Numbers and types of arthropods overwintering on common mullein, Verbascum thapsus L. (Scrophulariaceae), in a central Washington fruit-growing region

9 Pages·2003·5.4 MB·English

by  David R Horton

J.ENTOMOL.SOC.BRIT.COLUMBIA100,DECEMBER2003 79
Numbers and types of arthropods overwintering on common
mullein, Verbascum thapsus L. (Scrophulariaceae), in a
central Washington fruit-growing region
DAVID R. HORTON and TAMERA M. LEWIS
USDA-ARS, 5230KONNOWAC PASSRd.,WAPATO,WA, UNITED STATES98951
ABSTRACT
Densitiesandtypes ofarthropodsoverwinteringon common mullein, Verbascumthapsus
L., in a fruit-growing region of Central Washington were determined. Over 45,000
arthropods were collected from 55 plants (5 plants from each of 11 sites), dominated
numericallybyAcari andThysanoptera. Insectsrepresenting 8 orders and29 familieswere
identified, distributedboth in thebasal leafrosettes and inthe stalkmaterial oftheplants.
One specialist insect herbivore of mullein, the mullein thrips, Haplothrips verbasci
(Osbom), was abundant at all sites. Several pest and predatorytaxathat commonlyoccur
in orchards were also collected, suggestingthat mullein maybe a source ofoverwintered
pests or predators moving into orchards in early spring. Pest taxa included primarily
western flowerthrips {Frankliniella occidentalis (Pergande)), Lygus spp., andtetranychid
spidermites. Common predators includedphytoseiidmites andminutepiratebugs {Orius
tristicolor (White)). Sites that were geographically close to one another were not more
similar (in taxonomic composition of overwintering arthropods) than more distantly
separated sites.
Key words: common mullein, overwintering, orchardpests, predatoryarthropods,mullein
thrips, western flowerthrips, Orius tristicolor, mites
INTRODUCTION
Commonmullein, Verbascum thapsus L. (Scrophulariaceae), is abiennial herb native to
Eurasia (Munz 1959) but now common throughout North America. The species occurs in
open waste areas, along fence lines, in overgrazed pastures, and along river bottoms, often
found growing in large single-species stands. Common mullein has a biennial life cycle,
germinatingfromseedoftennearclunps ofthe deadparentalplants. Inthe firstyear, theplant
develops as a rosette ofsoft, bluish-gray leaves which are densely covered with silky hairs.
The followingyear, a stout, leafystalk is sentup fromthe centerofthe rosette, oftenreaching
m
a height ofmore than 2 inmid-summer. At the top ofthe stalk is a spike having 100-200
yellow flowers which are inbloomseveral at a time formuch ofthe summer.
There is a great deal of interest in managing or conserving non-agricultural habitats
adjacentto agriculturalhabitats to enhancebiological control orto reduce infestationofcrops
by pest species (Pickett and Bugg 1998; Ekbom et al. 2000). In Pacific Northwest fruit
growing regions, commonmullein often is abundant ontheperimeterofpome and stonefruit
orchards. Theplantisknowntoharborimportantpests oftree fruits duringthe growing season
(Thistlewood et al. 1990; Krupke et al. 2001), but it also is an important source ofcertain
predators (e.g., Campylomma verbasci(Meyer), the mulleinbug) thatmayprovidebiological
control of aphids and mites during the growing season. Less information is available
concerninguse ofmulleinbypests andpredators as anoverwintering habitat(McAtee 1924).
It is important to understand late winter and early spring population dynamics ofpests and
predators in pear and apple orchards, as that time ofyear is often crucial for pest control.
Thus, we need also to understand the overwintering biology ofarthropods both inside and
outside ofthe orchard, including a determination ofwhere pests and predators overwinter
80 J.ENTOMOL.SOC.BRIT.COLUMBIA100,DECEMBER2003
(Hortonand Lewis 2000; Hortonetal 2002).
