Detecting early mealybug infestation stages on tomato plants using optical spectroscopy

Mealybugs (Hemiptera: Pseudococcidae) are important pests in agricultural and ornamental crops, including the tomato. Damage by mealybugs is characterized by a reduction in plant photosynthesis and growth due to sap feeding and also, as a result of honeydew excretion, from sooty mould development and virus transmission. The effectiveness of mealybug control strategies, including the application of insecticides and biological control, depends on the ability to detect the infestation at an early stage. Monitoring by visual observation is not very effective and is time-consuming. Optical spectroscopy represents a potential tool for detecting plant biotic stresses, including that caused by insect pests. In this study, we tested the feasibility of using optical spectroscopy for the early detection of mealybug infestation of tomato plants. An experiment was carried out using potted plants under field conditions, with 15 replicates per treatment and a randomised design. Two treatments were considered: 1) infested plants inoculated with three mealybug egg masses; and 2) control plants without mealybugs. The distance between pots was kept at 80 cm and the plants were frequently inspected to ensure control plants were not infested with mealybugs. The following parameters were recorded weekly over 5 weeks for each plant: 1) reflectance of marked leaves was measured with a USB4000 spectrometer across the wavelength 400-1,000 nm; 2) plant height; 3) leaf size; 4) mealybug density; and 5) presence and density of other pests. Results of principal component analysis (PCA) second derivative of the leaf reflectance showed a clear distinction between control and infested plants and a separation of components in the near infrared (NIR) region on the last day of the analysis (57 days). The reduction in absorption in the NIR region may be due to an increase in the quantity of air spaces within the leaf's mesophyll, changing the spatial distribution of the leaves' refractive index and, as a consequence, the light scattering contribution to the reflectance spectra. When tracking the evolution of the leaves' absorbance, infested leaves relative to control leaves had a tendency over time to have reduced absorbance in photosystem II and NIR plateau wavelengths. The evolution over time of the reflectance of analysed leaves at each wavelength fitted a quadratic curve, the coefficients of which discriminated between infested and control plants. This methodology has the potential to provide an objective measure of the degree of infestation by pests and the potential impact on the crop.