Studies on the spread capacity of Pityophthorus juglandis
The walnut twig beetle, Pityophthorus juglandis (Coleoptera: Curculionidae: Scolytinae) is the vector of Geosmithia morbida (both EPPO A2 List), a pathogen causing the thousand cankers disease on walnut (Juglans spp.). Both organisms were found for the first time in Veneto region, Italy in 2013 (EPPO RS 2014/001) and represent a serious threat for walnut orchards in the EPPO region. A study was conducted in Veneto region to survey the annual increase of the invaded range of P. juglandis and to develop a model assessing its spread capacity according to specific environmental parameters.
A survey for P. juglandis was conducted for 8 consecutive years (2013–2020) in 106 walnut (J. nigra and J. regia) orchards and showed that out of the 106 sites monitored, 44 walnut orchards were found to be infested. It was also found that P. juglandis had a mean annual dispersal of 9.4 km, with peaks of approximately 40 km. Pest dispersal was affected by distance of suitable hosts from the nearest infested site, number of walnut orchards in the surroundings (both infested and healthy), orchard size and walnut species in the orchard. Observations made in Veneto confirmed that P. juglandis has a preference for J. nigra (black walnut) over J. regia (English walnut).
Based on this data, a model was developed to calculate the colonization risk for a specific walnut orchard, taking into account its characteristics. For example, it was calculated that a medium-size orchard (5 000 trees) of J. nigra located at 25 km from the nearest infested orchard had a 50% probability of becoming infested. According to this model, using a 2 km radius to delimit a buffer zone around the infested site corresponds to an infestation risk higher than 80% and 40 % for medium-sized orchards of J. nigra and J. regia, respectively. The authors considered that an increase of the buffer zone radius could be used with other control methods (e.g. use of repellents) to control the spread of P. juglandis. They also noted that other studies had determined that the active flight distance covered by P. juglandis was less than 2 km. This showed that factors, other than active flight, should be taken into account to explain the spread observed in Veneto, in particular the human-mediated movement of walnut logs or wood products, and possibly wind. Considering the outbreaks that have been reported in other Italian regions, as they are distantly located (more than 200 km away from the original finding site in Veneto), the hypothesis of multiple introductions seems to be the most plausible one but remains to be confirmed by further studies.
Sources
Marchioro M, Faccoli M (2021) Dispersal and colonization risk of the walnut twig beetle, Pityophthorus juglandis, in southern Europe. Journal of Pest Science. https://doi.org/10.1007/s10340-021-01372-5