Effects of exposure to sun on the frequency of attack by the azalea lacebug
Michael J. Raupp
Department of Entomology, University of Maryland
Abstract — The occurrence of the azalea lacebug, Stephanitis pyrioides (Scott), was monitored on azaleas, Rhododendron spp., in 4 suburban Maryland communities. Azaleas planted in full sun for all or part of the day were significantly more likely to be infested with lacebugs compared to those planted in shade. Furthermore, on a communitywide basis, the proportion of azaleas infested with lacebugs was directly related to the proportion of azaleas planted in full sun. Azalea growers can reduce the lacebug occurrence and associated pesticide applications by planting azaleas in the proper site.
Several recent studies indicate that azaleas (Rhododendron spp.) are the most widely used woody ornamental plant in suburban Maryland home sites where they comprise 10-20% of the total woody ornamentals encountered (2, 3, 6). The azalea lacebug, Stephanitis pyrioides (Scott) is a major pest of azaleas throughout the eastern United States. (4). Due to the widespread use of azaleas and their susceptibility to lacebug attack, the azalea lacebug perennially achieves key pest status in home sites. For example, over a 3 year period from 1980 through 1982 in a variety of communities, this insect ranked as high as first and no lower than fourth in overall importance (2, 3, 6).
A striking feature of azaleas in suburban plantings is the variability with which they are attacked by the azalea lacebug. One environmental factor reported to affect the susceptibility of azaleas to lacebug attack is their exposure to sun. Bowers (1), Johnson and Lyon (4) and Holmes and Davidson (3) suggest that azaleas planted in full sun are more likely to be lacebug infested than those planted in shaded locations. Yet, to date, there has been little empirical evidence to support this contention.
In this report, I examine the possibility that azaleas planted in full sun are more likely to be infested with S. pyrioides than those planted in shade. Furthermore, variation in lacebug attack is reported among communities of homeowners. The implications of these findings with respect to lacebug management are discussed.
Materials and Methods
The study was conducted with 68 homeowners in 4 suburban Maryland communities. Over a 12 week period from June through August of 1982, all azaleas at each home-site were inspected at biweekly intervals for signs of S. pyrioides. Chlorotic foliage and black fecal deposits on the undersurface of leaves were helpful indicators of lacebug activity. An infestation was confirmed by direct observation of nymphs, adults, or their damage on the current season's foliage.
In addition to recording the presence of lacebugs, the degree of exposure to the sun was determined for each plant or plant unit (plant units are here defined as tightly clustered plants with coalesced canopies). Plants and plant units were placed in 1 of 4 categories, those in full morning or afternoon sun, those in full sun all day, and those in shade all day.
Frequencies of lacebug attack among categories of plants were compared with a log likelihood contingency test (6-test, 12). Furthermore, on a community wide basis the relationship between the % of azaleas planted in full sun and the % of azaleas infested by lacebugs was examined with linear regression analysis (12).
Results and Discussion
The total number of azalea plants and plant units studied was 135. Results of the contingency analysis clearly indicate that exposure to sun and frequency of lacebug infestation were not independent (x2/3 = 14.-570, p < 0.001). Azaleas planted in full sun all day and during the afternoon were more than twice as likely to be infested by lacebugs as those planted in shade (Figure 1). Azaleas receiving sun in the morning were slightly less frequently infested than those in afternoon or full day sun.
|Figure 1. Frequency of lacebug attack for 4 groups
of azaleas planted in different exposures to the sun.
Prior to regression analysis, for each community percentages of azaleas in full sun and azaleas with lacebugs were transformed using an arcsine transformation as recommended by Zar (12). A regression of the proportion of azaleas planted in full sun for all or part of the day with the proportion of azaleas infested with lacebugs revealed a highly significant linear relationship (r = .977, p<.01; Figure 2). From this analysis it is obvious that communities with large proportions of azaleas planted in full sun will be much more likely to incur lacebug attack compared to communities with azaleas planted in the shade.
|Figure 2. Relationship between the percent of azaleas
planted in full sun and the percent of azaleas with lacebugs
in 4 Maryland communities.
These results confirm contentions held by previous authors regarding the effect of exposure on the susceptibility of azaleas to lacebug attack (1, 3, 4). Both Bowers (1) and Holms and Davidson (3) suggested that full sun may stress azaleas and make them prone to lacebug attack. Rhoades (8) proposed that a variety of physical factors such as drought, temperature extremes, and pollution stress plants. This may reduce their natural defenses and increase their susceptibility to attack from pests such as lacebugs. In addition, White (9, 10, 11) has suggested that stress may increase the proximate nutritional quality of plants thereby making them more susceptible to herbivore attack.
The findings of this study contrast with those of others who have reported differential herbivory between plants in sunny and shady locations. Maiorana (5) found individuals of several plant species sustaining greater herbivory when in shaded compared to sunny habitats. She suggested that shaded habitats may serve as refuges for herbivores. Consequently, plants in shade incur greater herbivory due to the concentration of herbivores there (5). Rice et al. (7) reported similar findings for the perennial evergreen Satureja douglasii under attack by the banana slug Ariolimax dolichophallus.
The implications of these results with respect to managing the azalea lacebug are clear. Since azaleas planted in full sun are more likely to be infested with S. pyrioides, these plants will require more frequent inspections to detect incipient lacebug outbreaks. To prevent aesthetic damage caused by lacebug feeding, sunny plants may require more treatments with insecticides. As part of an integrated approach to pest management, professional landscapers, institutional ground keepers, and homeowners should avoid planting azaleas in full sun. Other landscape plants with the same ornamental effect should be substituted for azaleas in sunny locations. By selecting the proper site for azaleas, lacebug infestations and associated pesticide can be reduced.
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