Detection of plant stress responses in aphid-infested lettuce using non-invasive detection methods

Abstract: Lettuce cultures are prone to aphid infestations, but consumer tolerance for aphids in fresh lettuce is close to zero. To avoid losses of harvest due to aphids, lettuce plants are routinely sprayed with pesticides. Broadcast spraying of pesticides are costly, pollute the environment and may lead to pesticide residues in lettuce. Typically, early aphid infestation of lettuce cultures is not uniform, but shows a heterogeneous distribution. An early identification of aphid-infested lettuce plants prior to widespread infestation of whole fields would allow for selective spraying which reduces the use of pesticides. Although aphids have been shown to trigger physiological adjustments in plants, optical detection methods for aphid-infested lettuce plants based on their biotic stress response have yet to be developed. For other crop plants, aphid infestation has been shown to change the optical properties of leaves and canopies. Spectral reflectance measurements of wheat and soybean revealed an increased reflectance in the visible spectrum, but decreased reflectance in the near-infrared (NIR) due to changed chlorophyll content and structural characteristics of leaves. In our first experimental approach, we compared spectral reflectance of field-grown plants with and without aphid infestation. Here, we used aphid-proof enclosures with control plants and plants subjected to manual infestation with the potato aphid Macrosiphum euphorbia, a generalist aphid commonly found on lettuce plants. Three weeks after initial infestation, we counted an average of 130 aphids per plant, which were mainly found on the abaxial side of outer lettuce leaves, while control plants were mainly uninfested. A comparison of leaf-level spectral reflectance of control and aphid-infested plants revealed minor variation in spectral reflectance patterns. We conclude that despite high infestation levels, plants did not exhibit a strong systemic stress response to aphid infestation. This experiment is only the beginning of a row of field and laboratory experiments to find a non-invasive detection technique suitable to indicate the biotic stress response of lettuce plants, and will be followed by experimental approaches using plant- and canopy-level spectral reflectance measurements, chlorophyll fluorescence measurements and thermal imaging.