POTENTIAL FOR GYPSY MOTH PROBLEMS IN SOUTHWEST OREGON

by

Gary E. Daterman, Jeffrey C. Miller, and Paul E. Ranson

ABSTRACT—Gypsy moth could become established insouthwest Oregon. Numerous plant species native tothe area appear to be suitable hosts, the oaks andPacific madrone being particularly susceptible.Damage potential is highest in the interior valleysand surrounding foothills, with damage to coniferspecies likely limited to those growing in mixedstands with broadleaf hosts such as oaks, madrone, andalder.

KEYWORDS—Gypsy moth, Lymantria diaper, conifers,western host plants.

INTRODUCTION

The gypsy moth, Lymantria dispar (L), is a majordefoliator of hardwood forests in Europe, westernAsia, and northeastern North America (Doane andMcManus 1981). Until recent years the western UnitedStates has been free of this pest. Since about 1980,however, detections of isolated infestations of gypsymoth have increased in the West, particularly inWashington, Oregon, and California. This trendcoincides with a major four—year population explosionof gypsy moth in 1980-1983 when the insect defoliatedmore than 29 million acres in the northeastern UnitedStates. This major outbreak supplied a largereservoir of egg masses for accidental transport onvehicles, boats, outdoor furniture, etc., to new sitesof introduction.

The situation in Oregon became particularly serious in1984 when some 20,000 gypsy moth males were capturedby pheromone—baited detection traps, and heavy eggmass deposits were discovered on rural forest landnear Pleasant Hill, Oregon (Lane County). As part ofa plan to eradicate the extensive Oregon infestations,over 220,000 acres of rural and urban lands weresprayed in 1985 with multiple applications of themicrobial insecticide Bacillus thuringiensis (B.t.).The effectiveness of those sprays is still beingevaluated, but already in 1985 the annual detectionsurvey with pheromone—baited traps has turned up a newinfestation of this pest near Glide, Oregon (DouglasCounty).

Because of an established gypsy moth population in theeastern United States and a continuing flow oftravelers from that area, it is clear that thepressure of new introductions of this pest willcontinue, regardless of the effectiveness oferadication programs against isolated, localinfestations. These circumstances make it realisticfor the western states to consider where and how gypsymoth may pose a threat within their boundaries. Inthis paper we have attempted to evaluate the potentialproblems that may be caused by gypsy moth in southwestOregon. By necessity some statements are speculative,although we have a degree of predictive capabilitybased on activities of this insect elsewhere and oncurrent research on host susceptibility beingconducted at Oregon State University.

SUITABILITY OF NATIVE PLANTS AS HOSTS FOR GYPSY MOTH

More than 300 plant species are known to be adequatehosts for gypsy moth (Leonard 1981), but little ispublished about the suitability of plants native to

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western North America as potential hosts for this

insect. Jobin (1981) found that second instar andolder larvae would feed and complete development onDouglas-fir, Pseudotsuga menziesii (Mirb.) Franco, andthat all larval stages fed and completed developmenton western hemlock, Tsuza heteroph ylla (Raf.) Sarg.Studies of host suitability and preference were begunat Oregon State University in 1982. In this paper wehave summarized preliminary findings about(1) suitability of selected western tree and shrubspecies for feeding and development and (2) preferenceof larvae for difference species when offered a choicein small laboratory cages.

Host-Plant Evaluations

Eleven conifer species and more than 100 broadleavedspecies native to Oregon were evaluated. Many ofthese are native to southwest Oregon. In this test,clipped foliage supplied with water was placed in a1-pint plastic container with three larvae. Eachcontainer was supplied with one particular diet offoliage. If first instar larvae died, second instarswere then tested on the same host, and so on.

Results of this evaluation show gypsy moth can surviveon a variety of plant species native to southwestOregon. Among the conifers, members of the Pinaceaeappeared to be moderately suitable as host plants,whereas the Taxodiaceae were marginally suitable andthe Taxaceae and Cupressaceae unsuitable. There wasmuch variation among the broadleaf species evaluated,with the extremes represented by the highly suitableFagaceae and the very unsuitable Oleaceae. Table 1provides specific examples of southwest Oregon treeand shrub species according to their relativesuitabilities for gypsy moth growth and development.The different categories of host suitability are basedon rate of larval development and pupal weightsattained. The "very suitable" host category containsthe top 25 percent of the plant species for rate ofdevelopment and pupal size, whereas the "moderatelysuitable" category represents host plants thatsupported insect growth and development throughpupation but with a slower rate of growth and lower

TABLE 1.

pupal weights. The "unsuitable" category includesplants the larvae would not touch or on which theyfed only lightly and failed to complete developmentto adulthood.