McAtee (1924), inMaryland, conducteda cursorysurveyofthe insects associatingwith
common mullein, and found that the dense basal rosette ofleaves provided overwintering
habitat for several taxa ofarthropods, including both pest and predatory insects. Thus, in
addition to being a source ofpest and beneficial arthropods during the summer and fall
growing season, commonmulleinmayalsobe a source ofoverwintered arthropods moving
into Central Washington orchards in late winter and early spring. Objectives ofthe present
studywere to determine types anddensities ofarthropods overwinteringoncommonmullein
ina fruit-growingregion ofCentral Washington. The studywas designedtoprovide amore
thoroughlookatthe arthropodcommunities inmulleinthanprovidedbyMcAtee (1924), and
toprovidedataforawestemUSpopulationofmullein. Wealsocomparedtypesandnumbers
ofarthropods overwintering inthebasalrosette ofleaves to numbers andtypes occurring in
the leaves, dried flowers, and seed capsules ofthe stalk. Lastly, we looked for geographic
patterns in the taxonomic composition ofarthropod communities overwintering in mullein,
testing the hypothesis that arthropod communities wouldbe more similarbetween sites that
occurgeographicallynearone anotherthanbetween sites thatwere more widelyseparated.
MATERIALS AND METHODS
The study was done in and adjacent to Yakima, Washington, USA. Eleven sites, each
having stands offullymature commonmullein, were selectedinNovember2000 forsampling.
Plants that were sampled had bloomed the previous summer and were composed ofdead
leaves andseed-ladenstalks atthe time ofcollection. Allofthe sites were alongroadsides that
occurred immediatelyadjacentto orchardhabitatorwithin 1 kmoforchardhabitat. Straight-
line distancesbetween sites rangedbetween0.5 to46.4 km. InDecember 2000 and January
2001, we collected five fully mature plants (1.5 to 1.8 m tall) from each site by cutting the
plantsjustbeneath the soil surface. Plants wereplaced in large plastic bags for transportto
the laboratory. Bags and plants were placed in a large walk-in cooler (2 ''C) until the
arthropodswereextracted. To extractthe arthropods, theplantmaterialwasdistributedamong
25 Berlese-TuUgrenfunnels (Southwood 1980), keepingstalks and leafrosettes separate. We
used 40 watt lightbulbs to force the arthropods into 75% ethanol.
Arthropods were thenseparated fromtheplantdetritusbyfirstslowlypouringthe alcohol
throughveryfine (0.2 x0.2 mm) organdymesh. Themeshappearedto capture allbutthevery
smallest mites (mostly immature Tydeidae and Tenuipalpidae). These specimens were
discarded withoutbeing identified, thus summaries providedbelow forAcari underestimate
total numbers of certain small-bodied taxa. The arthropods remaining on the mesh were
removed fromthe meshandplantdetritus withforceps, insectpins, orsmallpaintbrushes, and
transferredto fresh 75% ethanol for later counting and identification.
Insects other than Lepidoptera (all ofwhich were in the caterpillar stage) and parasitic
Hymenoptera were identified at least to family. Known important predators and pests in
orchards were identified to species. Most samples contained very large numbers of
Thysanoptera, anditwasnotfeasibleto identifyeachspecimen. A subsampleof50thrips was
removed fromeachsample foridentificationto genusbeneatha dissectionmicroscope, using
the key ofMound and Kibby (1998). Immature thrips were not classified. Results for the
subsample were thenextrapolatedbackto thefullsampletoprovideestimatesoftotalnumbers
ofthrips for each genus. Representative examples ofeach genus were sent to an expert in
thrips identification(Steve Nakahara; Beltsville, MD) to confirmour identifications.
