Susceptibility Of Selected White Azaleas To The Azalea Lace Bug
Peter B. Schultz and Mark A. Coffelt
Virginia Polytechnic Institute and State University
Virginia Beach, Virginia
The azalea lace bug (ALB), Stephanitis pyrioides (Scott), is a major pest of azaleas and is most damaging to azaleas grown in full sunlight (2). The nymphs and adults feed on the underside of the leaf and injure the foliage by mechanical removal of chlorophyll. The damaged leaves have a stippled or blanched appearance depending on the severity of the injury. Caste skins and tar-like excrement are found on the underside of infested leaves.
Personal observations and reports from horticulture professionals indicate that differences in susceptibility of azalea cultivars to ALB exist. Since consumer buying patterns, and therefore grower marketing strategies, emphasize flower color as the primary criterion, evaluations between azaleas with a common color were undertaken.
Three azalea cultivars with white flowers, 'Effie Bunce', 'Delaware Valley White' and 'Indica Alba', grown at the Virginia Truck and Ornamentals Research Station in Virginia Beach, Virginia were selected for this study.
'Effie Bunce' also known as 'Conversation Piece' is a Robin Hill hybrid with 3½ inch diameter blooms. The cultivar flowers mid to late season and has white flowers that often have a prominent blotch spotting from pink to purplish red (1). The underside of the foliage is glabrous. 'Indica Alba' and 'Delaware Valley White' are in the Mucronatum group and are characterized by having 3 inch diameter blooms that flower in early season. The leaf underside of these cultivars is pubescent. Plants approximately 30cm (12 in) tall of each cultivar were compared by placing a plant of each cultivar beneath a screened cage 45 x 60 x 60 cm (18 x 24 x 24 in). Approximately 50 adult ALB were collected from foundation plantings and placed in a 150 ml (5 oz.) plastic container. The container was placed equidistant from the azaleas beneath the cage. Water and fertilizer regimes were consistent for all varieties. After 72 hours the number of ALB on each plant was counted. The study was conducted four times between May and August, 1985, with four replications per variety.
Evaluation of each date indicated that 'Effie Bunce' had significantly higher numbers of ALB in May and June, 1985, than 'Delaware Valley White' and 'Indica Alba' (Table 1). In August 'Effie Bunce' had higher ALB adults than the other cultivars but the differences were not significant. When the means of the four dates was combined, 'Effie Bunce' had significantly greater ALB. The seasonal change in results suggests that environmental factors may be affecting the results. The greenhouse temperatures are greater in August which could account for the decrease in ALB totals when compared to May and June. Since the decrease occurred only in 'Effie Bunce' it is likely that changes to the foliage due to high temperatures reduced susceptibility. Further research is needed to determine if additional azalea cultivars are highly susceptible to ALB.
Table 1. Mean number of S. pyrioides on three cultivars of azaleas. Cultivar May 7 June 27 Aug 9 Aug 20 Mean 'Effie Bunce' 16.8a* 22.0a 8.8a 3.5a 12.8a 'Delaware Valley White' 3.0b 1.3b 3.5a 2.5a 2.6b 'Indica Alba' 0.3b 1.3b 6.0a 2.3a 2.4b * Means followed by the same letter are not significantly different using Duncan's multiple range test, p=0.05.
1. Galle, F.C. 1985. Azaleas. Timber Press, Portland, OR. 486 p.
2. Raupp, M.J. 1984. "Effects of exposure to sun on the frequency of attack by the azalea lace bug." J. Amer. Rhod. Soc. 38:189-190.
Peter Schutz and Mark Coffelt are associated with the Virginia Tech and Ornamentals Research Station of VPI & SU at Virginia Beach, Virginia.