From hickory to walnut

Back in 2010 researchers of the Italian National Research Council found an unknown leafmining moth in southern Italian walnut orchards. It soon appeared to belong to the North American genus Coptodisca, very small but beautiful moths of about 2.5 mm length. However, establishing the identity of the species was less easy. It looked similar to Coptodisca juglandella, the only genus member known to attack walnuts (Juglans spp.), but an unambiguous identification couldn't be made because of the insufficient characterization of this group of leafminers.

Around the same time a researcher from the Naturalis Biodiversity Center in Leiden, Netherlands had collected North American larvae of another leafminer that was invading North Italian vineyards (Antispila oinophylla). Surprisingly, when both research groups compared their results, DNA Barcodes of North American Coptodisca larvae collected on hickories appeared to be very similar to those of the Italian walnut feeding moths. Hickories (Carya spp.) belong to the same family as the walnut tree - Juglandaceae. All these similarities and the fact the entire group was essentially understudies triggered the curiosity of the researchers and they began to study moths from several North American collections and finally decided on the basis of both morphological and genetic studies that the mysterious species is Coptodisca lucifluella, described in the 19th century from hickory. 

The leafminer Coptodisca sp. (Lepidoptera: Heliozelidae), recently recorded for the first time in Europe on Italian black and common walnut trees, is shown to be the North-American Coptodisca lucifluella (Clemens) based on morphological (forewing pattern) and molecular (cytochrome oxidase c subunit I sequence) evidence. 

These moths have a beautiful color pattern consisting of silver, black and yellow spots, and measure only 4 mm in wingspan. The larvae make short galleries in leaves and at the end cut out a small shield, leaving a characteristic hole at the end of the leafmine. The larva attaches the shield to the leaves or other surfaces and pupates inside. 

Coptodisca lucifluella naturally occurs in eastern North America. Its larvae feed on several species of wild hickory and commonly on pecan in orchards, It has never been considered more than a minor pest and in North America the moth has never been found on walnut. However, in Italy the species is widespread and happily feeding on Walnut.

The phylogenetic relatedness of three species feeding on Juglandaceae suggests that C. lucifluella has likely shifted, within the same host plant family, from its original North-American hosts Carya spp. to Juglans spp.

It is likely that the insects hitchhiked as larvae or pupae as those protected by their shield attach easily to leaves or stems. Global trade provides excellent transportation. It is interesting that the moth shifted to a new host plant once arrived in Europe, a phenomenon that has been observed with other invading insects. 

Considering its widespread presence and rapid reproduction, C. lucifluella might quickly increase its abundance and distribution in Italy and other European countries. Nevertheless, as also observed for other Heliozelidae and excluding the outbreaks, the level of damage does not seem to be worrisome. The possibility remains, however, that damage by C. lucifluella, if continued for a number of years, might lead to a progressive reduction in carbohydrate reserves and a gradual decline in tree health and performance. Based on the high mortality recorded and on the large number of parasitoids being collected, it would be preferable to encourage biological control rather than performing chemical treatments.