# Near-Real-Time Detection of Insect Outbreaks in Urban Trees Using a PlanetScope Time Series
## 4 Discussion  
Monitoring insect infestations through remote sensing is a well-established practice [49; 50; 51]. Accordingly, in this research, we monitored insect infestation considering vegetation indices determined from high-resolution satellite data. The complexity of urban patterns poses a challenge for studying green spaces within densely built-up areas [52]. In this context, although PlanetScope images have been widely employed for vegetation monitoring [18; 30; 53], their spatial resolution (3 m) and temporal resolution (1 day) make \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{Summer RDVI\({}_{\text{sus}}\) Threshold = 33.1} & \multicolumn{4}{c}{Threshold Classification} \\ \cline{3-5}  & & Infested & Healthy & Total \\ \hline \multirow{3}{*}{Field survey classification} & Infested & 1596 & 1 & 1597 \\ \cline{2-5}  & Healthy & 6 & 232 & 238 \\ \cline{2-5}  & Total & 1602 & 233 & 1835 \\ \hline \hline \end{tabular} Overall accuracy = 0.996, producer accuracy = 0.999, and user accuracy = 0.996.  
\end{table} 
Table 4: Confusion matrix considering RDVI\({}_{\text{sus}}\) = 33.1 as threshold. Figure 5: Normal distribution of RDVI\({}_{\text{sus}}\) of healthy and infested areas. Figure 6: False-color PlanetScope images (in the red channel is displayed the NIR band) of Villa Pamphili of 2022, 2020, and 2019. White circles indicate areas with the presence of pine trees. them very promising for use in studies focused on urban contexts. Compared to Sentinel-2 images, PlanetScope images allow for an increase in spatial detail of more than 10 times and a reduction in revisit time from 5 days to 1 day, with a historical series that reaches 2014 [54]. In detail, the spectral information provided in the PlanetScope data was used for the first time to test the utility of the RDVI index in monitoring _T. parvicornis_. The suitability of the index was evaluated to describe infestations [55], compared to other vegetation indices already used for this purpose, such as NDVI, EVI (Enhanced Vegetation Index), EVI2, NDMI (Normalized Difference Moisture index), and Modis ET (Modis Evapotranspiration time series) [56; 57]. [PERSON] et al. [58] demonstrated that the average NDVI trend for _P. pinea_ peaks in the winter and declines in the summer, indicating a slowdown in plant activity during warmer months. The RDVI index shows a similar temporal trend to the NDVI, but, when comparing the RDVI values of healthy and insect-affected areas, the decrease between late spring and early autumn is significantly more pronounced in infested plants (Figure 4), which are profoundly debilitated, showing more accentuated minimum values. Analyzing a time span of 48 months--longer than other studies of this kind [59; 60; 61; 62]--allowed for the identification of recurring trends. In particular, the distribution of the July/August RDVI averages (RDVI\({}_{\text{sua}}\)) clearly distinguished healthy plants from infested plants, but the deterioration tended to decrease gradually over time. For instance, the lowest average peak index values (indicative of higher degradation) were 21.9 in 2020, 24.6 in 2021, and a minimum peak of 26.6 in 2022. This observed increase in lower peaks may be attributed to the extensive implementation of endotherap treatments across the municipality of Rome since the autumn of 2020. Research conducted in Italy has demonstrated that endotherapy is the most effective method for managing pest invasion [59; 60; 61]. Additionally, this approach is particularly suitable for urban environments, as it minimizes public exposure to chemical agents [63]. The length of the considered observation period is one of the aspects that makes this research useful in supporting the management of _T. parvicornis_ infestations in urban areas. Considering the reproduction periods of _T. parvicornis_[15; 17], there are three ovideposition cycles within a year: between the second half of April and May, between July and early August, and between September and November.