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Title: Information Sheet, Hollies, Scale Insects, and the GU Urban Forests, Washington, D.C.
Instructors' Note
A student in Forest Ecology (fall 2002) at Georgetown University (GU), Washington, D.C., produced this report as an individual class project which had a limit of about 20 hours.
Goals of the projects included:
(1) learning about a relatively natural forest (Glover-Archbold Park which is adjacent to Georgetown University, in Washington, D.C.), the "open urban forest" of GU Campuses, and the "urban forest" in residential neighborhoods near GU.
(2) providing information about these subjects to interested parties via the Internet.
These are short projects that lay groundwork for further investigation in their respective areas.
Therefore, these are pilot projects.
To see all 12 of the 2002 projects, please use the keyword FE2002R on this Website.
Projects of future Forest Ecology students might continue lines of investigation of the 2002 projects.
E. M. Barrows and N. Bakkour, instructors
Wax Scales (Coccoidea) on Hollies (
Ilex) on Georgetown University Main and Medical Campuses
Rachel I. Adams
Department of Biology
Georgetown University
Forest Ecology 355
Fall 2002
Abstract
Hollies are many of the ornamental trees and shrubs on the Main and Medical Campuses of Georgetown University which can be viewed as artificial “open forests.”
Hollies variable in both appearance and form and are usually evergreen, making them excellent choices for landscape architecture.
Wax scales (small insects which produce a thick, white layer around themselves as adults) are common holly parasites.
On Georgetown’s Campuses, it is valuable to know which hollies are more susceptible to scales and what landscape habitat is correlated with a high scale infestation.
I observed that less than 40% of examined hollies looked infested with scales in fall 2002.
Certain holly cultivars and species were not more likely to have wax scales than others; however, most hollies with scales were in a physical environment that restricted air flow.
Introduction
Hollies (
Ilex spp.) are important plants in artificial “open forests” such as college campuses.
Most hollies are evergreen plants that can be trimmed into many forms, including small and large hedges, bushes, and large trees.
Their leaves are usually a brilliant dark green and are variegated in some cultivars.
Hollies can grow in dry through moist habitats and even in rock gardens.
Hollies often have red fruit, but also have black, orange, white, or yellow fruit, depending on the species, variety, or both.
Some hollies have very fragrant flowers.
Some hollies make excellent hedges because their leaf spines are deterrents to people who might walk into or try to walk through a hedge.
In addition, holly cultivars are easy to develop, and hybrid hollies are frequently used in landscaping.
Yet above all these advantageous traits that make hollies excellent for landscape architecture, it is perhaps their evergreen leaves that allow them to serve as a constant backdrop of vegetation for deciduous plants (Galle 1997).
Many holly cultivars and species adorn the Main and Medical Center Campuses of Georgetown University (Table 1).
Next to the Jesuit Cemetery, two large American Holly trees are about 50 ft high with branches that grow out and lean toward the ground, creating a dense wall of dark green, spiny leaves.
Two Japanese Hollies flank the entrance to Lauinger Library; these 15-ft high and 20-ft wide hedges serve as nice, large ornamentals.
Blue Hollies form a 2-foot high hedge around the periphery of the Dahlgren Quadrangle.
Also, deciduous hollies are on the hill slope on the south side of Yates.
Hollies are not free of diseases and parasites, and because of their importance in landscaping, there
is a large body of information about their pest control.
Many diseases and parasites affect hollies, for instance, aphids, leaf feeders including leaf-miners, mites, Wax Scales (
Ceroplastes spp.), and whiteflies.
Wax Scale females make holes into holly stems and suck out their sap.
The Indian Wax Scales (
Ceroplastes ceriferus, IWS) is not host-specific and is a common holly pest.
Adult females look like large white drops of wax on dark holly stems.
The actual insect, which is pink, is underneath her waxy covering.
When their population sizes are low, the scales are hardly noticeable.
When their population sizes are large, entire trunks and branches can be covered in white spots, and it can appear, at a quick glance, that clumps of snow have fallen on a plant.
A female wax scale can produce up to 3000 eggs in late spring, and they usually hatch in May and June over a 2-week period.
Scales disperse as first instar larvae (“crawlers”), which have legs and antennae and exist for about 24 hours.
The second instar larva is called the “cameo stage” because of it has prominent white wax on the margins on its back.
The third instar larvae are called the “dunce-cap stage” because its wax secretion on its back blends into the wax on its body margins forming an unbroken layer over the entire insect.
A female wax scale remains in this stage until she becomes an adult in late summer or fall(McComb, 1986).
Scales are parasites, feeding on holly juices that they usurp through their long rostrums inserted into phloem.
