BIOCIDAL SIDA (MALVACEAE)

6 Pages·1991·2.3 MB·Norwegian

by  Charles L Burandt

BIOCIDAL SIDA (MALVACEAE)
CHARLESL. BURANDT,JR.
DepartmentofBiology, TexasA &I University
Kingsvtlle, TX 7H363. U.S.A.
ABSTRACrr
ThesecretionsofglandularrrichomcsoftwoSouthAmericanspeciesofSic/aarchighly
toxictoantsandcockroaches.Innaturalhabitat,thesesecretionsmostlikelyprovideresist-
ancetoherbivorybyinsectorothersmallarthropods.
INIRODUCnON
DuringpreparationofamonographictreatmentofSiciasectionOltgan-
drae(Malvaceae)twospecieswereobservedwithaninterestingvestitureof
stalked glandular trichomes (Fig. 1). One species, Sidajatrophoides L
Heritier, is a xerophyte occurring in scattered populations from coastal
PeruandtheGalapagos.Theother,SidapalmataCavanilles,isamesophyte
occuringalongtrailsidesmnorthernPeruandsouthernEcuador(Burandt
1992). Touchingtheyoungstemsorpetiolesofeitherspecieslefttheskin
coveredwithamoistbut notstickyfilm, evidentlythesecretionofthese
trich(HTies.
Severalobservationscoincidedtosuggestandsupportahypothesisthat
thetrichomesecretionsinthesespeciesmightfunctiontorepelorpossibly
killwalkingarthropods: 1.Thedropletsofsecretionsformedattheeleva-
ted tips ofthe trichomes (Fig. I) would undoubtedly contact an insect
walking on theplants. 2. The trichomes were moredensely distributed
(especially in youngS. jatrophoides) in the inflorescencebranches(Fig. 1)
and inthepetioleareajustbelow theblades. Awalking insectwouldbe
obligedtocrossaveritableminefieldofdropletstogettothepresumably
moredelectableleafbladesandflowers. 3. Plantpopulationsstudiedinthe
heldappearedrelativelyfreeofinsects,andtheleaveswerefreeofevidence
ofinsect herbivory.
Thelethalityofmorphologicallysimilarglandulartrichomestoarthro-
podshasbeenfrequentlyreported(JuniperandSouthwood 1986). While
toxiccompoundsmayalsobepresentmtrichomesecretions(Carterctal.
1989, Walters etal. 1989, Dimock and Kennedy 1983, Gerhold etal.
1984,andWilliamsetal. 1980), theirroleinpestresistanceiscomplica-
tedbythefactthatmosttrichomesecretionsareadhesiveandtheprincipal
modeofactionisbytrapping-larvaeoradultsareimmobilizeduntilthey
SiDA 14(3):4()5~410. 1991.
406
die. Conversely, the trichomesecretionsoiS/c/asect. Ol/i^a^u/rae-drcnon-
adhesiveandanyantibiosesobservedforthethesesecretionswouldlikely
beattributable to theeffectsoftoxinsalone. Thisstudypresentsexperi-
mentalevidencewhichstronglysupportsthehypothesisthatnaturaltoxins
in the secretions in S/cIa section Oi/{^a>u/rae potentially function as
deterrents to arthropod herbivory.
Ml-rilODS
Plantsoi.V. pulnu/taandS.jatrophiiicksweregerminated fromscarified
seeds placed in 7^ to95 mm plastic pots filledwithsand/clay/peatmix.
Seedlings were kept in a growth chamber illuminated with fluorescent
"gro-lights"andsetfora 12hrphotopcriodwith 17°nightand27°Cday
temperatures.Theyweresuppliedwithdcionizcdwaterandcommercially
availablefertilizers. Toencourageflowering,fertilizerswerediscontinued,
soilswereleachedbyexcessprovisionofdeionizedwater, andplantswere
allowedtobecomewater-stressed. Inflorescenceswerewelldevelopedafter
six monthsofgrowth, and trials werebegun.
Toxicity of the secretions ofthese species was assayed using tire ants
{SolaiojisLsnivictaL.)andGermancockroaches{BlattdlagermankuBuren).
Antswerecollectedasneededfromfieldcolonies,whereasimmaturecock-
roaches(5— 10 mm in lengthexcludingantennae)weregathered from a
laboratory-maintained colony. Active specimens of these insects were
subjected to various topical applicationsasdescribed below. After treat-
ment,testandcontrolspecimenswerehousedincovered500mlglasscon-
tainers forobservation.
SmA I'ALAiAiA assay:
Trialno. 1. Fifteenantswereindividuallygatheredwithanartist'ssmall
paintbrush and repeatedly pressed against secretory trichomes of S.
pahthitci. Antswerethenplacedcollectivelyinacontainer.Asacontrol, 15
ants were "jostled" with aclean paintbrush and placed collectively in a
separatecontainer. Mortality was recorded at 21 hrs.
Trial no. 2. Tenantsweresimilarlytreatedbutplacedin 10individual
containers. Asacontrol, 10antswererepeatedlypressedagainsttrichome-
bearingpartsofadriedspecimenof.V.palmataandplacedin 10individual
containers. A second control consisted of 10 otherwise unmolested ants
placed in 10 individual containers. Mortality was recordedat 21 hrs.
Trialno. 3. Tenantswereindividuallygatheredwithapaintbrushand
gently placed on plant parts bearing numerous secretory trichomes. \i
necessary, theywerecoaxedtowalksufficientlytocomeintocontactwith
trichomes. Ants that fell were retrieved and again placed on trichome-
407
1.2!5!(;m.mI.dAibamoevtee:r)b.ranchesotleatyinflorescenceolS.lalrnphdiJcs:below:petioleoi\S,jutrophotdes(ca.
