The twospotted spider mite, Tetranychus urticae (Acari: Tetranychidae), is one of the most destructive mite pests of greenhouse-grown horticultural crops, feeding on over 300 plant species ( Jeppson et al., 1975 ; van de Vrie, 1985 ), and, as
Twospotted spider mite, Tetranychus urticae (Acari: Tetranychidae), is a major arthropod pest of greenhouses, feeding on over 300 plant species ( Jeppson et al., 1975 , van de Vrie, 1985 ). Twospotted spider mite (TSM) feeds within leaf cells
Abbreviations: DAT, days after treatment; ERM, European red mite; Pn, net photosynthesis; SLW, specific leaf weight; TCHL, total chlorophyll; Tr, transpiration; TSM, twospotted spider mite; WUE, water-use efficiency. 1 Graduate Student. 2 Associate
did not produce comparable results until the end of the third week ( Table 1 ). Table 1. Mean ( se ) number of twospotted spider mites (TSM) per sampled leaf for each species and cultivar are shown for each sample date ( N = 90 for all treatments
and James Higgins for statistical advice, and Marci Spaw and Sara Schumacher for providing support in rating ivy geranium plants for twospotted spider mite damage. We gratefully acknowledge Fischer USA, Inc., Boulder, Colo., for providing the plant
Resistance to the twospotted spider mite (Tetranychus urticae Koch) of 18 cultivars of ivy geranium [Pelargonium peltatum (L.) L’Her ex Ait.] was evaluated under greenhouse conditions by mass screening, by measuring fecundity on isolated leaves, and by paired leaf tests for selected cultivars. ‘Sybil Holmes’, ‘Yale’, and ‘Cornell’ were among the most susceptible, whereas ‘Double Lilac White’, ‘Sunset’, and ‘Salmon Queen’ were relatively mite-free. ‘Madeline Crozy’ and ‘Cornell’ were preferred for oviposition over the resistant ‘Double Lilac White’. Mite resistance was not related to density of simple or glandular leaf trichomes, but there was an apparent relationship between mite susceptibility and severity of oedema.
Thirty clones of the beach strawberry, Fragaria chiloensis (L.) Duch., that were known to be resistant to strawberry aphid, Chaetosiphon fragaefolii (Cockerell), were evaluated for resistance to twospotted spider mite, Tetranychus urticae Koch. AH clones had fewer mites per leaf than the cultivar ‘Totem’. Two clones, BSP-14 and LCM-19, had totals of only 6 and 82 mites, respectively, on 4 dates compared to 1741 mites on ‘Totem’. The ‘Del Norte’ clone of F. chiloensis, previously used as a source of aphid resistance in breeding, was more susceptible than BSP-14 and LCM-19 to the mite.
Twospotted spider mites (Tetranychus urticae Koch) can be a serious pest on Rubus, primarily red raspberry, in the Pacific Northwest. As we expand the Rubus germplasm in our breeding program, we must evaluate new material for its susceptibility or resistance to pests. Populations of 40 Rubus sp., representing the Malachobatus, Idaeobatus, Eubatus, and Anoplobatus, as well as `Meeker' and `Marion', were evaluated for mite presence in either a replicated or observation trial. Fully mature leaves, 10 in the replicated trial and 15 in the nonreplicated trial, were harvested at random from 15 to 60 cm above the soil surface. Mature, motile mites were counted on the lower surface of the leaves using a dissecting microscope. Because leaf size was extremely variable from species to species, leaf area was measured and a mite density calculated. There were significant differences in mite density among the species in the replicated trial. The average mite density was 0.03 mites/cm2, and mite densities were high on one population of R. Iambertianus and R. hunanensis, 2.7 and 1.6 mites/cm2, respectively. In the nonreplicated trial, R. glabratus had the highest mite density (2.3 mites/cm2). A population of R. corchorifolius (1.0 mites/cm2) and R. parviflorus (0.9 mites/cm2) had fairly high mite densities. In both plantings, many of the populations had few or no mites.
Scanning and transmission electron microscopy were used to investigate damage to strawberry (Fragaria ×ananassa Duch.) leaves caused by twospotted spider mite (Tetranychus urticae Koch.). Mites damaged epidermal cells on the lower leaf surface, but did not damage major vascular elements of the leaf. Mite-damaged spongy and palisade parenchyma cells had coagulated protoplasts, with some cells devoid of cellular contents. Mesophyll cells adjacent to damaged regions showed no ultrastructural distortion or disruption of chloroplasts.
Mite resistance of strawberry, Fragaria ☓ ananassa Duch, based on the ability to support low populations of the twospotted spider mite, Tetranychus urticae Koch, was assessed in the progeny of 15 crosses at two locations. The best parents were Washington State University selections 72–83M, 75-7-5 and 75-57-3 and the poorest parents were the susceptible cultivars ‘Totem’ and ‘Rainier’. General combining ability estimates for mite resistance were higher than specific combining ability estimates. Heritability estimates, based on parent/offspring regression, were high and ranged from 0.62 to 0.92. These observations suggest that additive genetic factors play a major role in the inheritance of mite resistance.