Synchronized Oviposition Triggered by Migratory Flight Intensifies Larval Outbreaks of Beet Webworm

Yun Xia Cheng,Thomas W Sappington,Xing Fu Jiang,Li Zhi Luo,Matt Hayward

doi:10.1371/journal.pone.0031562

Abstract

Identifying the reproductive consequences of insect migration is critical to understanding its ecological and evolutionary significance. However, many empirical studies are seemingly contradictory, making recognition of unifying themes elusive and controversial. The beet webworm, Loxostege sticticalis L. is a long-range migratory pest of many crops in the northern temperate zone from 36°N to 55°N, with larval populations often exploding in regions receiving immigrants. In laboratory experiments, we examined (i) the reproductive costs of migratory flight by tethered flight, and (ii) the reproductive traits contributing to larval outbreaks of immigrant populations. Our results suggest that the beet webworm does not initiate migratory flight until the 2nd or 3rd night after emergence. Preoviposition period, lifetime fecundity, mating capacity, and egg hatch rate for adults that experienced prolonged flight after the 2nd night did not differ significantly from unflown moths, suggesting these traits are irrelevant to the severity of beet webworm outbreaks after migration. However, the period of first oviposition, a novel parameter developed in this paper measuring synchrony of first egg-laying by cohorts of post-migratory females, for moths flown on d 3 and 5 of adulthood was shorter than that of unflown moths, indicating a tightened time-window for onset of oviposition after migration. The resulting synchrony of egg-laying will serve to increase egg and subsequent larval densities. A dense population offers potential selective advantages to the individual larvae comprising it, whereas the effect from the human standpoint is intensification of damage by an outbreak population. The strategy of synchronized oviposition may be common in other migratory insect pests, such as locust and armyworm species, and warrants further study.

Highlights

Migration is an important life history option for adapting to seasonal and temporal changes in habitat [1,2,3]
We evaluated the effects of migratory flight on general reproductive traits and a novel parameter that serves as a measure of population-level synchrony of egg-laying, the period of first oviposition (PFO)
Tethered flight distance on each day tested was comparable to that of S. exigua [23], a noctuid species with one of the greatest documented expanses traversed in migratory flight [43]

Summary

Introduction

Migration is an important life history option for adapting to seasonal and temporal changes in habitat [1,2,3]. Migration may impose reproductive costs on the migrant relative to residents [6,7,8,9]. Development of a flight apparatus in migrants, such as flight muscles or even wings, along with the expense of fuel usage during the energetically demanding process of flight may reduce lifetime fecundity [6,7,9] by diverting resources from egg development. In the Glanville fritillary butterfly, Melitaea cinxia, lifetime fecundity was higher in the more dispersive females than in the less dispersive individuals [14]. The reproductive consequences after flight may be substantially different from those of unflown migratory-phase adults, because reproduction and its governing physiological processes usually do not occur until after migration [2,12,13]. In the wing-dimorphic cricket Gryllus texensis, the mating capacity and ovary weight of the flight-capable morph is generally poorer than that of the short-winged morph, but is comparable when measured after the former has experienced flight [11,16]

Methods

Results

Conclusion