Research Article |
Corresponding author: Aline Nunes-Silva ( alinenunes420@gmail.com ) Academic editor: Walter Boeger
© 2020 Aline Nunes-Silva, Isabela de Lima-Keppe, Robson V. de Souza, Gilmar Perbiche-Neves.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Nunes-Silva A, Lima-Keppe I, Souza RV, Perbiche-Neves G (2020) Survival of the copepod Mesocyclops longisetus during simulations of transport from hatchery to target areas for biological control of mosquito larvae. Zoologia 37: 1-4. https://doi.org/10.3897/zoologia.37.e37658
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Copepods have been successfully used in many countries for the biological control of larvae of mosquitoes that vector diseases. In Brazil, this line of research has been focused on the use of the copepod Mesocyclops longisetus (Thiébaud, 1914) for the biological control of the mosquito Aedes aegypti (Linnaeus, 1792). The transportation of the copepods from the place where they are reared to where they will be used often involves long distances for extended periods of time. This study assesses the survivorship of M. longisetus during simulation of transport under different conditions. Different loading densities (20, 30, 40, 80, and 120 ind.L) and stirring times (30 minutes, one hour, two hours, and four hours) were tested. Survivorship was high, with 75% of the results equal or higher than 90% survival. Reduced mortality was observed when transportation time was up to 120 minutes and densities were up to 40 ind.L. In higher densities or longer transportation times, the mortality rate was significantly affected.
Aedes aegypti, Copepoda, Culicidae, density
The mosquito Aedes aegypti (Linnaeus, 1792) is a vector of viruses that cause several globally important diseases, such as dengue fever, yellow fever (
The chemical control of mosquitoes using insecticides and larvicides raises concerns for the health of the people who have to apply the products (
The technology for the large-scale cultivation of M. longisetus is already available (
Copepods were reared in one-liter plastic containers. The containers were filled with mineral water fertilized with 0.02 grams per liter of NPK (10:10:10). The goal of the fertilization was the creation of a phytoplankton biomass to feed the copepods. The transport was simulated by fulfilling 1.5- and two-liter plastic bottles with a mixture of culture medium containing copepods and mineral water (same proportion 1:1) and stirring the bottles using an orbital shaker model SL-180/A (2.5 shakes per second). Different copepod densities were tested: 20, 30, 40, 80, and 120 ind.L. The transport times varied from 30 minutes to one hour, two hours, and four hours. All treatments were tested in triplicates. Copepod survivorship was estimated after a resting period of 12 hours. Survivorship of the control treatment, consisting of three bottles that were not stirred, was also estimated for each of the tested densities.
The effect of copepod density and transport simulation time (explanatory variables) versus copepod survival (response variable) were evaluated separately. First, we evaluated the effect of transportation duration versus copepod survivorship separately for each copepod density, and then the effect of copepod density versus copepod survivorship separately for each transport duration. After that, the combined effect of the explanatory variables was evaluated through multiple linear regression analysis. The analyses were performed using R Cran Project 3.3.0 software (2016).
Mean copepod survivorship after transport simulations was 95%, ranging from 70 to 100%. Additionally, 75% of the results were equal to or higher than 90% survival. The mortality of copepods that were not subjected to stirring ranged from 90 to 100%, with a mean of 98.75%. When the effect of transport duration versus copepod survival was evaluated separately for each of the densities tested, significant correlations were obtained only for the highest densities: 80 and 120 ind.L. In both cases, a negative correlation was obtained, with a reduction in copepod survival as copepod density increased. A weak correlation was obtained for 80 ind.L (R2 = 0.23) and a moderate for 120 ind.L (R2 = 0.57) (Fig.
The dots represent the observations and the lines the linear regression models for significant correlations. The first row of graphs demonstrates the effect of transport time over copepod survival separately for different loading densities and the second demonstrates the effect of the loading density over the copepod survival, separately for different transport durations. Significant correlations: Density 80 ind.L-1 – Model: Copepod survival = 95.81 - 0.039*transport time (p-value = 0.038, Adjusted R2 = 0.23); Density 120 ind.L-1 – Model: Copepod survival = 97.58 - 0.071*transport time (p-value = > 0.001, Adjusted R2 = 0.57); Transport time 240 min – Model: Copepod survival = 100.97 - 0.186*copepod density (p-value = 0.0003, Adjusted R2 = 0.46).
When the effect of copepod density versus copepod survival was evaluated separately for each of the transport times tested, a significant correlation was obtained only for the longest time, 240 minutes. This was a negative and moderate correlation (R2 = 0.46) with decreasing copepod survival rate as the transport time increased (Fig.
When the combined effect of density and transport time were evaluated, the importance of the interaction between the two explanatory variables was evidenced. Significant correlations were obtained for both models considering and not considering this interaction (p-values of 0.015 and <0.001, respectively). The correlation was weak for both and the models explained 20% and 8.1%, respectively, of the variance in copepod survivorship. A higher R2 (0.20) was obtained when the interaction was considered, compared to the model that disregarded the interaction between explanatory variables (R2 = 0.08) (Figs
The dots indicate the observations and the surface plot the multiple linear regression models not considering (2) and considering (3) the interaction between explanatory variables. Models: (2) Copepod survivorship = 97.65 - 0.023*transport time - 0.03*copepod density (p-value = 0.015, Adjusted R2 = 0.084); (3) Copepod survivorship = 93.40 + 0.024*transport time + 0.035*copepod density - 0.0008*transport time*copepod density (p-value = > 0.001, Adjusted R2 = 0.20).
To our knowledge, this is the first study that investigated the survivorship of copepods under transport conditions. Similar studies on other crustacean species are available in the scientific literature, especially on the transport of younglings. Most of the studies indicated that crustaceans are very resistant to transport conditions.
In the case of the copepod Penaeus monodon (Fabricius, 1798), for example, the survival rate of post-larvae after eight hours of transport in a density of 600 ind.L was up to 95% (
The present study tested loading densities of up to 120 ind.L under a maximum transport time of 240 minutes. The survivorship of M. longisetus was high, with 75% of the results equal or higher than 90% survival. The results also showed that the minimum mortality is expected for transport times up to 120 minutes and loading densities up to 40 ind.L. For higher densities or longer transport times, however, mortality seems to be significantly affected. Our results also indicated that the loading density and duration of the transport act synergistically in the reduction of survivorship of copepods. Future studies are important to test higher densities and longer transport times to confirm these tendencies.
Mortalities of at least 10%of the copepods were obtained for all the treatments, including the control groups. It is possible that the act of transferring copepods to the transport vials was at least in part responsible for this mortality. Future studies are needed to investigate ways to minimize the damage of the copepods during the transference to the transport vials.
Based on our results, it is possible to suggest that M. longisetus can be successfully transported from hatchery to target areas for the biological control of A. aegypti mosquitoes in densities up to 40 ind.L during 120 minutes with minimum mortality. This information can be useful to the establishment of transport strategies in programs for biological control of A. aegypti. More experiments are needed to find out how long these copepods can survive under transport conditions.
We thank to Conselho Nacional de Desenvolvimento Científico e Tecnológico for the financial support (CNPq projects 425799/2016-6 and 142793/2018-3).