Mites were veryabundantinthe leafrosettes andmuchlessabundantinthe stalkmaterial,
thus we limited acarine identificafions to those mites inhabiting the leaf rosettes. The
identificationswere confinedto sixofthe 11 sites. We firstseparatedGamasida fromthe total
sample, for laterexamination. Fromthe remaining sample, a subsampleofapproximately 50
,
J.ENTOMOL.SOC.BRIT.COLUMBIA100,DECEMBER2003 81
to 500mites(dependingupontotalnumbers inthe sample) weremountedinHoyer's solution
onmicroscope slides (Krantz 1978). The mites were then identified to genus (Tetranychus
spp. only) orto familyunder a compoundmicroscope usingkeys inKrantz (1978). Counts
for Tetranychus spp. were then extrapolated back to the total sample to obtain estimates of
absolute densities for Tetranychus spp. Fromthe Gamasidawhole sample, subsamples of25
to 85 miteswerethentakenforthose siteshavinglargenumbers ofgamasidmites(>90mites).
The mites were mounted on slides in Hoyer's solution and identified to family (all but
Phytoseiidae) orto species(Phytoseiidae). Species identifications forthe Phytoseiidae were
made using keys in Schuster and Pritchard (1963) and Chant et al (1974). We then
extrapolatedresults forthe subsamplebacktothe full Gamasidasample toprovide estimates
ofabsolutenumbers forphytoseiid species.
Straight-line distances between sites were obtainedusing a Vista globalpositioning unit
(Garmin; Olate, KS). Wetestedwhethersitesthatwere geographicallynearone anotherwere
more similar than those more distantly separated by calculating taxonomic (family-level)
similaritybetweenallpossible sitepairs, using the following formula:
Relative absolute distance = ^Absolute value [(%/^Xy)- {xnJ^Xy^t)]
wherej and k are two sites, is the abundance ofthe insect family at sitey, and xn^ is
abundance ofthe insect family at site k (Ludwig and Reynolds 1988). The analysis was
limitedto families ofInsecta. The indexvariesbetween0 and2, with0 indicatingmaximum
similarityand 2 indicating complete dissimilaritybetweenthe two sites.
RESULTS
Atotalof46,712 arthropods wascountedfromthe 11 sites, ofwhich44.7%were collected
fromthe leafrosettes and the remaining 55.3% were collected from the stalk material. The
sampleswere dominatednumericallybytheThysanoptera andAcari(Table 1), accounting for
over90% ofthe arthropods frombothleafand stalkmaterial. Forthe Insecta, 29 families in
8 orders were identified, withspeciesofThysanoptera, Coleoptera, andHeteropterabeingthe
most abundant. There was considerable site-to-site variation in counts (Table 1, Range) for
virtuallyall commontaxa.
' Miteswereveryabundantinthe leafrosettes(exceeding 500per5-plantsample) andmuch
less abundantinthe stalkmaterial(Table 1). Specimens fromleafrosettes atsixofthe 11 sites
were identified, andwere foundto include large numbers ofGamasida andActinedida (Table
I 2). Gamasida were composedprimarilyofPhytoseiidae, including some importantpredatory
I species (Amblyseius spp.; Typhlodromus caudiglans Schuster; western predatory mite,
I
Galendromusoccidentalis (Nesbitt)). Spidermites(Tetranychidae) were relativelyuncommon
(Table 2), and includedprimarily Tetranychus spp. (47 of64 tetranychids in subsamples).
Thysanoptera included two families, Thripidae and Phlaeothripidae (Table 1), the latter
apparentlybeingrepresentedbya single species, Haplothrips verbasci(Osbom), the mullein
thrips. This species was very abundant inboth stalks and leafrosettes, reaching densities of
over800 adultsperstalkatone site. Thripidae included species ofThrips (apparentlymostly
ThripstabaciLindemanbutincluding T.fallaciosusNakahara), Caliothrips, andFrankliniella
(apparentlyall western flowerthrips, F. occidentalis (Pergande)).