Broadleaf-Conifer Rost Preference Evaluations

Because of their high timber value, Douglas-fir andwestern hemlock were the two coniferous speciesemphasized in this feeding experiment. Oregon whiteoak or Garry oak, Ouercus garrvana Dougl., and redalder, Alnus rubra Nutt., were the two broadleafspecies selected because of the likelihood of theirbeing suitable host plants for gypsy moth, theiroccurrence in mixed stands with several westernconifers, and their abundance near urban areas wheregypsy moth introductions are most likely to occur.

Choice tests with first instar larvae were conductedutilizing foliage of Oregon white oak, red alder,Douglas-fir, and western hemlock. Larvae were placedin a 1-pint plastic container with two types offoliage offered in paired choice tests. Four twigs,two of each foliage type, were placed in thecontainers in a crosswise pattern. Two larvae wereplaced on each of the four twigs. The relativeamounts of defoliation, the frass production, and thelocation of the larvae were observed at days 3, 6,and 9. Each trial was replicated twice.

The results demonstrated that the two broadleafspecies, in particular Oregon white oak, were highlypreferred to the coniferous species. For the twoconifers, Douglas-fir was preferred to hemlock, andparticularly the one-year-old needles of Douglas-fir. These laboratory findings were partiallyvalidated by field observations in Lane County,Oregon, where most evidence of gypsy moth feeding wasfound on oak, with visible defoliation to Douglas-firbeing generally limited to trees with foliageadjacent to or mixed with oak foliage. Red alder andwestern hemlock were not present in the heavilyinfested area. In summary, the insect prefers Garryoak to red alder, red alder to Douglas-fir, andDouglas-fir to hemlock.

RELATIVE SCITA3ILITIES OF SELECTED SOUTHWEST OREGON TREES AND SHRUBS AS HOST PLANTS FOR GYPSY MOTH.

Very suitable Moderately suitable

Unsuitable

Oregon white oakPacific madroneCanyon live oakaTanoakaRed alderManzanitaWillowHazelCalifornia black oakPoison oakHuckleberrySalal

MaplesAlders (other than red)Oregon grapeChokecherryPoplarsRose (sweetbriar)HawthornDouglas-firPinesWestern hemlockCoast redwoodSpruceThimbleberryTrue firsPacific bayberry

Himalayan blackberryDogwoodOregon ashCeanothus app.Rhamnus spp.SnowoerryCalifornia juniperPacific yewWestern redcedarPort-Orford-cedarIncense-cedarCalifornia laurel

aSmaller larvae require new foliage in order to complete development.

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SUITABILITY OF CLIMATE

Are there climatic characteristics unique to Oregon,and southwest Oregon in particular, that might checkgypsy moth infestations? Certainly the Oregon •weather patterns vary greatly by subregion, but it isdoubtful that climate will prove a serious barrier toestablishment of this insect in much of the State.The gypsy moth already has demonstrated its abilityto multiply to heavy densities in the western Oregon,Lane County, infestation. Also, the existence andpest status of this insect over much of Europe,western Asia, parts of North Africa, and NorthAmerica (Leonard 1981) indicate its adaptability andgeneral success in coping with a wide variety ofphysical environments. In the extreme, we can makesome marginal predictions about regionalsusceptibility and climate. Surely the harsherconditions at higher elevations will be lesshospitable to gypsy moth than the interior valleysand lower slopes of the Cascade Range, the SiskiyouMountains, and the Coast Ranges. The cool, damp,marine air in much of the Coast Ranges may influencesuccess of the gypsy moth, but a determination willlikely have to await the test of time and the moth'sperformance in such areas.

