threatens biodiversity and ecosystem functioning. Promoting ecosystem services, such as biological pest control, could help to reduce pesticide inputs while simultaneously sustaining a high productivity. The highly intensive rice production in Southeast Asia, where more than 20% of the world's rice yield is produced, is challenged by devastating losses each year due to rice hoppers. This poses a great threat to the more than 3.5 billion people depending on rice as staple food. Egg parasitoids are among the most important natural enemies of rice hoppers and might be promoted with effective habitat management. However, empirical studies that focus on the management of parasitoid populations to enhance biological pest control in rice agroecosystems are largely lacking. We therefore analysed the effects of the availability of diverse habitats on hopper parasitoid performance, parasitism rates and pest control services, hypothesising that egg parasitoid abundance and pest control is positively influenced by diverse non-crop areas, which provide food resources as well as retreat areas for the fallow season. We experimentally tested the efficiency and abundance of egg parasitoids of Nilaparvata lugens and Nephotettix spp. in three study sites representing different levels of floral resources over the course of two rice growing seasons. We used mixed effect models to test whether habitat diversity positively influenced parasitoid abundance and subsequently reduces hatching rates of the hopper nymphs. Nephotettix spp. eggs were parasitized by Gonatocerus spp. and Paracentrobia spp. by 92.5%; Nilaparvata lugens eggs were parasitized by Oligosita spp. and Anagrus spp by 93%. In contrast to our hypothesis, we could demonstrate that additional floral resources do not significantly enhance parasitoid abundance and pest control in rice agroecosystems. Up to six times more parasitoids hatched from the bait plants exposed in the monoculture compared to the non-crop areas (p<0.05). All parasitoid genera except Anagrus spp. were found in rice fields in triple or quadruple numbers compared to the non-crop habitats (p<0.05). This was true also during the fallow period (p<0.05), where only ratoon rice was available to sustain hopper populations. In contrast to our hypotheses and findings from temperate productions systems, we found higher parasitoid abundance during all crop stages and increased hatching rates in the monocultures than in the non-crop habitats. The structural and temporal heterogeneity of rice crops and ratoon rice within the production area seemed to be sufficient to sustain high densities of parasitoid populations. We conclude that ecological intensification schemes should implement asynchronous planting cycles in rice systems to maintain or enhance parasitoids populations and their biocontrol services. In combination with reduced pesticide inputs, these measures might help to avoid yield losses due to rice hoppers in the future.

Publisher URL: www.sciencedirect.com/science

DOI: S0167880917305327

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