Dispersal and colonization risk of the Walnut Twig Beetle, Pityophthorus juglandis, in southern Europe

The Walnut Twig Beetle (WTB), Pityophthorus juglandis Blackman, is a small bark beetle native to Mexico and Southwestern USA recorded for the first time in Europe (NE Italy) in 2013. WTB attacks walnut (Juglans spp.) and wingnut trees (Pterocarya spp.) and is the vector of Geosmithia morbida Kolarík et al., a pathogen causing the thousand cankers disease (TCD). WTB and TCD represent a serious threat for walnut orchards in Europe. Spatiotemporal data of the WTB-TCD infestations recorded from an 8-year-long (2013–2020) monitoring conducted in 106 walnut orchards of NE Italy were used to develop a model in order to analyze: (i) the effective dispersal capacity of WTB, (ii) the factors affecting dispersal and (iii) the colonization risk of healthy walnut orchards. We registered a mean annual dispersal of 9.4 km, with peaks of about 40 km. Pest dispersal is 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. Using the model, it was also possible to calculate the colonization risk of a specific walnut orchard according to its characteristics showing, for instance, that a medium-size (5,000 trees) black walnut orchard located at 25 km from the nearest infested orchard has an infestation risk of about 50% of probability.

Geosmithia morbida (thousand cankers disease).

Thousand Cankers Disease is a disease complex native to the western United States that affects many Juglans and Pterocarya species, i.e. walnut and wingnut trees. It is caused by the fungus Geosmithia morbida, which is vectored by the walnut twig beetle (Pityophthorus juglandis), and possibly by other insects. The beetle carries fungal spores that are introduced into the tree during gallery construction, and the fungus then causes cankers in the inner bark that disrupt the flow of nutrients throughout the tree, often leading to its death. In recent years the disease has been reported in several eastern states, and also in Italy. Long-distance spread is thought to be a result of the movement of infected and infested wood.

Assessment of Semiochemical Repellents for Protecting Walnut Trees From Walnut Twig Beetle (Coleoptera: Curculionidae) Attack in a Commercial Orchard Setting in California

The walnut twig beetle, Pityophthorus juglandis Blackman, the vector of thousand cankers disease (TCD), poses a significant threat to North American walnut (Juglandaceae Juglans) trees. Despite discovery of TCD-related tree mortality over a decade ago, management options are lacking. This study represents the culmination of several years of investigating the chemical ecology of P. juglandis in hopes of developing a semiochemical repellent to disrupt the beetle’s host colonization and aggregation behaviors. Numbers of P. juglandis landing on semiochemical-treated Juglans regia L. trees in a commercial walnut orchard were compared based on captures on sticky traps. Two repellent combinations were tested: R-(+)-limonene and trans-conophthorin (LimeCon), and R-(+)-limonene, trans-conophthorin, and R-(+)-verbenone (LCV). Both repellents reduced P. juglandis aggregation (captures) equally; thus, we proceeded with the LimeCon combination to reduce potential treatment cost. Subsequent trials included a 2× dose (Dual) of LimeCon. Both LimeCon and Dual significantly reduced the number of P. juglandis caught compared with the baited control, however, only for the lower of two trap positions. Beetle landings were modeled by trap distance from repellent placement on each tree. Beetle responses to the pheromone lure were surprisingly localized and did not bring the whole tree under attack. LimeCon, LCV, and Dual treatments averaged fewer than a single beetle caught for all trap distances; however, performance of the repellents beyond 150 cm is not clear due to the localized landing response of P. juglandis to pheromone lures. Further testing is required to fully analyze the zone of inhibition of the LimeCon repellent.

Eastern Black Walnut (Juglans nigra L.) Originating From Native Range Varies in Their Response to Inoculation With Geosmithia morbida

Thousand cankers disease (TCD) is caused by the walnut twig beetle (Pityophthorus juglandis) vectoring the fungal canker pathogen Geosmithia morbida, which can result in severe dieback and eventual death to species of walnut (Juglans spp.) and wingnut (Pterocarya spp.). This disease is most devastating to the highly valued species J. nigra (black walnut). This species is primarily grown and harvested for timber production in the Central Hardwood Region of the United States, which comprises part of its native range. Management options for TCD are limited; therefore, finding resistant genotypes is needed. Initial studies on black walnut susceptibility to G. morbida documented some genetic variation and suggested potential resistance. Furthermore, G. morbida is thought to be native to the United States, which may have allowed for co-evolution. To capture the representative genetic diversity and screen for resistance to G. morbida, J. nigra families were collected from across the native range. These wild trees, in conjunction with seedlings developed in a black walnut timber improvement program, were planted in a common garden in Fort Collins, Colorado and repeatedly inoculated with G. morbida over the course of four years and three growing seasons. Improved seedlings exhibited larger cankered areas than wild J. nigra of the same provenance. Cankers induced by G. morbida in wild germplasm were smaller on J. nigra collected from the western and central portions of the native range compared to those collected from the eastern portion. Although trees from the western and central part of the range still incurred cankers, our findings indicate that variation in genetic resistance to G. morbida is present in black walnut. This study was performed with G. morbida independent of the walnut twig beetle, but our results suggest the limited G. morbida resistance observed in J. nigra will prevent the full compromise of black walnut to TCD. Results from this study should be taken into consideration in future black walnut breeding programs.