POTENTIAL IMPACTS OF THE GYPSY MOTH

In North America the gypsy moth has been knownprimarily as an urban pest of ornamental hardwoodtrees in the northeastern part of the United States.There is little doubt that the insect could alsobecome an urban pest in southwest Oregon, damagingornamental trees and shrubs and possibly causing mildhealth problems from allergic reactions to theinsect's hairs. Further, however, is the possibilityof the insect causing other kinds of direct economicand environmental impacts. Such a possibility is ofparticular importance in southwest Oregon because ofthe significance of the forest to the local economy.The following is a list of potential impacts thatcould result from gypsy moth establishment in an arealike southwest Oregon:

Quarantines on shipments of restrictedcommodities (Christmas trees, firewood, nurseryornamentals, logs, lumber)

Increase in agricultural costs (additionalpesticide treatments)

Increase in costs to timber industry (specialpackaging or treatments to meet regulations forexport or interstate shipment)

Forest damage (tree mortality and growth loss)

Urban nuisance problems (ornamental damage;malodorous fecal material and dead insects; larvae inyards, on cars, in and on houses, etc.)

Health problems (allergic skin reactions fromurticaceous larval hairs, respiratory problems forsome individuals)

7. Increase in pesticide use (increased use ofpesticides by public agencies and private individuals

to minimize all the above impacts)

Quarantines and increased agricultural and timberproduction costs could result from the detection of

only light and scattered infestations of gypsy moth.Any product entering interstate commerce or export toanother country can be discriminated against becauseit may harbor gypsy moth eggs. Consequently,selected products may be banned from certain routesof commerce merely because of light infestations atthe source location which provide the possibility ofpest introduction. Such circumstances can lead topartial loss of markets, or costly precautionarytreatments necessary to certify pest—freecommodities. An obvious example of such a commodityis the Christmas tree. Christmas tree growers in theLane County area met with varying degrees ofdifficulty in 1984 when marketing their trees. Thesedifficulties originated from quarantine regulationscalling for fumigation of trees to be shipped, andfrom interpretations and resulting prejudices ofbuyers regarding the gypsy moth situation in Oregon.As a consequence, most Oregon Christmas tree growershave resorted to "insurance" sprays with carbaryl in1985. This is perhaps a classic example of how thepossible presence of the gypsy moth can (1) lead toquarantine regulations, (2) increase productioncosta, and (3) increase the use of pesticides.

The potential of gypsy moth infestation can also leadto increased costs to the timber industry and for theproduction of certain agricultural crops. Productsfor interstate commerce or for export, particularlyto Asian markets, may have to be treated prior toshipment. Logs may have to be debarked, mulchproducts may require heat treatments, and veneer andlumber may have to be treated in some way to ensurethat viable gypsy moth eggs will not be transportedwith the product. In the orchard industry, increasedsprays may be necessary to guard against potentialdamage to pears, apples, filberts, and possibly othercrops. Where gypsy moth populations reach outbreakproportions, we can expect forest damage by treemortality and growth loss; by the usual urbanproblems of high nuisance factor from larvae crawlingin and on houses, cars, outdoor furniture, etc.; bythe health hazard from the insect's hairs and bodyparts causing allergic reactions; and finally, by thedamage to shade trees and ornamental plants in thecommunity. Use of pesticides would again increase ashomeowners and public officials attempted to lessenthe various effects of gypsy moth infestation.

DISCUSSION AND SUMMARY

It is clear from the studies on host suitability andby the performance of the gypsy moth in Lane County,that parts of southwest Oregon are susceptible toinfestation by this pest. The interior valleys andlower elevations are perhaps most susceptible becausethese areas harbor a high proportion of the hardwoodsand evergreen trees and shrubs that are suitablehosts. Most of the human population in theseinterior valleys exist in the cities of Roseburg,Grants Pass, Medford, and Ashland. Accordingly, itis in these communities and nearby parks and otherrecreation sites that the gypsy moth is most likelyto be introduced by travelers from the heavily

infested eastern United States. Surrounding thesecommunities are habitats that are generally highlyhospitable to the gypsy moth because of theprevalence of oaks, madrone, tanoak, and other plantspecies categorized as suitable or very suitable. Inaddition, the towns and cities contain many otherexotic trees and shrubs that are favored as hostplants. Thus, the stage is set for continued

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introduction and establishment of gypsy moth in theinterior valleys and lower elevations of southwest

Oregon.

Projections of how damaging gypsy moth might beoutside the interior valleys of southwest Oregon ismore speculative. The insect's success in thevalleys and up to elevations of about 1,200 feetappears generally assured. The next level of 1,200to about 2,500 feet might be categorized as aprobable problem zone because of the prevalence ofbroadleaved species such as madrone and the oaksmixed with the conifers (Franklin and Dyrness 1973).We do not anticipate serious gypsy moth damage whereconifer species predominate on a site. For thisreason, it is doubtful that this insect will everbecome a pest in southwest Oregon at elevationshigher than 3,000 feet. In the coastal mountainswhere there is an influence of the cool, damp, marineair from the Pacific Ocean, the question of gypsymoth success is again speculative. Many suitablehosts exist in this area, but the influence of themild but wet physical environment is not known. Also,the synchronization of host plant phenology as itcorresponds to egg hatching and other critical pointsin the insect's life cycle may differ in this zone.

