Population Colonization, Re-Establishment and Development of the Tomato Leafminer (Tuta absoluta)

The tomato leafminer (Tuta absoluta), one of the most destructive insects of Solanaceae crops, is a worldwide quarantine pest. T. absoluta was first recorded in open field in northwestern China, Ili, Xinjiang in 2017, and detected in greenhouses in southwestern China, Lincang, Yunnan in 2018. At present, T. absoluta has rapidly spread to more than 20 provincial-level regions in China.

The colonization, re-establishment and development ability of invading populations of T. absoluta were evaluated under indoor caged conditions using adults as the research object.

The female fecundity and percentage of larval hatching of each propagule were tested by introducing 5 different numbers of propagules (2, 4, 6, 8, and 10 individuals) of T. absoluta adults (sex ratio = 1∶1) into different cages. Subsequently, the effect of propagule size on the ability of population colonization of T. absoluta was evaluated. During the population re-establishment and development starting from one pair of adults (2 individuals, sex ratio = 1∶1), the number of individuals (including adults, eggs, and larvae), the percentage of individuals (including different developmental stages and larval instars) and the richness (totally 6 developmental stages and larval instars) of T. absoluta were investigated. Subsequently, the population re-establishment and development starting from the minimum number (one pair of adults) of adult individuals of T. absoluta were evaluated.

Introducing adults with 5 propagule sizes (i.e., 2, 4, 6, 8, and 10 individuals; sex ratio=1∶1) into each cage, the numbers of eggs laid by all females were 93.1, 194.9, 271.3, 311.5, and 400.2 eggs/cage, respectively, significant differences were detected between the 5 propagule sizes. The average fecundity was 93.1, 97.4, 90.4, 77.9, and 80.0 eggs/female, respectively; significant differences were detected between them with that the average fecundity (per female) detected in propagule sizes 1♀∶1♂, 2♀∶2♂, and 3♀∶3♂ was significantly higher than that of propagule sizes 4♀∶4♂ and 5♀∶5♂. The percentage of larval hatching in all the 5 propagule sizes was 91.3%-92.7%, all of which were relatively high. Comprehensive evaluation, based on above three indexes, showed that the population colonization ability of the initial number of propagules 5♀∶5♂ was the highest, and then followed by the 3♀∶3♂, the next was the 4♀∶4♂, and then the 2♀∶2♂, that of 1♀∶1♂ was the lowest. One pair of T. absoluta adults (2 individuals, sex ratio=1∶1) could re-establish a population; after ~90 d (3 generations), the population could be in a stable state. Three distinct population peaks were observed in the initial 0-91 d of population re-establishment and development (~3 generations). The numbers of total individuals (pupa not included) of T. absoluta at the three peak days increased by 77.8-, 60.5-, and 60.1-fold compared to the initial number of propagules (2 individuals), and adults increased by 6.4-, 4.1-, and 7.7-fold, respectively. Afterwards, all developmental stages, including eggs, 1-4 instar larvae, and adults, occurred simultaneously showing that the phenomenon of overlapping generations was obvious. The population tended to stabilize and in fluctuating growth with the total population size of 96.6-140.6 individuals and the ratios of different developmental stages (egg and adult) and larval instars (1-4 larval instars) around 16.67%, and the richness index R was stable at 1.011-1.094.

One pair of T. absoluta adults had high reproductive and survival abilities. The larger the initial number of invaded individuals, the greater the likelihood of successful colonization of T. absoluta. One female and one male adults of T. absoluta could successfully re-establish a population and maintain long-term stability of the population. The introduction of one pair of T. absoluta adults could likely cause significant risk of invasion and might generate enough progenies to meet the needs of population development and expansion. In the work of prevention and control, the monitoring and warning of T. absoluta should be strengthened and it should be eliminated as early as possible.