Abstract
The beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), is a serious polyphagous insect pest worldwide. This species is known as a long-distance migrant, and previous studies on its migration have been mostly carried out in regions where it can overwinter. However, what pattern of seasonal migration this species exhibits in regions where it cannot overwinter (i.e., the ‘summer breeding region’) remains unknown. Here, we present data from 14-years of monitoring on a small remote island located in the center of the Bohai Strait, in northern China, by means of searchlight trapping and ovarian dissection. We found that the population size of this overseas migration varied significantly among years, with very large migrations in 2005, 2007, 2009, and 2014 that resulted in annual total catches of more than ten thousand individuals. In addition, nightly catches exhibited a significant inter-month variability, with the vast majority of S. exigua moths being trapped in August and September, (81.1 ± 3.6%), making S. exigua one of the most frequently encountered species in that period. The mean time from the earliest trap capture to the latest capture within a given year was 113 ± 22 d (range 57 d [2003] to 138 d [2008]). The sex ratio (females: males) was significantly less than 1:1 in each month, but the proportion of females showed an upward trend from June to October. The majority of trapped females in summer were mated (94.4 ± 10.7% in June, 80.0 ± 6.4% in July) and sexually mature (88.9 ± 11.1% in June, 61.8 ± 12.3% in July), suggesting the onset of mating and/or sexual maturation does not terminate the migration behavior in this species. These findings provide a good starting point for study of the trans-regional migration of S. exigua across different climate zones.
Highlights
Since the late 1980s the beet armyworm, Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae), has been a destructive pest attacking >90 plant species in Asia, including several major crops, such as sugar beet, cotton, soybean, and potatoes [1,2,3]
The occurrence range of S. exigua in east Asia can be divided into three zones: (1) the ‘year-round breeding region’ (< 25 ̊N), mainly including the tropical climate zone (TCZ) and the southern part of subtropical climate zone (STCZ)
S. exigua moths were regularly captured in the searchlight trap from May to October every year on BH (Fig 3A; S1 Fig), which strongly suggests that these moths frequently immigrate from the mainland rather than emerging locally, a distance of at least 40–60 km
Summary
Since the late 1980s the beet armyworm, Spodoptera exigua (Hubner) (Lepidoptera: Noctuidae), has been a destructive pest attacking >90 plant species in Asia, including several major crops, such as sugar beet, cotton, soybean, and potatoes [1,2,3]. Successful management of S. exigua is complicated by its broad host range, rapid growth rate, high fecundity, strong capacity for long-duration flight, and the rapid evolution of resistance to pesticides [6,7,8,9,10,11] Serious outbreaks of this pest in many regions of Asia, Africa, Europe, and America have been reported in the past two decades [7, 12,13]. (2) the ‘overwintering region’ (25–35 ̊N), mainly ranging from the northern part of the STCZ to the middle part of warm temperate climate zone (WTCZ) In this region, S. exigua overwinter as pupae in the soil [9]. S. exigua overwinter as pupae in the soil [9]. (3) the ‘summer breeding region,’ mainly including regions north of 38 ̊N, and S. exigua cannot overwinter in this region [7, 9, 21,22] (Fig 1)
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