Ochlerotatus togoi is a salt-tolerant euryhaline mosquito that lays its eggs in rock pools. Although it is a pest that transmits flaviviruses and filarial worms to humans, ecological studies have not been actively conducted because of its limited habitat. Rising sea levels have created a favorable environment for Oc. togoi to survive; additionally, the incidence of  Oc. togoi-borne diseases may increase with an increase in population of Oc. togoi. We examined oviposition and growth rates in the range of 0–35 psu to obtain ecological data for Oc. togoi. Ochlerotatus togoi exhibited the highest oviposition preference at 0 psu; however, the hatching rate was highest at 10 psu, the pupation rate was highest at 25 psu, and the emergence rate was highest at 5 psu. Ochlerotatus togoi showed the highest rate of growth into adults at 25 psu. The results were verified for statistical significance using Mann–Whitney U and Kruskal–Wallis H tests (post hoc test: Bonferroni), and a regression equation was generated for the incidence of adult Oc. togoi based on the change in salinity (y = −14.318 + 9.821x; y = adult incidence rate; x = salinity). The oviposition habits and developmental conditions of Oc. togoi were confirmed and the number of Oc. togoi was predicted based on the changes in sea level and ocean salinity. The results of this study are expected to be used for controlling salt-tolerant vectors and responding to vector-borne diseases.

1. Collection and Classification

Samples of Oc. togoi were collected from Geoje, Jeju, and Pohang, South Korea. Larvae were collected from stagnant water of artificial structures or rock pools near the coast (Table 1). The collected samples were classified according to the salinity of the habitat, and transferred to the insectary (temperature 27°C ± 1°C, humidity 70%, L:D = 12:12) of the Animal Systematics & Taxonomy Laboratory of Kyungpook National University. They were raised under the same salinity in insectary as the salinity at the collection points. The hatched adults were then used for the oviposition preference experiment and the eggs laid by the adults were used for growth rate experiments. The collected Oc. togoi were morphologically classified according to the taxonomic keys described previously (Lee, 1987; Ree, 2003). Molecular identification (forward primer: 5′-AGG ACA CAT GAA CAC CCA CA-3′; reverse primer: 5′-AGG CGG TGG AGT GTA TGG-3′) was performed as described previously (Bang, 2021).

2. Oviposition preference

Adult Oc. togoi females (n = 100) collected from the same collection points were randomly selected and set as one group. The experiment was conducted in nine groups according to nine collection points (Table 1). The Oc. togoi group was placed into an adult mosquito cage containing 10% concentration sugar cotton and fed for 3 days. An acrylic case containing eight salinity concentration (0, 5, 10, 15, 20, 25, 30, and 35 psu; psu: practical salinity unit) was made by aquarium salt with distilled water was then established. Then the number of eggs laid for 24 h in each salinity concentration was counted (Fig 1). Each salinity concentration was replicated three times producing 27 experimental units, and 27 oviposition preference experiments were performed.

3. Growth rate

As with oviposition preference, the experiments were conducted in an acrylic case divided into eight salinity concentration. The growth of Oc. togoi was determined by placing 100 eggs into each salinity concentration. All eggs were oviposited under the same conditions in insectary (temperature 27°C ± 1°C, humidity 70%, L:D = 12:12) and all developing larvae were provided the same amount of food. The hatching rate was determined using the cumulative number of larvae hatched from the eggs, the pupation rate was determined using the cumulative number of pupae from the larvae, and the emergence rate was assessed using the cumulative number of adults emerging from the pupa. Finally, the adult rate (the number of eggs who made it to adult) was determined using the cumulative population of adults grown from eggs. A total of nine growth rate experiments were conducted for Oc. togoi collected from Geoje, Jeju, and Pohang, in triplicate. Experiments were conducted with Oc. togoi collected from as similar salinity as possible to minimize variables that are feared to be due to differences in salinity at collection points (Geoje: 0.5 psu; Jeju: 1 psu; Pohang: 1.2 psu).

4. Control group

To compare the salt-tolerance ability of Oc. togoi, a freshwater species, Aedes albopictus, was used as the control group. Aedes albopictus was raised individually in the insectary of the Animal Systematics & Taxonomy Laboratory of Kyungpook National University. The oviposition preference and growth rate experiments were conducted in the same way as that for Oc. togoi and repeated three or five times, respectively.

5. Statistical analysis

Significance was verified by comparing the average growth rates of Ae. albopictus and Oc. togoi using Mann–Whitney U test. Using Kruskal–Wallis H test, the average growth rates of Oc. togoi collected from Geoje, Jeju, and Pohang and Ae. albopictus were compared to establish statistical significance among the four groups. To confirm which of the four groups exhibited statistical significance, a post hoc analysis was performed using Bonferroni’s multiple comparison test. Finally, a simple linear regression analysis was performed based on the adult rate.