Homoptera were composed primarily ofaphids and psyllids (Table 1), including pear
psylla, Cacopsylla pyricola (Foerster), a pest of pears. Heteroptera were dominated
numericallybyAnthocoridae andMiridae (Table 1). The 75 Tingidae thatwere collected all
occurred inthe leafrosettes ata single site. Anthocoridae were almost exclusively minute
piratebug, Orius tristicolor(White) (164 total specimens), but includedalso five specimens
ofXylocoris umbrinus VanDuzee. Overwintering Coleoptera were dominated numerically
1 H 1
82 J.ENTOMOL.SOC.BRIT.COLUMBIA100,DECEMBER2003
Table 1
Mean(averagedover 11 sites)numbers ofarthropodsper5 mulleinplantsinleafrosettes and
stalkmaterial. Numbers inparentheses indicate percentage oftotal arthropods composedof
that taxon [(tr) indicates less than 0.1%]. Range shows minima and maxima across the 1
sites. Familymeans maynotsumto ordermeans due topresence ofunidentifiedarthropods
inthatorder (particularlytrue forThysanoptera, forwhich immatures were notidentified).
Leaf rosettes Stalks
ItIC^II IIUlllUvl9 i.TlC<tll llUlllUvi3
npr^nl$intc nprSnIfinfK
^^^^ 123-1455 34.0(1.4) 3-167
111^3(11IVJJJICIa 1i^2y6jy6j.Qy^y\6j\6j.7/^j 30-3936 2263 5C96 S^k 210-5516
11111L/lvJuV^ 515.6 0-2362 590.4 49-1655
1IlldCUUlIipiUdC 7106 2S 16^2 1i4to67/.SJ fxAui.1lA
nUliHJ|JlCIa 1 2Q 4 Sf'O2^
/ApillUlUdC J.J 0yj-114 06 0\j-1j
("'prpf^niflap 0.1 0-1 0.0 0
l^lLaUCIllUaC yj.j n 1 00 AU
rayiuuac t.J 0 14 J3.8O 0u-oR
Het6roptera 30.5 (1.6) 4-87 8.9(0.4) 0-32
Anthocoridae 7.5 0-22 7.7 0-30
Delyuuac 0v.11 0 1 0\).\0J A\j
Lygaeidae 1.6 0-16 0.0 0
IVl11IVJUV.' 10.6 0-64 0.8 0-6
0.1 0-1 0.0 0
1ClUalUIIllUaC 06 00 A\j
RIvpHiLliVvi11iLiHdpep 0.1 0-1 0.0 0
RivlhInVrJ\L/^dlliIHv;J^up^ 2.3 0-12 0.1 0-1
T1i1n1ic^jiirlU^a^cp 6.8 0-75 0.0 0
Colcoptera JJ.OV 11-1107 jj.jyy.'^) 0-70
Anthicidae nu.nu AU n 1 n 1
07 0-1 0.0 0
r(\JrC*CrI*iHnCplllliIHUcajCp 0U.JS n 1 00 Au
\^UnIr\y7i1uripniHll1Ur1cipC 57 0 37 0.6 0-6
27.5 1 130 32.5 6-98
Dermestidae 0.0 0 0.1 0-1
Staphylinidae 0.8 0-3 0.0 0
Neuroptera 0.5 (tr) 0-2 0.1 (tr) 0-1
Hemerobiidae 0.5 0-2 0.1 0-1
Lepidoptera 3.9(0.2) 0-17 0.3(tr) 0-2
Diptera 2.7(0.1) 0-16 0.4(tr) 0-2
Cecidomyiidae 1.5 0-11 0.2 0-2
Chironomidae 0.2 0-2 0.1 0-1
Mycetophilidae 0.1 0-1 0.0 0
Syrphidae 0.1 0-1 0.0 0
Hymenoptera 10.2(0.5) 0-73 0.5(tr) 0-2
Parasitoids 3.5 0-7 0.5 0-2
Formicidae 6.5 0-69 0.0 0
Vespidae 0.1 0-1 0.0 0
Araneae 7.4(0.4) 0-21 0.8(tr) 0-3
Opiliones 0.1 (tr) 0-1 0.0(tr) 0
Chilopoda 0.2(tr) 0-1 0.0(tr) 0
Isopoda 0.1 (tr) 0-1 0.0(tr) 0
1 J11
J.ENTOMOL.SOC.BRIT.COLUMBIA100,DECEMBER2003 83
Table 2
Taxonomic compositionofmite subsamples (Oribatida, Acaridida, Actinedida) andabsolute
numbers ofGamasida inmullein leafrosettes ateachof6 sites.
