Open field surveys to evaluate the susceptibility of red raspberry genotypes to raspberry gall midge, Lasioptera rubi Schrank (Diptera, Cecidomyiidae) – a 4-year results

The raspberry gall midge (RGM), Lasioptera rubi Schrank (Diptera, Cecidomyiidae) is an economically important pest, especially in the raspberry plantings where canes are not removed after harvest or mechanically damaged under inter-row or intra-row growing, harvest or by hail. The objective of the study was determination of the harmfulness of RGM in an untreated raspberry cultivar plantation at the Fruit Research Institute, Serbia. For the assessment, canes were collected from a planting of raspberry cultivars established in 2002. The sampling was performed on five genotypes i.e. cultivars Willamette, Meeker, Latham, Tullameen and hybrid K 81-6, and the rows were selected at random. Five meters of each of the chosen rows were marked. Canes displaying morphological changes in gall form were collected from the marked row sections. Sampling was performed on August 2nd, 2006 and July 6th, 2007 (we did a second control, on October 12th, on the artificially wounded canes in July). Cane samples were collected on July 15th, 2008 and July 22nd, 2009. Collected canes were brought to laboratory where the diameter of canes was measured, and diameter, length and width of galls and the number of larvae in galls were recorded. The features were measured using an Inox calliper, precision +/- 0.05 mm. No galls were evidenced in Meeker during 2006 to 2008, Willamette and K81-6 in 2007, in Latham and Tullameen during the 2009-research period. Galls were evidenced in the Tullameen only in 2008. The largest number of infested canes in 2006 was found in K81-6 (16%), in 2007 in Latham (12.5%). Similar number of infested canes in 2008 was found on hybrid K 81-6 and Willamette which had 33.33 and 30% of canes infested – respectively. In 2009 growing season the largest number of infested canes was found on hybrid K81-6 which had 51.11% of canes infested. The calculated values show that the highest larval pressure was found in Tullameen (24.5 larvae per gall during 20080 and smallest in Willamette (4.5 larvae per gall during 2009). Different levels of harmfulness of RGM were recorded over the four years of study on different raspberry genotypes. During 2006 to 2009 growing period we registered different number of infested canes per genotype, galls/canes, larvae/gall and eggs on the genotypes. In the collected canes, galls occurred in different forms i.e. elongated, roundish or canker wound forms. Galls were observed along the entire cane length, at different cane sections (lower, medium and upper parts) as well as on petioles. Our results demonstrate that in conditions of high temperatures and lower humidity raspberry gall midge has a very extended flight period or maybe two generations per year.