The Sex Ratio Indicates the Conclusion and Onset of Population Cycles in the Beet Webworm Loxostege sticticalis L. (Lepidoptera: Pyralidae).

Beet webworms, Loxostege sticticalis L. (Lepidoptera: Pyralidae), are one of the most destructive pest insects in northern China, and their populations outbreak periodically. Developing an indicator that defines the ending and beginning of the occurrence period cycle is urgent for the population forecast and theoretical study. The sex ratio can be a primary pathway through which species regulate population size. We measured the maximum mating potential of both females and males and the population net reproductive rate under different sex ratios (e.g., 3:1, 2:1, 1:1, 1:2, 1:3). The maximum mating frequency of males was 2.91 times that of females. The progeny contribution per mating decreased with increased mating times in males. The variation in population net reproductive rate affected by the sex ratio fits the parabolic curve analysis and peaked at 1.82 for females vs. males. Our results illustrate the quantitative connection phenomenon shown by the historical data: population outbreaks occur at a sex ratio of two or more and collapse at a sex rate lower than one. Simultaneously, the sex ratio may be utilized as a definite indicator for the beginning and end of the future occurrence cycle in the beet webworm.

Drying soil in North China drove the outbreak range expansion of meadow moth by facilitating long-distance migration.

Studies of the mechanism underlying the range expansion of organisms have mainly focused on environmental conditions at the edges of species' distributions, potentially ignoring other possible factors. Here, we demonstrated the outbreak range expansion of meadow moth from North China to Northeast China in the past three outbreak periods. We found that the negligible infestation in Northeast China in the 1950s could not be explained by local climatic conditions. However, the soil moisture in North China decreased distinctly from 1951 to 2011 and was significantly and positively correlated with the timing of the first adult peak on plateaus, meaning that the deterioration of habitat conditions could result in earlier peaks of adults in areas of high-elevation by stimulating the short-distance dispersal of adults from the plains to the plateaus. Adults peaking earlier have a stronger tendency to emigrate due to mismatched phenology. Hence, drying soil in North China caused the frequent long-distance migration of meadow moth after the 1970s and drove the outbreak range expansion. This study suggests that, for a migratory species, the deterioration of habitat conditions in overwintering areas might also influence the distribution of this species in breeding areas at high latitudes by facilitating migration activities.