Dang Hoa Tran

Effects of low temperatures on pupal survival of the stone leek leafminer Liriomyza chinensis (Diptera: Agromyzidae)

Abstract The cold-hardiness of Liriomyza chinensis pupae was measured in the laboratory by observing pupal mortality at low temperatures. Pupal mortality increased with decreasing temperature and with extended cold exposure time. No recently pupated pupae (6 h) were able to survive after 16 days of chilling at 0, 2.5 and 5°C, but 42.9% survival was observed at 10°C. Pupae at different developmental stages showed significant difference in mortality, with very low levels of mortality observed for older pupae (4 and 7 days) after exposure to 0°C for 16 days. The lethal time for 50% survival (LT50) increased with increasing age of pupae. LT50 for 4-day-old pupae exposed to 0°C was 52.1 days. Analysis of mean temperatures at several localities indicated that L. chinensis is able to overwinter outdoors in southern regions, but is unable to overwinter in open fields in northern regions of Japan, suggesting that overwintering in these regions would only occur in greenhouses.

Hydrological and Agricultural Impacts of Climate Change in the Vu Gia-Thu Bon River Basin in Central Vietnam

This paper summarizes some of the climate (change) impact modeling activities conducted in the Land use and Climate Change interactions in Central Vietnam (LUCCi) project. The study area is the Vu Gia-Thu Bon (VGTB) river basin in Central Vietnam, which is characterized by recurrent floods during the rainy season, but also water shortages during the dry season. The impact modeling activities, such as the validation of the models are hindered by the scarcity of hydrometeorological data and an unfavorable distribution of the observation network, i.e., station data is available only for the lowlands. In total, two different process-based and distributed hydrological models are applied in concert with climate change and land use projections. Based on that, the magnitudes and return periods of extreme flows are estimated. The modeling results suggest increases of extreme high flows due to climate change. A multi-objective agro-economical model was developed for a typical irrigation scheme in the region in order to optimize the area for cropping, irrigation-techniques and schedules. The model results suggest the irrigation technique Alternate Wetting and Drying, which has the potential to increase the benefits for the farmers and help to mitigate greenhouse gases at the same time. In addition, the regional-scale crop model GLAM is applied for groundnut under rainfed conditions, which is capable to identify regions suitable for cropping in the future. The paper further synthesizes recommendations for local stakeholders in Central Vietnam.

Rice-Based Cropping Systems in the Delta of the Vu Gia Thu Bon River Basin in Central Vietnam

Despite high pressures for agricultural land conversion, increasing competition for water, and the relatively low net benefits of rice production, rice is still by far the predominant farm occupation in the Vu Gia Thu Bon basin in Central Vietnam . This study examined the reasons for such persistence by surveying and analyzing a comprehensive set of qualitative (planting and harvesting dates) and quantitative data (yields , labor and nonlabor inputs, prices) for all the crops present in the cropping systems of 113 farms in the region. The net benefit derived from rice production was on average 23 M VND ha−1, with a relatively low labor input of 144 man-days per ha−1. The net benefits generated by vegetable production are more than 9 times higher (ca. 215 M VND ha−1) with a labor demand of ca. 928 man-days ha−1. Despite the very high net benefits of vegetable production, in this region they do not translate into an equivalently high added value per ha and man-day. These values are ‘only’ nearly double than those for rice, and not much higher than those for watermelon, chili, and groundnut. The results indicate that farmers’ decisions for not rushing in diversifying production to vegetables are wise when looking at the high risks of vegetable production, shortage of on-farm labor resources, and high opportunity costs of nonfarm labor opportunities. Rice is a robust crop and a pillar of families’ food security, demanding low labor inputs. Under current conditions, farmers will most probably continue predominantly cropping rice. There is nevertheless the need to improve the rice system. Technical efficiency of rice production in the delta of the VGTB basin is 78 %, a low figure if compared to recent average estimations of 86 % for the Vietnamese Mekong and Red River deltas. The small scale of production, land fragmentation and irrigation challenges due to salinity intrusion are the main factors impacting on technical efficiency in the region.

Neochrysocharis okazakii (Hymenoptera: Eulophidae) as a Major Parasitoid Wasp of Stone Leek Leaf Miner Liriomyza chinensis (Diptera: Agromyzidae) in Central Vietnam

Liriomyza chinensis is a major pest of Welsh onion Allium fistulosum in Asia but little is known about the abundance of its natural enemies. A field survey was made to explore the major parasitoids of L. chinensis leaf miner in central Vietnam. An eulophid parasitoid, Neochrysocharis okazakii, comprised more than 95% of parasitoids reared from leaf miner larvae collected in the onion field and 98.3% of leaf miner parasitoids found during searches of onion plants. The mean number of female N. okazakii on plants was greater in onion fields with a higher density of L. chinensis, and, during searches, a greater proportion of N. okazakii was found on onion with more L. chinensis mines, suggesting density-dependent parasitoid aggregation. Melanized dead larvae of L. chinensis were more frequently found in onion fields with more parasitoids, demonstrating that melanized leaf miners are a good indicator of parasitoid activity. Mean instant rate of host encounter by N. okazakii in the field was estimated at 0.077, and the likelihood of a parasitoid finding a host increased with host density. Taken together, these results show that N. okazakii is the major parasitoid of L. chinensis. Usefulness of this parasitoid in stone leek leaf miner management is discussed.