Control of IWS and other scale insects is clearly an important issue for persons who maintain hollies.
A balance needs to be created between preserving the equilibrium of nature and maintaining the aesthetics of the landscape.
Enlightened people use Integrated Pest Management (IPM) which basically works on the principle of “the environmentally right treatment at the right time.”
Mr. Mike Pontti, Director of Facilities and Recycling at Georgetown University, uses IPM on the Campuses.
The right treatment is the one that effectively rids the pest while causing the least negative environmental consequences.
The IWC control method is simple with little through no environmental impact – an oil-spray when the scales are crawlers in May and June.
The oil spray is a natural oil that smothers IWCs to death and quickly biodegrades.
It does this no plant damage or only minimal damage to its environment.
The oil can be distributed by a low-volume sprayer at light pressure which lightly coats a holly’s trunk and branches, in contrast to using a high-volume sprayer that works at such high pressure that oil enters the plant.
Spray programs can solve a scale problem completely within 2 years.
Timing is another aspect of IPM, not just time of the year but also timing of when to treat infested plants.
A campus with many hollies will have scales, but not all hollies should be treated for scales every spring.
GU Facilities must determine a threshold, somewhat arbitrarily, at which point a plant will be treated.
Below this threshold, Facilities will do nothing but watch a holly.
If wax scales become so abundant that a plant is unsightly that its value as an ornamental is lost, the plant must be treated (Michael Pontti, personal communication).
The goal of my survey was to ascertain the status of wax scales on Georgetown University's Main and Medical Campuses.
I tested the hypothesis that holly species influence wax-scale infestation level and plant confinement influences wax-scale infestation level.
Materials and Methods
I recorded holly forms and sizes, locations, degrees of isolation from other plants, and degrees of infestation on Georgetown’s Main and Medical Campuses.
Mr. Pontti identified hollies.
For small hedges, I looked for scales at the bases trunks and branches.
For large shrubs with a dense walls of foliage, I often had to locate an opening in the leaves and branches and appeared to sneak into the bushes.
Large hedges were large enough that I was able to stand inside them and look at their trunks and branches.
For shrubs, I pushed branches aside to look for scales either on branches or trunks.
Holly trees were large problems because most branches grew well above my height.
However, most had branches that drooped down toward the ground.
If I found no scales on these low branches, I assumed that the entire tree fostered no scales.
I estimated the abundance of scales on a plant.
After looking at several plants, it seemed that there were four infestation levels.
I assigned 0 to a plant with no wax scales, 1 to a plant with a few scales on its branches, 2 to a plant with many scales, and 3 to a plant with abundant scales.
Results and Discussion
Hollies fell into three air-flow categories:
no air flow (hollies surrounded by walls), restricted air flow (hollies next to walls), and large air flow (hollies in large open areas). No-air-flow hollies were more infested with scales than restricted-air-flow and large-air-flow ones (Table 2).
The two Campuses have about 200 hollies.
Only about 20% of hollies had wax scales.
There are over 50 wax-scale species; many of these are host specific.
My project did not identify wax-scale species.
Therefore, this study cannot conclude if certain hollies are more susceptible to certain scales than others.
However, I found that a holly’s physical environment may influence a plant’s level of wax-scale infestation.
Plants in confined places with restricted air flow tended to have more scales than plants in the open and in even
in somewhat restricted air-flow environments.
For example, the hollies in Dahlgren Quadrangle are surrounded on 3 1/2 sides by tall buildings, and these were
clearly the most infested hollies on the Campuses.
The row of plants near Yates Field House that is also badly infected with wax scale is composed of ‘Texas’ Japanese Quinces,
Chaenomeles ‘Texas,’ not hollies.
In another instance, the large holly shrubs near the wall of Yates had scales.
The scales were obvious when the inside branches were investigated, yet far away the scales could hardly be noticed.
Between the Intercultural Center (ICC) and the Jesuit Auditorium, scales were common on the row of ‘China Boy’ and ‘China Girl’ hollies.
These hollies are in a valley that may restrict air currents.
My sample sizes are small, and they suggest a trend only.
In wax-scale IPM, only plants that need treatment will receive it.
Facilities does not treat all plants (infested and noninfested) on a strict schedule.
Mr. Pontti has determined that his threshold for treatment is when the scales are completely infesting a plant and blatantly conspicuous.
So, for instance, the Front Gate hollies will not receive treatment, but the ones in Dahlgren Quadrangle will next spring.
It would be interesting to know how well oil treatment of plants in spring 2003 will control their scales.
Another interesting study would be to see if there is a correlation between the size of the holly and the degree of wax-scale infestation.
My data suggested that smaller hollies had higher wax-scale infestations than larger hollies.
