Study On The Migration Of Heliothis Viriplaca(Lepidoptera: Noctuidae) Across The Bohai Strait

Long-distance migration plays a key role in the sudden outbreaks of crop pests and plant diseases, which has become a bottleneck problem in the sustainable development of agriculture in China. Bohai Strait is the major pathway for the seasonal north-ward and back-ward migration of many insect species. Heliothis viriplaca(Lepidoptera: Noctuidae), is one of the major insect pests threatening cotton, soybean, maize, alfalfa, etc. in northeastern China. It is hard to prevent the damage and control of H. viriplaca because this species can silk and roll in plant leaves, as well as it has been developed resistance to the chemical pesticides. In the 21 st century, the outbreaks of H. viriplaca occurring more frequently and causing serious yield losses. In order to determine(1) whether H. viriplaca is a long-distance migrant, and if so,(2) what pattern of seasonal migration this species exhibits in East Asia, the combination of searchlight trapping, entomolocial radar monitoring and ovarian dissection was carried out on an isolated small island in the center of the Bohai Strait during 2003-2014. The major results obtained are summarized as follows:1. During 2003-2014, no H. viriplaca larvae were found on BH by field investigations. However, H. viriplaca moths were regularly captured in the searchlight trap, which strongly suggests that this species immigrate from the mainland rather than emerging locally, and they migrated at least 40-60 km to reach the trapping site across the Bohai Strait. The results confirmed that H. viriplaca can take long-range migration driven by prevailing winds of the East Asian monsoon airflows, and the over-sea migration of this species is a regular ecological behavior rather than an accidental incident. The vast majority of catches(83.49 ± 11.80%) was trapped in the summer. The mean migration period was 116.50 ± 5.63 days from 2003 to 2014, with the shortest time span of 74 days in 2013 and the longest of 144 days in 2005.2. The seasonal pattern of H. viriplaca migration across the sea was changed significantly, and five migration periods were divided by Fisher’s optimal segmentation method, namely the preliminary period of north-ward migration(P1, 01 April to 02 July), the peak period of north-ward migration(P2, 03 July to 21 July), the transitional period of migration(P3, 22 July to 11 August), the peak period of back-ward migration(P4, 12 August to 15 September), the final period of back-ward migration(P5, 15 September to 31 October). The number of H. viriplaca captured in the searchlight trap during the back-ward migration period increased approximately 10 times compared to the north-ward migration period, which showed that the pole-ward migration by the prevailing southwest winds in spring and summer, brought significantly ecological and reproductive benefits to the population.3. From May to June during 2009-2014, the average mating rate and the index of ovarian development of H. viriplaca females were significantly higher than that in other months, which might be due to these moths emigrating from sites far away from the trapping site and therefore having several successive nights of migratory flight, because active flight results in a significant increase in body temperature and juvenile hormone biosynthesis. However, the majority of the trapped H. viriplaca females from August to September have little or no ovarian development, supporting the idea that the onset of migration is initiated by sexually immature individuals.4. The majority of H. viriplaca migration across the sea flying at the altitude between 100 m and 250 m above the ground level, and air temperature is one of the key factors affect their flying height. The maximum volumn density of H. viriplaca appeared at the top of the inversion layer. Analysis of the track direction of the targets registered on the PPI showed that H. viriplaca moths were heading toward the downwind direction, which was commonly the east-northeast in spring and summer, rather than being displaced randomly. The displacement direction was linearly related to the downwind direction, and subtracting the wind velocities from the insect displacement vectors showed that this species had some degree of collective orientation. The moths showed a tendency to head with an acute angle counterclockwise or clockwise to the wind direction in the four mass migration events.