oiie 1 oiie z oiie J oiie 4 oiie J oiie o
Oribatida 49 1 5 27 0 1
Acaridida 2 35 5 7 6 1
Actinedida /O 1i1z1i JJ 1 J/ z3/
Anystidae 3/ 1 3-> / '7
A A A
Baelliaae z 14 0
Camerobiidae 10 4 4 0 4 17
Cunaxidae 0 1 7 0 0
Raphignathidae 3-> 21 1 0 1 0
Smarididae 2 1 0 0 0 0
1arsonemidae 0 0 0 128 0 0
A A A
Tenuipalpidae 3 U u u 11 u
Tetranychidae z J111 13 4/I A
Tydeidae 11 J 1 11 ZUo
Unidentified 0 0 0 0 34 0
Countedbutnot classified 0 235 90 841 92 364
Gamasida 238 94 58 29 52 379
Ameroseiidae 0* 0* 0 4 0 0*
Ascidae 1* 1* 38 2 0 0*
Laelapidae 0* 8* 7 0 0 0*
Phytoseiidae 83* 17* 13 23 51 81*
Unidentified 1* 1* 0 0 1 3*
* Results for a subsample ofmites taken fromthe Gamasida total.
byanunidentifiedweevil thatappearstobe associatedwiththe flowering andseedingmullein
stalk also during the growing season. For the remaining taxa, spiders and ants were fairly
abundant (both exceeding five specimens per 5-plant sample) inthe leafrosettes. Over90%
ofthe ants were collected at a single site.
Knowntree fruitpests overwintering inmulleinincludedpearpsylla, spidermites, westem
flowerthrips, Lygus hesperus Knight, andLygus elisus VanDuzee (Table 3). Westem flower
thrips was especiallyabundant inthe stalks, where densities reached 1000 thripsper 5- plant
sample. Lygus spp. hada densityofover 10bugsper5-plantsample overwintering inthe leaf
rosettes (Table 3). Beneficial arthropods knownto occurinorchards and foundoverwintering
inmullein includedprimarilyphytoseiid mites and minute pirate bugs (Table 3), with much
lower numbers ofa few other species. Ph3^oseiidae included species of Typhlodromus, G.
occidentalism andunidentifiedAmblyseius. Minutepiratebugswere common, havingadensity
ofalmost 15 bugs per 5-plant sample.
Taxonomic similarity (basedupon insect families) between sites showed no relationship
with distancebetween sites (Fig.l).
DISCUSSION
A large and diverse community ofarthropods usedboththe leaf rosettes and stalks of
commonmulleinas overwinteringhabitat. The communities were dominatednumericallyby
Thysanoptera (leaves and stalks) andAcari (leaves), butothertaxa including Heteroptera and
Coleoptera were also relativelycommon. Atcertain sites, insects andmites overwintering in
mulleineasilyexceeded a densityof1000 arthropods perplant, numbers considerablylarger
84 J.ENTOMOL.SOC.BRIT.COLUMBIA100,DECEMBER2003
Table3
Mean densities (averaged over 11 or 6 [Acari] sites) ofarthropods found overwintering in
mulleinthatare alsoknownpestornaturalenemyinhabitantsoforchards. Densitiesexpressed
as numbersper 5 mulleinplants. Mites not identifiedforstalksamples.
Leaf
rosettes Stalks
ORCHARD PESTS
Acari Tetranychidae 25.8
Tetranychus (urticaegroup^) 19.0
Thysanoptera Frankliniella occidentalis^ 280.0 733.4
Homoptera Cacopsyllapyricola 3.5 3.6
Heteroptera Lygus spp. 10.5 0.6
ORCHARD BENEFICIALS
Acari Phytoseiidae 124.0
Galendromus occidentalis^ 106.5
Typhlodromus spp.^ 11.3
Amblyseius spp.^ 6.2
Heteroptera Orius tristicolor 7.2 7.7
Deraeocoris brevis (Uhler) 0.1 0.2
Geocoris spp. 0.6 0.0
Coleoptera Stethoruspicipes Casey 0.0
0^^2
' Mean density estimated by extrapolating from subsamples (Table 2); spider mites were
identifiedusing keys inBaker and Tuttle 1994.