Vacuum Steam Treatment Effectiveness for Eradication of the Thousand Cankers Disease Vector and Pathogen in Logs From Diseased Walnut Trees

Logs of high-value eastern black walnut (Juglans nigra L.) are commonly exported from the United States for production of veneer and lumber. Veneer logs are not debarked to minimize degradation of wood quality and reduce moisture loss. Thousand cankers disease (TCD) is caused by the walnut twig beetle (Pityophthorus juglandis Blackman) and the fungal pathogen, Geosmithia morbida M. Kolarik, E. Freeland, C. Utley and N. Tisserat sp. nov., which colonize the inner bark of Juglans species. Effective eradication of these organisms by heat or chemical fumigation treatment is required for walnut logs prior to export. Because vacuum steam is an effective and efficient means of heating round wood, its use in eliminating the TCD causal agents was evaluated using Juglans logs (12- to 44-cm small end diameter and 1.7- to 1.9-m length) from TCD-symptomatic trees in Oregon and Washington State. Five replicate trials with three logs per load were conducted in a portable vacuum chamber to test two treatment schedules: 60°C for 60 min and 56°C for 30 min. Complete elimination of P. juglandis and G. morbida was achieved when using a minimum of 56°C at 5-cm targeted depth from bottom of bark furrow into the sapwood and held for 30 min. Treatment cycle time ranged from 298 to 576 min depending on log diameter and initial log temperature. Artificial inoculation of J. nigra trees with G. morbida within the TCD range in Pennsylvania was minimally successful in producing adequately colonized logs for experimental trials.

Trapping Failure Leads to Discovery of Potent Semiochemical Repellent for the Walnut Twig Beetle

The walnut twig beetle, Pityophthorus juglandis Blackman, and its associated fungal pathogen that causes thousand cankers disease, currently threaten the viability of walnut trees across much of North America. During a 2011 assessment of seasonal flight patterns of P. juglandis with yellow sticky traps baited with the male-produced aggregation pheromone component, 3-methyl-2-buten-1-ol, dramatically reduced catches were recorded when Tree Tanglefoot adhesive was used to coat the traps. In summer 2011, two trap adhesives were tested for potential repellency against P. juglandis in a field trapping bioassay. SuperQ extracts of volatiles from the most repellent adhesive were analyzed by gas chromatography–mass spectrometry, and limonene and α-pinene were identified as predominant components. In field-based, trapping experiments both enantiomers of limonene at a release rate of ~700 mg/d conferred 91–99% reduction in trap catches of P. juglandis to pheromone-baited traps. (+)- and (‒)-α-Pinene reduced trap catch by 40 and 53%, respectively, at the highest release rate tested. While a combination of R-(+)-limonene and (+)-α-pinene resulted in a 97% reduction in the number of P. juglandis caught, the combination did not consistently result in greater flight trap catch reduction than individual limonene enantiomers. The repellent effect of limonene may be valuable in the development of a semiochemical-based tool for management of P. juglandis and thousand cankers disease.

Bursaphelenchus juglandis n. sp. (Nematoda: Aphelenchoididae), an associate of walnut twig beetle, Pityophthorus juglandis, the vector of thousand cankers disease

Summary Bursaphelenchus juglandis n. sp. was isolated from the walnut twig beetle, Pityophthorus juglandis, and walnut trees, Juglans spp. with symptoms of thousand cankers disease, in California, USA. Based on analysis of three rRNA genes and morphological features (three lines in lateral field, small arched vulval flap in female, broad spicule with two lines along blade and small cucullus, digitate dorsally bent condylus, male tail pattern of five papilliform papillae and one pair of glandpapillae (P5), and curved conical female tail), the new species belongs to the Abietinus group within Bursaphelenchus. It differs from similar species of this group by the presence of a cephalic disc with lateral labial sensilla at the disc border, and in having thick spicules with the capitulum surface almost parallel to a virtual direct line extending from the spicule end. An emended diagnosis, tabular polytomous identification key and compendium of species with the lists of their vectors, plant hosts, and distribution are provided for the Abietinus group. The diagnostics of the propagative developmental stages is given, including sex differences; the transmission dauer stage was identified as the third stage and its description given with sexual differences. A molecular phylogeny of Bursaphelenchus is provided based on partial 18S rRNA, ITS rRNA and the D2-D3 expansion fragments of 28S rRNA gene sequences. A PCR with a species-specific primer was developed for detection of B. juglandis n. sp.