Final report

Evolution of insect-plant relationships has been one of the central topics of science for more than 150 years. Charles Darwin pointed out the interplay between flowering plants (angiosperms) and plant-eating (herbivore) insects as early as 1859, in 'Origin of Species'. In fact, when Darwin saw the long spur on an orchid from Madagascar, he predicted the existence of a pollinating moth with a very long proboscis. Final evidence for the coevolution involving the orchid and the long-tongued moth, named Xanthopan morganii praedicta, did not appear until the 1990s.

Angiosperms, which occurred 130 million years ago, have always required a specific form of pollination. Among all Earth's terrestrial organisms that interact with angiosperms, insects are the most important group. Cultivated plants include to a large extent angiosperms and are therefore fundamentally dependent on these small creatures. On the other hand, certain herbivorous insects represent a serious problem for agriculture just because they are plant-eaters. Although these pests make up only 1% of all insects, they are responsible for a significant loss of crops. To establish a sustainable agriculture, it is necessary to minimize the extent of insect pests and at the same time preserve the insects' biological diversity. This requires, above all, increased knowledge about insects.

In this project, which lasted from 2019 to 2022, we have studied the olfactory system of the noctuid moth, Helicoverpa armigera – one of the most serious insect pests in the world. In line with the project title - 'Insect–plant relations: imaging of CO2, pheromones and plant odors in the olfactory paths of herbivorous insects', we contribute new knowledge that has been published both in recognized scientific journals and in popular science channels.

New findings on the olfactory pathway linked to male-specific reproductive behavior:

• A unique target area for pheromone signals in the male brain: We have acquired new knowledge about the pheromone system by identifying a group of second-order olfactory neurons ending up in a delimited brain area, completely separated from the target area of the previously described and well-known 'sister neurons'. The sister neurons resemble the human Mitral cells and, like them, end up in areas linked to memory formation. We hypothesize that the new population of neurons is specifically involved in the characteristic flight behavior of a male seeking a potential mate. The article is published in the renowned journal, Frontiers in Cellular Neuroscience, 2020, https://www.frontiersin.org/articles/10.3389/fncel.2020.00147/full. In a subsequent article, we compare the two populations of male-specific pheromone neurons with two groups of visual neurons that make up the mammalian ventral ('what') and dorsal ('where') streams.

• Valence-based odor processing in higher-order brain areas: In an article in the internationally renowned journal eLife, 2021, we present how female-produced odor signals linked to attraction and rejection, respectively, are coded in higher-brain centers in the male. We demonstrate that the two categories of scent signals, each linked to a specific behavior – attraction versus repulsion – are processed in separate brain areas. The findings provide new insight into higher-order olfactory pathways linked to reproductive behavior. The data are very relevant to the entire field of olfactory research as we describe neural principles linked to the 'pleasure/displeasure' classification typifying the olfactory system. https://elifesciences.org/articles/65683

New scientific data on neural networks processing input from CO2 and plant odors:

• Mapping the peripheral CO2 pathway: Like other species of lepidoptera, the moth has a separate sensory organ on its mouthparts for the detection of CO2. By using different color techniques, we have carried out a precise mapping of the peripheral sensory pathway for CO2. The data is published in the journal Frontiers in Physiology, 2020, https://www.frontiersin.org/articles/10.3389/fphys.2020.00202/full.
• Second-order CO2 pathways in the moth brain: We demonstrate, for the first time, how input from CO2 is represented in second-order neurons. This type of signal essentially follows different tracts than the ordinary olfactory neurons. However, the CO2 projections end up in a brain area that also includes terminals of plant odor neurons - which makes sense as the insect uses CO2 input to detect the most nutritious parts of a host plant. https://www.nature.com/articles/s41598-020-76918-1

The new knowledge from this project will contribute to promoting human awareness of the advanced nervous system of insects - which is a prerequisite for the conservation of species diversity and sustainable development. The data will also enable new strategies for reducing pests where use of traditional pesticides is replaced with control mechanisms based on biologically relevant signal.