A future study should attempt to determine which scale species infest which holly cultivars and species.
Acknowledgements
I could not have done this study without the assistance of Mr. Mike Pontti, Director of Facilities, Georgetown University.
Also, I would like to thank a DPS officer for making sure I "was doing alright" as I poked my head into a holly hedge at the Front Gates.
Literature Cited
McComb, C. W.
1986.
A Field Guide to Insects Pests of Holly.
Holly Society of America, Inc., Baltimore, Maryland.
Galle, F. C.
1997.
Hollies: The Genus
Ilex.
Timber Press, Portland, Oregon.
Table 1. Location of Hollies and Leaf Miner Activity on the Georgetown University Main and Medical Center Campuses, Fall, 2002.
=20
| =20
Species
|
=20
Common name
|
=20
Location
|
=20
Leaf miner
|
=20
Comments
|
=20
| =20
Ilex aquifolium
|
=20
English Holly
|
|
|
|
=20
| =20
Ilex aquifolium=A0 x cornuta 'Nellie R. Stevens'
|
Nellie Stevens Holly
=20
Behind Copley, Healy
|
=20
No
|
=20
Cultivar, very glossy leaves
|
=20
| =20
Ilex aquifolium 'Berkeley'
|
|
=20
By Yates Field House
|
|
|
=20
| =20
Ilex aquifolium 'Yellow=20
Umbrellas'
|
=20
Yellow Umbrella
|
|
|
=20
Very shiny, leaves turn over, yellow=20
fruit
|
=20
| =20
Ilex x =A0aquipernyi 'Dragon=20
Lady'
|
=20
Hybrid of English and pernyi (Asian)
|
|
|
|
=20
| =20
Ilex x =A0aquipernyi 'San=20
Jose'
|
=20
Aquipern Holly
|
|
|
=20
Glossy, spiny leaves
|
=20
| =20
Ilex cornuta
|
=20
Chinese Holly
|
=20
Harbin Hall, in front of the football=20
field
|
=20
No
|
=20
Spiny, stiff, glossy leaves
|
=20
| =20
Ilex cornuta 'Burfordii'
|
|
=20
White Gravenor, in front of Lauinger=20
Library
|
=20
No
|
=20
Like the 'Dwarf Burford,' except it grows larger, about 20 by 20 feet.=A0=
Open=20
arca.
|
=20
| =20
Ilex cornuta 'Dwarf=20
Burford'
|
=20
Dwarf Chinese Holly
|
=20
Harbin Hall patio
|
=20
No
|
=20
Single terminal spine on each leaf
|
=20
| =20
Ilex crenata
|
=20
Japanese Holly
|
|
|
|
=20
| =20
Ilex crenata 'Helleri'
|
|
=20
New North Hall
|
=20
No
|
=20
Flatter and shinier leaves than I.=20
crenata
|
=20
| =20
Ilex crenata 'Rotundifolia'
|
=20
Japanese Holly
|
|
|
=20
Few spines, black fruit, lots of scale=20
insects in 2002.
|
=20
| =20
Ilex opaca
|
=20
American Holly
|
=20
Between Harbin Hall and the ICC, also=20
by the Observatory
|
=20
Yes
|
=20
Most likely, the leaf miner is Phytomyza ilicicola, Ameri=
can Holly=20
Leaf Miner. a =A0
|
=20
| =20
Ilex opaca 'Miss=20
Helen'
|
=20
Large fruit, heavily fruited
|
|
|
|
=20
| =20
Ilex opaca 'Vera'
|
=20
American Holly
|
=20
Harbin Hall, in front of the field
|
=20
No
|
|
=20
| =20
Ilex penduculosa
|
=20
Longstalk Holly
|
|
|
=20
Native, soft leaves
|
=20
| =20
Ilex siberia
|
|
=20
By Observatory
|
=20
No
|
|
=20
| =20
Ilex verticillata 'Red=20
Sprite'
|
=20
Common Winterberry
|
=20
No
|
|
|
=20
| =20
Ilex x=20
aquipernyi 'Lydia Morris'
|
=20
Aquipern Holly, Wire Holly
|
|
|
|
=20
| =20
Ilex x=20
attenuata 'Fosterii No. 2'
|
|
|
|
|
=20
| =20
Ilex x=20
meserveae 'Blue Princess'
|
=20
Blue holly
|
=20
Dalghren Chapel patio
|
=20
No
|
|
=20
| =20
Ilex x=20
meserveae 'China Girl' and 'China Boy'
|
|
=20
By Healey Gates
|
=20
No
|
=20
Hybrid
|
a Mr. Michael Pontti (personal communication)
