In the garden and in agriculture, aphids can become a real plague. To determine the infestation, exploit the aphids’ preference for the color yellow and set up yellow sticky traps. But why is this color so attractive to crawlers? Researchers now present two new models for analyzing color vision in aphids, which may offer new opportunities for agricultural pest management.
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Whether as a model organism for science or as a plague in agriculture: aphids still keep a lot of people busy with their sophisticated survival mechanisms. The small insects suck plant sap from the leaves and stems of a wide variety of crops, but also serve as food for other animals such as birds, lacewings and ladybirds. The latter even hunt the aphids so efficiently that they are bred specifically for biological pest control. Another method of catching aphids that is widely used in agriculture is to set up yellow traps. Because numerous field studies have shown that the small pests are much more attracted to pure yellow than to other colors.
Insects see differently than we do
But why the color yellow is preferred by aphids is still a mystery to scientists. “Despite the large number of studies on the reaction of aphids to paint, research into the underlying mechanisms has only begun in the last two decades,” explain agricultural scientists Thomas Döring from the University of Bonn and Sascha Kirchner from the University of Kassel. However, this is important because the color yellow has so far only been identified as the favorite color of aphids through a trial-and-error approach. However, the small insects perceive the color spectrum differently than we do, so that they hardly like the color we define as “yellow”, but most likely a range in the light spectrum that we humans do not recognize and delimit as a specific color.
In order to get closer to the mechanism of color vision in aphids and their reaction to plants, Döring and Kirchner placed a total of more than 200 color traps in a field in two consecutive springs, representing seventy different sections of the visible color spectrum. Using a light spectrometer, they measured the so-called reflection spectra of the individual colors – this reveals the relevant wavelengths of light for this colour. “The spectrometer allows us to objectively quantify the colors. This is important because humans perceive colors very differently from insects, so we cannot rely on our eyes,” explains Döring. For each trap, the team measured how many aphids were attracted and caught.
It stays with yellow – but why?
The reflection data and the behavioral data, i.e. the number of aphids caught per trap, then formed the basis for two mathematical models. The aim was to relate this data to one another – and thus find out which wavelengths are decisive for the reaction of the aphids. In the first model, the scientists incorporated already known information about which photoreceptor cells are stimulated by aphids when they search for food. The second model managed without the physiological data and only considered the behavior of the animals and the measured reflection data for each color.
The results of both models confirmed the aphids’ preference for the spectral range that we humans perceive as yellow. Furthermore, both models suggested a special neuronal mechanism that controls the visual behavior of the aphids. This is based on the so-called opposite color theory, which states that the photoreceptors do not perceive a single specific color, but instead detect the difference between two color opponents such as red and green or yellow and blue. The clarity of the results was also remarkable: “We were surprised that the results of both evaluations matched so much in the end,” says Kirchner. “But the result is so powerful that it cannot be a mathematical artifact.”
But the question remains unanswered as to why lice actually have such a preference for yellow – after all, their food is green plant leaves. “It can now be explained physiologically, but the benefit for the lice behind it is still unclear,” says Thomas Döring. Nevertheless, the new models can help to optimize existing agricultural methods that change the visual background of the crops and can thus “hide” the host plants from the pests. The results could also be relevant for attempts to change leaf color, for example through breeding or special fertilization. The current study already provides evidence that wheat leaves with low nitrogen fertilization are more attractive to aphids than if they were treated with more intensive nitrogen.
Source: Rheinische Friedrich-Wilhelms-Universität Bonn, specialist article: Philosophical Transactions of the Royal Society; doi: 10.1098/rstb.2021.0283