Tracking the invasive hornet Vespa velutina in complex environments by means of a harmonic radar

An innovative scanning harmonic radar has been recently developed for tracking insects in complex landscapes. This movable technology has been tested on an invasive hornet species (Vespa velutina) for detecting the position of their nests in the environment, in the framework of an early detection strategy. The new model of harmonic radar proved to be effective in tracking hornets either in open landscapes, hilly environments and areas characterised by the presence of more obstacles, such as woodlands and urban areas. Hornets were effectively tracked in complex landscapes for a mean tracking length of 96 ± 62 m with maximum values of ~ 300 m. The effectiveness of locating nests was 75% in new invasive outbreaks and 60% in highly density colonised areas. Furthermore, this technology could provide information on several aspects of insect’s ecology and biology. In this case, new insights were obtained about the mean foraging range of V. velutina (395 ± 208 m with a maximum value of 786 m) and flying features (ground speed), which was 6.66 ± 2.31 m s−1 for foraging individuals (hornets that are not carrying prey’s pellet) and 4.06 ± 1.34 m s−1 for homing individuals.


Supplementary Fig. S3 Tracking session in the outbreak of Ameglia (18-19 September 2018).
Hornets were tagged in the apiary (yellow square), tracked with the harmonic radar from six positions (triangles) until nest position was discovered (red dot). Background map by Google Maps (maps.google.com).

Supplementary Fig. S4 Tracking session in Dolceacqua (7-20 November 2018).
Hornets were tagged in the apiary (yellow square), tracked with the harmonic radar from two positions (triangles) until nest position (red dot) was visually discovered by chance with the use of binoculars, therefore tracking session was interrupted. Background map by Google Maps (maps.google.com).

Supplementary Fig. S5 Tracking session in the outbreak of Finale Ligure (4-9 October 2018).
Hornets were tagged in the apiary (yellow square), tracked with the harmonic radar from four positions (triangles) until nest position was discovered (red dot). Background map by Google Maps (maps.google.com).

Supplementary Fig. S6 Tracking session in Ospedaletti (1-2 August 2018).
Hornets were tagged in the apiary (yellow square), tracked with the harmonic radar from three positions (triangles) but nest position was not detected in this tracking session. Background map by Google Maps (maps.google.com).

Supplementary Fig. S7 Tracking session in the outbreak of Riccò del Golfo (19 September -3
October 2018). Hornets were tagged in the apiaries (yellow squares), tracked with the harmonic radar from six positions (triangles) but nest position was not detected in this tracking session. Tracks are pointing towards a dense vegetated area that prevented the possibility to carry on both harmonic radar tracking and on foot inspection of the area. Background map by Google Maps (maps.google.com).

Supplementary Fig. S8 Tracking sessions in the village of Calvo of Ventimiglia in 2018 (24-27
July the first tracking session, 8-9 August the second and 12-17 October the third). Hornets were tagged in the apiary (yellow square), tracked with the harmonic radar from two positions for each tracking session (position 1-2 for the first session, 3-4 for the second and 5-6 for the third) until the position of three nests was discovered (red dots) at the end of each tracking session. Background map by Google Maps (maps.google.com).

Supplementary Fig. S9 Tracking sessions in the village of Latte of Ventimiglia in 2018 (21-23
August the first tracking session, 5-6 September the second and 19-25 October the third). Hornets were tagged in the apiary (yellow square), tracked with the harmonic radar (position 1 for the first session, 2-4 for the second and 5-7 for the third) but nest position was not detected in the area. Background map by Google Maps (maps.google.com). Supplementary Table S1 Characteristics of the localities in Liguria (Italy) in which hornets have been tracked with the harmonic radar technique. The district of each locality is reported in brackets: La Spezia (SP), Savona (SV), Imperia (IM). Land cover indicates the percentage of open terrains (O), urban areas (U) and woodlands (W). Detected nests are the number of V. velutina nests detected with the harmonic radar tracking. For the high-density colonised areas, the range of nest density in the period 2016-2018 is reported (in these localities, the density was estimated considering the nests reported within a buffer area of 1 km). In outbreaks, the presence of a single nest prevents the possibility of providing values for nest density.   * in the locality of Camporosso, hornets were tagged when feeding on flowers. In this case, the distance corresponds to the distance between the nest and the feeding point (this value was not included when estimating the mean distances of nests from apiaries). Supplementary Table S4 Results of a GLMM analysis on the relationship between the length of the tracks recorded with harmonic radar and the environmental characteristics. The model with the best fitting is a model that considers all the environmental variables as fixed effects and uncorrelated random intercepts (based on locality) with random slopes (based on slope degree of the study areas). For each variable the following is reported: the estimated coefficient (β), the standard error (SE), Z and P values. The slope degree is the main variable that negatively affects the length of the tracks, while urban areas has a positive influence on the response variable. Woodlands, elevation and road distance do not affect tracking length.