^ Mean density estimatedby extrapolating fromsubsamples.
^ Mean densityestimatedby extrapolating fromsubsamples ofGamasida (Table 2).
thanthose reportedbyMcAtee (1924) inMaryland (who appears to have ignoredAcari and
Thysanoptera inhisbriefstudy). Itis ofinterestthataplantnative to Eurasiawouldhost such
substantial numbers ofphytophagous arthropods inNorthAmerica. However, several ofthe
most common species that we collected are cosmopolitan or Holarctic in distribution,
including westem flower thrips and mullein thrips, and it is likely that these species have
geographic ranges in Europe and Asia that overlap the native range ofcommon mullein.
Indeed, themulleinthrips isknovmto specialize onspeciesofVerbascum inEurope andNorth
America(Bailey 1939). Baileyrecordsthisthrips as overwinteringbothinthe leafrosette and
in the seed capsules or stalkmaterial of V. thapsus, as shown alsohere.
The westem flower thrips was very abundant overwintering in mullein (Table 3),
suggesting that this plantmaybe an important source offlowerthrips moving into orchards
during early spring. Westem flower thrips is a major source ofearly season damage on
nectarines and certain apple varieties in the Pacific Northwest (Madsen and Jack 1966;
Bradley and Mayer 1994; Pearsall and Myers 2000). Pearsall and Myers (2000, 2001)
concluded that location ofnectarine orchards in relation to wild habitats strongly affected
early-season densities ofthrips in orchards, with those orchards adjacent to other orchards
(rather than adjacent to non-orchard habitats) having the lowest spring densities ofthjips.
These authors suggestedthatcertainherbaceous and shmbbyplant species innative habitats
of the Pacific Northwest are a source of flower thrips that colonize nectarine orchards.
Pearsall and Myers (2000) did not include common mullein in their list ofimportant plant
species, butresults reportedhere indicate thatcommonmullein should also be consideredto
be an important source ofwestem flower thrips in tree fmit growing regions ofthe Pacific
Northwest.
Other potential pests oftree fmits, including pear psylla, Lygus spp., and spider mites,
J.ENTOMOL.SOC.BRIT.COLUMBIA100,DECEMBER2003 85
werepresent inmulleinbutatconsiderably lowerdensities than flowerthrips. Pearpsylla is
knownto overwinterinavarietyoflocations outside ofthepearorchard (Kaloostian 1970).
Lygus spp. are sporadic pests in pear, apple, and stone fruit crops (Beers et al. 1993).
Commonmulleinhasbeenreportedtobe ahostplantofLygus lineolaris (PalisotdeBeauvois)
ineasternNorth-America (Young 1986), butitisnotclearwhetherthe two speciesrecovered
inthepresentstudy{L. hesperus andL. elisus)usemulleinasmorethanoverwinteringhabitat.
McAtee (1924) recorded that Lygus sp. overwintered on common mullein leafrosettes in
Maryland. Spider mites are often abundant on broad-leaf plants within and adjacent to
orchards, andtheseplants appeartobe sources ofpestmitesmoving into fruittreesduringthe
summer(Flexneretal 1991; Alston 1994; Cohetal. 1994). Ourresults indicatethatcommon
mullein growing near orchards in the Pacific Northwest may be a source ofoverwintered
spidermites thatcould eventuallydisperse into fruittrees.
Several importantpredator species also overwintered on commonmullein, including the
highlyabundant O. tristicolorandpredatorymites. Orius tristicoloris animportantpredator
ofthrips, mites, andothersmall soft-bodiedprey(Askari and Stem 1972; Salas-Aguilar and
Ehler 1977). This and other species of Orius are known sources ofbiological control in
orchards (Westigard etal. 1968; Niemczyk 1978; McCaffreyandHorsburgh 1986), and our
results suggestthatmullein couldbe a source ofearly springpopulations ofO. tristicolorin
orchards. McAtee (1924) collected Orius insidiosus (Say) overwintering oncommonmullein
inMaryland.
