Preliminary studies on Colaphellus palaestinus Achard in the Maltese islands

Abstract

Colaphellus palaestinus Achard is an alien invasive species belonging to the Chrysomelidae family, which feeds on plants from the Brassicaceae family. The main objectives of this preliminary study were to establish the distribution of this species in Malta and Gozo, determine the host plants this species is associated with, and assess the damage done to the host plant. Field surveys lasted from 17th of October to 23rd November 2019. Observations were maintained occasionally up to 15th December 2019, after which date no more adults were recorded from the field. Sweep netting was the main technique used to sample the insect. Photographs were taken to document the areas sampled and the damage incurred by the host plants.An area of 100m2 from each of 62 sectors was sampled in Malta and from each of 19 sectors in Gozo. Each sector had an area of 2.5km2. In Malta, C. palaestinus individuals were found in 59 sectors, whilst in Gozo C. palaestinus was only found in 3 sectors that were located close to Mgarr Harbour. Larval individuals were found in 16 sectors in Malta, that were concentrated in the northern and western regions, whilst in Gozo no larval individuals were recorded. In Malta, a total of 3130 adult individuals and 311 larval individuals were recorded throughout the study, whilst in Gozo only 5 adult individuals were encountered. The main host plants were Brassica rapa subsp. sylvestris, Diplotaxis erucoides and Diplotaxis tenuifolia. Two occurrences of C. palaestinus were also recorded from Sinapis sp. Furthermore, a few individuals of the insect were also recorded on Matthiola incana subsp. incana outside the main sampling programme. The recorded damage was restricted to the leaves, with larvae causing the most damage. Many adults were also noted to congregate around the inflorescences of the first 3 plant host species mentioned above, with damage also being noted on the flower petals although it was not extensive. B. rapa subsp. sylvestris and D. erucoides were the two plant species that were affected most with regard to leaf damage. Rearing, which was undergone in an attempt to understand the life cycle of C. palaestinus, was unsuccessful for all 9 trials undertaken. The insect individuals died after a few days, and many adult individuals were noted to have burrowed into the soil present at the bottom of the container in which they were being kept. Further studies should be undertaken, given the dearth of information on the local ecology and impacts of C.palaestinus. Preferably, such studies would be undertaken over several years, and include further assessment of its occurrence and distribution in the Maltese Islands, its life cycle, feeding habits and economic impacts.