In our previous article on the project “Essential oil distillation waste streams as a potential source of sustainable plant-based repellent products” we looked at the path of plant processing by-products, from seemingly low-value biomass to valuable water extracts, this time let’s explore the next step in the process – insect testing. Kristīne Berķe-Ļubinska, Head of the Laboratory of Applied Entomology of the Institute of Environmental Solutions, will guide us through this section of the project.
Let’s start from the very beginning. Tell us how the idea of developing plant-based insect repellents came about!
The market niche of insect repellents for agricultural use is extremely narrow. Repellents for ticks, mosquitoes and other insects intended for humans and animals can be easily purchased, herbivore repellents can be found as well, but insect repellents for agricultural use are rarely available. Of course, various insecticides can be used. However, in the first place, only few insecticides are allowed for use in organic farming. Secondly, many insecticides used in conventional agriculture (especially when use instructions are neglected) are harmful to the environment and biological diversity. By creating a plant-based insect repellent, our goal is not to kill the insect – we don’t want it to touch down on the plants. This way insects cannot not reproduce on the leaves of the plant, are unable to lay eggs on them, and do not transmit various viruses to the plant. Insects getting accustomed with insecticides is also a problem – what works today may have no effect tomorrow.
What criteria did you use to select the insect species included in the study?
We tried to select the insect species that cause the most damage and yield loss to farmers every year.
Greenhouse whitefly (Trialeurodes vaporariorum) mainly destroys plants growing in greenhouses and other enclosed areas. Cucumbers, tomatoes, peppers and other popular greenhouse plants are attacked by whiteflies. The insect not only directly damages the plants by sucking their sap, but is also an active vector for mosaic viruses.
Greenhouse whiteflies at the Applied Entomology Laboratoy of IES. Photo: IES.
Peach aphid (Myzus persicae) is a very “universal” pest – it preys on a wide variety of plant species. Peach aphids are considered the main plant virus vector in the world – the insect transmits more than 100 different viruses. This is possible because aphids are highly resistant to various weather conditions and can be transported very far in wind streams. Aphids’ direct damage mostly affects potato yields, but plants from 40 different families, including Brassicaceae, Solanaceae, Poaceae, Leguminosae, Cyperaceae , Convolvulaceae, Chenopodiaceae, Compositae, Cucurbitaceae and Umbelliferae suffer as well.
Peach aphids at the Applied Entomology Laboratory at IES. Photo: IES.
Both whiteflies and aphids release honeydew on plant leaves. It is a sticky, sugary solution in which sooty mould easily develops.
Damage of the cabbage white (Pieris brassicae) is easily observed on crop fields. The larvae literally gnaw plantations in giant areas – the yield of cabbage, rape, mustard, horseradish and other cruciferous plants suffers to a great extent.
Cabbage white at the Applied Entomology Laboratory of IES. Photo: IES.
Tell us, where were the insects collected and where are they grown and reared?
Although our goal is a repellent for agricultural use, the insects were collected in the surrounding greenhouses and gardens. Insect populations of all three species are maintained in the Laboratory of Practical Entomology of the Institute of Environmental Solutions. The laboratory ensures adequate air temperature and humidity. The light mode also mimics the natural conditions. Food plants are provided for each species. Whiteflies live on cucumber plants, peach aphids on cabbage plants, while cabbage whites need cabbage seedlings for laying eggs and as a source of larval food. Adult cabbage whites feed on sugar syrup. Cucumber and cabbage seedlings are grown in the institute’s climate chamber, where the plants are provided optimal growing conditions throughout the year.
And what about insect reproduction?
For example, we have bred 17 generations of whiteflies. But for cabbage whites, we give each specimen a “name” – a unique number by which we can trace its ancestors. During the past year, 641 cabbage whites were grown in the laboratory. The maximum lifespan of a cabbage white in our laboratory is 42 days. This is the maximum margin life expectancy in the nature, but more often whites live in nature for only 3-5 weeks. In turn, the aphid population needs to be regularly reduced in order for the specimens to develop large and healthy.
Reproduction and cultivation of insects is only one part of the job. Tell us how to test repellents for insects!
Our goal is to simulate the spraying of the repellent in the field under laboratory conditions. A special spray chamber will be used for this purpose. Two cucumber plants will be placed in two glass vases. One plant sprayed with repellent, the other – unsprayed. Purified air will be passed through both vases (at constant temperature and humidity to ensure repeatability of the experiment). The air from both vases will be fed to the test maze, where the insect will have the choice to go in the direction of the normal cucumber smell or in the direction of the sprayed cucumber smell. Observing a convincing tendency that insects choose not to go in the direction of the sprayed plant, we’ll be able to assume that the particular repellent formula has potential! It is important that each insect is used only once in the tests to avoid possible effects of the insect’s experience.
The air-humidifying system of the device used for insect smell tests (olfactometer). Photo: IES.
Y-shape insect test maze. Photo: IES.
Sounds like hours of insect watching!
Not in our case! To avoid wasting time, we will provide video surveillance of the maze. The movement of the insects will be recorded, and the result will be visualizations of the insect tracks. By analyzing the tracks, we will be able to understand the path of thought of the insects!
The research is a part of the project “Essential oil distillation waste streams as a potential source of sustainable plant-based repellent products” (No. Nr. 18.104.22.168/20/A/096). It is developed as a part of the European Regional Development Fund programme 22.214.171.124 measure “Support for applied research” and specific objective 1.1.1 “Improve research and innovation capacity and the ability of Latvian research institutions to attract external funding, by investing in human capital and infrastructure”.