The mosquito Aedes aegypti were considered to be one of the most dangerous medical insects in the world 1 , as they were vectors of several globally important vector-borne diseases, including dengue virus (DENV) 2 , yellow fever virus 3 and chikungunya virus (CHIKV) 4 . In the past 50 years, the dengue epidemic caused more than one billion infections and one million deaths, and the number of cases have increased 30 times without any signs of slowing down 5,6 . Moreover, in Asia and the Americas, the burden of dengue was approximately 1300 disability-adjusted life years (DALYs) per million population, which was similar to the other childhood and tropical diseases in this regions 7 . Since 1978, dengue has been detected for nearly forty years in China and it also occasionally erupts in Guangdong Province every year. Geographically, the dengue outbreaks have gradually expanded from Guangdong, Hainan, and Guangxi in Southern coastal regions of China to other regions including Fujian, Zhejiang and Yunnan Provinces 8 . Due to the outbreak of dengue fever in Guangdong Province in 2014, more than ten thousands of people were infected by the dengue epidemic and the local government spent >$30 million US dollars to control mosquito populations 9 .
Until now, because of no vaccine or specific treatment for dengue fever, many indirect prevention and control measures have been taken, such as population surveillance, disease prevention and rapid outbreak response are used to improve mosquito vector control1. The use of insecticides has often been the only feasible method of disease control. For instance, wide-scale house spraying of DDT during the 1950s to 1960s dramatically reduced malaria incidence in Asia, and the pyrethroids were introduced to Cambodia in the late 1980s for malaria and dengue control5. In China, more than 27, 000 kg of pyrethroids were used for ultra-low volume spraying in the dengue control of Guangzhou city in 2014, and a large amounts of temephos and fenthion were also used as larvicides to prevent the mosquito spread10. However, the use of chemical pesticides not only polluted the environment but also caused great harm to non-target insects including butterfly, honeybee, bumblebee and other pollinators. The widespread use of chemical pesticides could easily interfere normal physiological behavior of butterfly and bee, such as visiting flowers, foraging, pollination and other ecological function11,12.
Insect growth regulators (IGRs) have been widely used in pest control in many areas, which were highly effective larviciding agents of mosquito larvae and showed low mammalian toxicity and more safety to most of non-target organisms13,14. Pyriproxyfen and S-methoprene were two kinds of effective juvenile hormone mimic with low toxicity to mammals, which could affect the hormonal balance in insects, thus strongly inhibit embryogenesis, metamorphosis and adult formation15,16. Diflubenzuron was also a readily available insect growth regulator, which could inhibit chitin synthesis and exhibited the ovicidal and larvicidal properties17. Besides, there were also some bioinsecticides used for larvae control, such as the microbial agent Bacillus thuringiensis (Bti) has become the most commonly used larvicide worldwide, which can form spores which contain crystals, predominantly comprising one or more Cry and/or Cyt proteins that have potent and specific insecticidal activity18,19. Plant-derived essential oils were also an environmentally friendly control agent, especially for repelling adult mosquitoes, indicating that they can be an important supplement for mosquito pest control20.
In addition, nematodes also played a key role in the field prevention of mosquito pests, such as Romanomermis iyengari (Mermithidae) was one of entomopathogenic nematodes which parasitized and killed mosquito larvae21. In our laboratory, the mermithid nematode R. wuchangensis has also been successfully used as an ecosystem-friendly biocontrol agent for mosquito control. Our previous study indicated that R. wuchangensis could infect C. quinquefasciatus and Ae. albopictus, and the infection rate and fatality rate of C. quinquefasciatus reached 49.18% and 100% in the field experiment29. The results demonstrated that the control of mosquito by nematodes had the characteristics of environmental friendliness and long-term sustainability, suggesting they maybe widely used in field biological control of mosquito pests.
Reactive oxygen species (ROS) produced by insect cells was the first line of defense against the invasion of insecticides and parasites, including hydrogen peroxide (H2O2), free hydroxyl (OH−) and superoxide anion (O2−)22. Insects mainly protected themselves by antioxidant enzymes, such as Superoxide dismutase (SOD) could effectively remove O2− and convert it into H2O2, Catalase (CAT) and Peroxidase (POD) work together to remove H2O2, which these antioxidant enzymes coordinated to regulate ROS in insects to keep them in dynamic balance23,24. Early studies have shown that in the process of parasitizing the host, the death of the host was usually caused by oxidative damage, including Sarcophagha crassipalpis, Nasonia vitripennis, Plutella xylostella,, Cotesia plutellae, Tenebrio Molitor and Scleroderma guani, etc25–27. However, there was no report about the oxidative damage effect of nematode on the mosquito, though it may play a vital role of biological control of mosquitoes.
In this study, we present statistics on the incidence of dengue fever in China from 2015 to 2018 prior to the novel Coronavirus outbreak. Besides, we compared the control effects of chemical insecticides, IGRs, B. thuringiensis and plant essential oil on the larvae and adult of Ae. aegypti. The mortality rate of Ae. aegypti was observed during parasitized by R. wuchangensis, and the activities of SOD, POD and CAT during the lethal period of the host were tested to determine whether oxidative damage was involved, then explore the potential cause of oxidative damage to the host through the content of Malondialdehyde (MDA). Simultaneously, Real-time quantitative PCR were used to investigate the expression patterns of SOD, POD and CAT genes of Ae. aegypti larvae under different parasitic period by R. wuchangensis. In addition, the effective components of plant essential oil was identified from Mentha haplocalyx and chemoecological functional analysis of olfactory protein (OBP1) of Ae. aegypti were also investigated.