College of Natural & Agricultural Sciences

As Florida health authorities work to respond to an ever-growing cadre of invasive tropical mosquitoes, a research team has sharpened an environmentally friendly tool increasingly deployed against a dangerous species that invaded the state two centuries ago. The mosquito is Aedes aegypti, vector of yellow fever, dengue fever, chikungunya, and Zika fever. It is one of the mosquitoes increasingly targeted with the sterile insect technique (SIT), in which male insects reared en masse are sterilized by gamma-ray or x-ray ionization and released to mate with wild females, which then produce non-viable eggs. Besides being environmentally innocuous, SIT is not hindered by insecticide resistance, a huge plus for pest managers.

Long used against flies and other agricultural insect pests, SIT has shown promise against disease vectors such as mosquitoes, but a few wrinkles have kept it from reaching full potential. Research on Aedes aegypti described in a new study published in June in the Journal of Medical Entomology has smoothed the process, showing that the success of SIT can be measurably enhanced if male mosquitoes are sterilized when they emerge as adults, rather than as pupae, the approach now in use.

“Sterile insect technique is an amazing tool for area-wide control that can be targeted to specific mosquitoes that can vector human pathogens,” says Dylan A. Tussey, Ph.D., a research assistant in entomology at the University of Florida at the time of the study and now a postdoctoral scholar at the University of California, Riverside.

“Improving the logistics of rearing and sterilizing mosquitoes can increase the production of sterile males, decrease the overall costs, and ultimately make mosquito SIT a viable control option in areas at risk for pathogen transmission,” he says.

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During the SIT rearing process, the pupal stage is the first at which males can be distinguished and separated from females for irradiation. Upon emerging, the males are chilled to dormancy and then packed into containers as tightly as possible without damaging them, for transport to release sites. (Research shared here at Entomology Today in 2018 described how 2,500 mosquitoes can be housed in a 10-milliliter syringe.) Since mosquitoes typically fly less than the length of two football fields in a lifetime, release must be exactly targeted, not always the case with other insects controlled with SIT.

A big drawback in the process to date has been that pupal development varies among individuals and age, so not all pupae respond the same way to radiation, making it difficult for the mass production of males SIT requires. Young adult mosquitoes, on the other hand, have wider windows for sterilization than pupae, allowing facilities to create fixed schedules for irradiation, streamlining the process and increasing economy.

Scientists conducting the experiments examined the impacts of all aspects of the process—chilling, compaction, and radiation dose—on the survival of mosquitoes and used what they learned to develop a protocol for irradiating adult males. It was clear from the experiments that, while males irradiated as adults had similar sterility compared to males irradiated as pupae, the former lived longer and were more sexually competitive. Their work, say the researchers, shows that “irradiating adult males can be a viable option to increase the efficiency of this operational mosquito SIT program.”

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Working with such tiny insects demands precision and delicacy. Researcher Rachel Morreale of the Lee County (Florida) Mosquito Control District describes how scientists handle such delicate insects: “Live mosquitoes from pupal to adult stages tend to be handled using brushes. A makeup fan brush and small paint brushes are gentle ways of letting us move mosquitoes without risking damage. While fine tip forceps may be used occasionally, they can damage the insects so are not preferred. If an adult is manipulated or transferred from one container to another and isn’t immobilized, it is aspirated [using a suction tube].”

Pupae are sorted using a device called a larval-pupal separator. Essentially, it is made of two rectangular glass plates, mounted on two aluminum legs, with a space between them. Its width, narrower on top, is controlled by two knobs on the front. When poured into the top, larger females are caught on top and males filter down below. Turning the knobs releases the whole batch for collection.

Producing enough mosquitoes for the research mimics to some extent how eggs hatch in the wild. Laboratories that raise mosquitoes generally trigger hatching by subjecting eggs to deoxygenated water. It appears that in nature low dissolved oxygen signals that water is chock full of anaerobic bacteria, on which mosquito larvae feed. Meanwhile, eggs used in the study were flooded with a nutrient slurry that substitutes for bacteria, promoting the synchronous hatching necessary for mass production.

Part of the study addressed how long it took for males to recover from cold temperatures needed to render them dormant. Says Morreale, “This was important because we needed to see how long we had to work with males that had been in the refrigerator for varying lengths of time before they started to move again. This factors into adult handling so that we can move the males from emergence containers to the irradiation containers where they will also be compacted. Males in chill coma are easy to work with and you don’t have to worry about them flying away.”

When recovering from chilling, mosquitoes are on their back, with their legs and wings jerking at first. Once they self-right, they are capable of coordinated movement, strolling about a bit before they take flight—hopefully, in this case, to help health authorities in the fight against disease.

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