Numerous negative impacts are associated with thegypsy moth. Economic impacts can result fromquarantine regulations restricting shipments ofcertain commodities; increased pesticide treatmentsrelated to gypsy moth problems are costly; heavyinfestations could reduce productivity in the forestswhere commercial timber species grow in mixtures withthe more susceptible broadleaved hosts; and healthand nuisance problems related to heavy infestationscan develop in communities. When populations havereached epidemic proportions in the eastern UnitedStates, gypsy moth impacts have negatively affectedproperty values. All such negative impacts result ingreater pesticide use in forest, agricultural, andurban environments.

On the plus side, there is the possibility that anestablished gypsy moth population could be partlybeneficial to southwest Oregon. The insect'spreference for certain shrub and tree species toothers could alleviate some of the brushfieldproblems in this area. Its preference for the oaksand red alder to the conifers, for instance, wouldappear to favor growth of species preferred by landmanagers. In a high density infestation, however, itis most likely the conifers would also suffer heavydamage where they were mixed with the moresusceptible host plants.

The Oregon Department of Agriculture and cooperatingpublic agencies are conducting very thorough surveysto detect new infestations of this pest. Theseannual surveys are designed to pinpoint newinfestations at an early stage when eradication maystill be a feasible option. Perhaps the most prudentrecommendation that can be made to land managers andresidents of the southwest Oregon region is tosupport this annual survey program to the extentpossible. This pest may ultimately becomeestablished throughout Oregon, as It has over much ofthe northern hemisphere. The potential damage it cancause, however, is sufficient to warrant ourconcerted efforts to keep it from permanentestablishment as long as possible.

REFERENCES CITED

Doane, C. C., and M. L. McManus. 1981. The gypsymoth: research toward integrated pest management.Northeastern Forest Experiment Station, Broomall,Pennsylvania. U.S. Department of AgricultureTechnical Bulletin 1584. 757 p.

Franklin, Jerry F., and C. T. Dyrness. 1973.Natural vegetation of Oregon and Washington. PacificNorthwest Forest and Range Experiment Station,Portland, Oregon. USDA Forest Service GeneralTechnical Report PNW-8. 417 p.

Jobin, L. 1981. Observations on the development ofthe gypsy moth, Lymantria disoar (L.), on Douglas—firand western hemlock. Department of the Environment,Ottawa, Ontario, Canada. Canadian Forestry Service,Bimonthly Research Notes 1(4):29-30.

Leonard, D. E. 1981. Bioecology of the gypsy moth.Pages 9-29 in The gypsy moth: research towardintegrated pest management. (C. C. Doane and M. L.McManus, eds.). Northeastern Forest ExperimentStation, Broomall, Pennsylvania. U.S. Department ofAgriculture Technical Bulletin 1584.

ACKNOWLEDGMENTS

This study was funded in part by a cooperativeresearch agreement from the U. S. Department ofAgriculture, Forest Service, Pacific NorthwestResearch Station, to Oregon State University. Weappreciate the manuscript review suggestions by JoeCapizzi and Jack Waisted of Oregon State Universityand Charles Sartwell and Leone Sower of the U.S.Department of Agriculture, Forest Service.

THE AUTHORS

Gary E. Daterman is a research project leader for theUSDA Forest Service, Pacific Northwest ResearchStation, Forestry Sciences Laboratory, Corvallis,Oregon 97331. Jeffrey C. Miller and Paul E. Hansonare associate professor and graduate assistant,respectively, in the Department of Entomology, OregonState University, Corvallis, Oregon 97331.

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OregonUniversity

Forest Pest Managementin Southwest OregonProceedings of a workshop held August 19-20,1985Best Western Riverside Conference Center,Grants Pass, OR

Editor: 0. T. Helgerson Published July 1986

Sponsored by the Adaptive Phase of Oregon State University's Fir Program(Southwest Oregon Forestry Intensified Research Program).

Forest Research LaboratoryOregon State UniversityCorvallis, Or 97331