Predatory mites were abundant in mullein, and included three genera {Galendromus,
Typhlodromus, Amblyseius) that are common in apple and pear orchards ofNorth America
(McGroarty and Croft 1978; Croft et al. 1990; Horton et al. 2002), where they provide
biologicalcontrolofpestmites (Hoyt 1969). As withspidermites inorchards, predatorymites
maycolonize fruittrees fromherbaceous orshrubbyvegetationwithinoradjacentto orchards
(McGroartyand Croft 1978; Johnson andCroft 1981; Alston 1994). Thus, the present study
suggests that commonmullein could act as a fairly important source ofbeneficial mites that
provide biological control ofpest mites inPacific Northwest orchards.
There was substantial variation among sites in densities ofarthropods overwintering on
mullein(range: 272 arthropodsperplantto 1986 arthropodsperplant). Anyofseveral factors
could have contributed to this variation, including host quality, types and amounts of
insecticides used innearbyorchards (e.g., Thistlewood etal. 1990), and local environmental
ormicroenvironmental conditions. We hypothesized atthe beginning ofthis study that sites
close to one another geographically would tend to have taxonomically similar communities
comparedto sites geographicallyseparated. Therewasno supportforthis hypothesis (Fig. 1),
possiblydue to the factthattwo taxa (Thripidae andPhlaeothripidae) were almost invariably
the most numerically dominant taxa at all sites, and comprised more than 80% of all
arthropods collected.
CONCLUSIONS
Boththe leafrosettes and stalks ofcommonmulleinprovided overwintering habitat to a
large and taxonomically diverse collection of phytophagous and predatory arthropods.
Because thisplant species commonly occurs in disturbed habitats adjacent to tree fruit
orchards inthe Pacific Northwest, it maybe an important source ofbothpest and beneficial
arthropods colonizing orchards. It is not possible to speculate on whether growers benefit
fromhavinglarge stands ofmulleingrowingneartheirorchards, as thepotentialbenefitsmust
bejudged relative to the possibleharmcausedbypests whichoverwinter onthe plant oruse
itas ahostduring summer. The neteffectinanorchardofbeing adjacentto stands ofmullein
woulddepend, ataminimum, onthe numbers ofpestandbeneficial arthropods overwintering
inthe stand (which appears to be highlyvariable among stands), as well as each species'
86 J.ENTOMOL.SOC.BRIT.COLUMBIA100,DECEMBER2003
0 10 20 30 40 50
Distance between sites (km)
Figure 1. Scatter plot showing relationship ofrelative absolute distance (i.e., taxonomic
similarity) andgeographic distance forallpossiblepairings ofsites. Smallervalues ofrelative
absolute distance indicate increasing taxonomic similaritybetweensites. Analysis limitedto
Insecta at family level.
tendencyto disperse into orchards. Untilwebetterunderstand factors affecting overwintering
densities ofspecific pestandbeneficial arthropods, andtheirpost-overwintering movements,
it is impossible to predict whethermullein is beneficial ordetrimental forgrowers.
ACKNOWLEDGEMENTS
WethankMerileeBayer, Deb Broers, IvanCampos, DanHallauer, andToniHinojosa for
field and laboratoryassistance. We are also verygrateful to SteveNakahara for assistance in
identifying our samples ofthrips. We thank GaryReed for loaning us his Berlese funnels.
The comments ofRick Redak and Gene Miliczky on an earlier draft ofthis manuscript are
appreciated. ThisresearchwaspartiallysupportedbytheInitiative forFutureAgriculture and
Food Systems (USDA-CSREES-IFAFS; award number 00-52103-9657), and by funds
obtained from the Washington State Tree Fruit Research Commission and the Winter Pear
Control Committee.
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