Response of hop cultivars to two-spotted spider mite infestation

The two-spotted spider mite (TSSM), Tetranychus, urticae Koch, is one of the most serious pests in regions where hops,Humu/us lupulus L., are grown. Resistant cultivars would be ideal to reduce plant damage caused by this pest. Studies on the biological interaction between cultivated hop cultivars and TSSM were conducted over three growing seasons to gain a greater understanding of plant mechanisms involved in the response to mite attack under both natural and controlled environmental conditions. In field observations, the results revealed a generalised pattern of growth and decline in naturally occurring mite populations infesting commercial hops during the growing season. The mites peaked after hops had formed the visible bases of inflorescences. Eggs were the predominant stage for most of the season and stage-specific percentages changed as the season progressed. The spatial and vertical distributions of the mites for each stage also varied with time. Natural enemies did not appear at levels sufficient to give control. Studies on the susceptibility of different hop genotypes to artificial and natural infestations by the mites indicated that all the genotypes tested expressed an intermediate to highly susceptible reaction, with M26 (Huller Bitterer) being the most susceptible. Despite no outstanding evidence of antibiosis, there were significant differences in susceptibility among hop genotypes in terms of tolerance, non-preference and plant avoidance in which plant growth exceeds the mite dispersive capacity. Significant differences in mite densities between the hop cultivars of European and American parentage were also detected in that the European cultivar was more susceptible than the American. Morphological variation in external and internal characteristics of hop leaves were found among the genotypes studied. Significant differences were found in ventral gland size, trichome density, trichome size, length of trichomes, stomatal density, stomatal size and moisture content. The morphological characteristics of the hop leaves collected from the same genotype may vary significantly according to leaf age and growing conditions. Under controlled conditions, none of these morphological characteristics were found to influence TSSM population reproductive parameters. In addition to genotypic effects, the findings demonstrated that environmental factors did influence life history traits of TSSM. These factors included temperature, relative humidity, light intensity and plant variables. Physiological and chemical responses of hop genotypes to mite feeding damage were examined in field and glasshouse experiments. Feeding damage by mites increased hop leaf resistance to CO2 uptake through stomatal closure and decreased photosynthetic rates. The stomatal response to infestation differed between cultivars. Analysis of chemicals extracted from hop leaves indicated that phenolics, alkaloids and volatile compounds in infested leaves were qualitatively and quantitatively similar to those detected in undamaged control leaves. Hop-canopy microenvironments do play an important role in seasonal population development of two-spotted spider mites. This role was amplified by hop susceptibility to the mites. For the hop genotypes studied; the populations of mites, and 'their predators, tended to increase more rapidly on sparsely leaved canopies than on densely leaved canopies. The overall results gtrongly suggest that TSSM performance was mainly regulated by the microenvircinments that exist within the canopies of different cultivars with (1) sparsely leaved canopies favouring population increase and (2) reduced temperature and light and increased humidity within dense canopies limiting population increase.