System with cheap and biodegradable materials allows controlled release of larvicidal compound and can be used in small volumes of water
Corn starch, an abundant, cheap and biodegradable raw material, was the basis used by researchers at Unicamp to develop particles capable of storing and controllingly releasing active compounds lethal to mosquito larvae. Aedes aegypti, transmitter of diseases such as dengue, zika, yellow fever and chikungunya.
The methodology was granted a patent through the Unicamp Innovation Agency (Inova) and was described in an article in the magazine Industrial Crops and Products.
In the work, supported by FAPESP and coordinated by Ana Silvia Prata, professor at the Faculty of Food Engineering (FEA) at Unicamp, thyme essential oil was tested as a larvicidal agent. This oil is also biodegradable and, at the concentration used in the research, does not pose any risks to human health.
“We managed to obtain a particle that behaves exactly like Aedes eggs. While the environment is dry, it remains inert and keeps the active agent protected. From the moment it comes into contact with water, it begins to swell to allow the larvicide to be released. After three days, a period in which the eggs hatch and the larval phase begins, the particle starts to release lethal amounts of the active ingredient into the water,” said Prata.
The idea of the project was to develop a controlled larvicide release system for small volumes of water, such as plant pots, tires, bottles and various debris that can become mosquito breeding grounds in the urban environment.
According to Prata, health authorities have been concerned about treating water tanks and other large reservoirs with larvicides, but epidemiological studies indicate that 50% of Aedes outbreaks are in small puddles.
“As the cost is low, the government could produce these particles and distribute them to the population, so that they could be spread in areas of the residence with the potential to accumulate rainwater, as a complementary measure to raise public awareness and the fight against dengue. ", he said.
Results of tests carried out at Unicamp indicate that the particles could remain functional during approximately five rain cycles. After the first contact with water they release only 20% of the thyme oil. “We tested letting the material dry and then rehydrating it and observed that the particles released the larvicidal agent normally again,” said Prata.
According to the researcher, the main active compound found in thyme oil – thymol – prevented the proliferation of microorganisms in the container containing the water, preventing the particles from spoiling quickly after getting wet.
production method
The life cycle of Aedes aegypti It is made up of four stages: egg, larva, pupa and adult mosquito. The total development period can range from five to 10 days, becoming shorter as the temperature increases. The larval phase, in which the insect is confined to the aquatic environment, is considered the most strategic for combat actions.
“Based on this information, we started thinking about what the particle should look like. One of our collaborators – Johan Ubbink [California Polytechnic State University, United States] – suggested producing it using a technique known as extrusion, the same used in the manufacture of corn snacks,” said Prata.
The method consists of forcing the moist and heated starch mass through a small hole. Normally, the action of temperature and pressure exerted by a screw causes the material to expand after passing through the hole.
“We adapted the process, adopting a milder temperature and a smoother screw rotation, so that the material did not expand. Otherwise, the particle would quickly soften upon contact with water, releasing the active ingredient all at once,” said Prata.
Another challenge for the group was finding the appropriate composition of the raw material. As the researcher explained, starch – whether from wheat, corn or any other source – is fundamentally composed of variable fractions of amylose and amylopectin. The amount of each of these components determines characteristics such as viscosity and structure (ability not to fall apart in contact with water).
“We tested formulations that had from 1,8% to 76% amylose. And we evaluated, in each case, what was the leaching behavior [how quickly the particle breaks apart] and swelling in the aquatic environment,” said Prata.
While evaluating these two aspects of the particle, measuring the amount of thyme oil released as a function of the time of contact with water, the researchers also observed the larvicidal activity of the active compound. The test consisted of measuring the concentration necessary to kill 99% of the larvae – a parameter known as CL99.
“The LC99 of unencapsulated thyme oil is approximately 70 micrograms per milliliter [µg/ml]. When we put this compound inside the particle, the value decreases to 31 µg/ml, that is, our controlled release system increased the larvicidal action”, said the researcher.
Even so, the LC99 of the natural compound remained much lower than that of synthetic agents, such as temephos. The advantage, according to Prata, is that because it has a complex chemical composition, with other active molecules in addition to thymol, it is more difficult for the insect to develop resistance.
The group also tested jambu extract as a larvicide. The result was similar to that observed with thyme, however, the cost was around 15 times higher.
“Thyme essential oil is a highly available material, sold commercially and represents only 5% of the particle composition – the other 95% is corn starch, which is very cheap. That’s why we consider the technique to be easily scalable”, said the FEA-Unicamp professor.
The FEA-Unicamp group is currently evaluating the possibility of using the same particles to encapsulate nitrogen-fixing bacteria, which help plant growth. The material could, in theory, reduce the amount of fertilizers used in agriculture. “This is a theory that we intend to test in a future project,” said Prata.
The article
The article Improved activity of thyme essential oil (Thymus vulgaris) against Aedes aegypti larvae using a biodegradable controlled release system, by Juliana Dias Maia, Roseli La Corte, Julian Martinez, Johan Ubbink and Ana Silvia Prata, can be read at: https://www.sciencedirect.com/science/article/pii/S0926669019301967?via%3Dihub#!.