408
bearingareas.Antswerethenplacedin 10individualcontainers.Asacon-
trol, 10antswereplacedindividuallyin 10containersand, usingaclean
paintbrush, swabbed with deionized water. Mortality was recorded at 7
hrs.
Trial no. 4. Twelvecockroacheswereplacedindividually inglasscon-
tainersandanesthetizedwithCO'gas. Secretionswerethencollected by
passinganartist'ssmallpaintbrushoverappropriateplantsurfacesuntil
bristlesweresaturated. Cockroacheswerethen"painted"ontheirventral
sideswiththesecretions.Asacontrol, 12roacheswereplacedindividually
inglasscontainers,anesthetizedwithCO%and"painted"ontheirventral
sides withdeionized water. Mortalitywas recorded at 5 hrs.
SmAjAiliornomiis assay:
Trial no. 1. Twentyants(two combined trials)antswere individually
gathered with a paintbrush and gently placed on plant parts bearing
numeroussecretorytrichomes. Ifnecessary, theywerecoaxedtowalksuf-
ficientlytocomeintocontactwithtrichomes.Antsthatfellwereretrieved
andagainplacedontrichome-bearingareas. Antswerethenplacedin 20
individualcontainers. Asacontrol,20antswereplacedmdividuallyin20
containersand, usingacleanpaintbrush, swabbedwithdeionizedwater.
Mortalitywasrecordedat 8 hrs.
Trialno. 2. Thirteencockroacheswereindividuallyplacedinglasscon-
tainersandanesthetizedwithCO'gas. Secretionswerethencollectedby
passinganartist'ssmallpaintbrushoverappropriateplantsurfacesuntil
bristlesweresaturated. Cockroacheswerethen"painted"ontheirventral
sideswiththesecretions.Asacontrol, 1.^roacheswereplacedindividually
inglasscontainers,anesthetizedwithCO',and"painted"ontheirventral
sideswith deionized water. Mortalitywas recordedat 5 hrs.
Trialno. 3. Sevencockroachesweretreatedasintheprecedingtrialbut
"painted"on theirdorsalsideswiththesecretionsofS.jatrophoides. Asa
control,7roachesweretreatedasabovebut"painted"withdeionizedwater
on theirdorsal sides. Mortality was recordedat 5 hrs.
RF.suI.TS
Insectbkhavior
Initial responses of both roaches and ants to application of plant
secretionswassimilar. Briefepisodicwhole-bodyconvulsionsandtremors
occurredsporadicallywithinthefirstfewminutes. Individuallegsbecame
sporadicallyorcontinuouslyrigidandweredraggedorremainedvariously
skewecitothesideortotherear.Rarely,anappendagewoulddisarticulate.
Effective walking gradually became impossibleand specimens collapsed
409
Tahlk1.LethalityofsecretionsotS.pahnataandS.jalrophotdestoantsandroaches.S=secretions
appHed,C = control.Foreach,thenumberdeadprecedesthenumbertreated(N).
TrialNumber: SI CSC2 CS3CSC4 SCTotal
-------
S.paImata
Ants 9(15) 1(15) 9(10) 2(10) 0(10) 6(10) 0(10) - - 24(35) 3(45)
Roaches 12(12) 0(12) 12(12) 0(12)
i'.jatrophoidei
Ants19(20) 2(10) - - - - - - - 19(20) 2(10)
Roaches13(13) 0(13) 7(7) 0(7) - _ _ - - 20(20) 0(20)
GrandTotals
Ants 43(55) 5(55)
Roaches 32(32) 0(32)
andcouldnotrightthemselves. Inantsandoccasionallyinroaches, these
symptomsappeared tosubsidewithin 30minutesoftreatmentbutlater
returned, however, with increasing dysfunction eventually involving all
appendages. Afewantsappearedtocompletelyrecover.
InsectMortality
Atnaturalconcentrations,secretionsoftheglandulartrichomesofboth
S.pahnataandS.jatrophoideswereverylethal. Roach andantmortality
pertrialandpercontrol(s)aresummarizedintable 1.Of35antsvariously
treatedwithsecretionsoiS.palmata,24diedwhereasonly3of45control
antsdied.All 12roachestreatedwithS,palmatadiedwhereasnoneofthe
12controlroachesdied. Of20antstreatedwithsecretionso(S.jatrophoi-
des, 19diedwhereasonly2of10controlantsdied.All20roachestreated
withsecretionso(S.jatrophoidesdiedwhereasnoneofthe20controlroaches
died.Totalingthetoxicityassaysusingants,43of55ofthosetreatedwith
plantsecretionsdiedwhereasonly5of55controlantsdied. Totalingthe
toxicityassays usingcockroaches, 32 of32 treated roachesdiedwhereas
noneofthe 32 control roaches died.
DISCUSSIONS
Allplantsexhibitpotentialresistancetoherbivory. Deterrentmorpho-
logiesrangefromthesimplemechanicalresistanceprovidedbytoughened
tissuestothehonedinjectiondevicesofthetrichomesofUrticaceae. An
arrayofmoleculardefensesisalsoavailabletoplants.Ordinarysap,exuded
asaresultofwounding, maybemildlyrepellent,orlethaltoxinssuchas
pyrethrinsand nicotinemaybeproduced.
410
Most plant structures and molecules serve several functions, however,
andtheirrolesinherbivoreresistanceisoftensubsidiarytootherfunctions
anddifficulttoestablish. Thedramaticlethalityoftrichomesecretionsof
Sidaisthusmoreremarkablesinceitappearstobeduesolelytotheeffectof
toxins. Thatsuchaspecializationmightevolvefromglandulartrichomes
with broadermethodsofantibiosis isnotsurprisingbutis, nevertheless,
apparently rareorlittlereported. Itshouldbemterestlngtosurveyaddi-
tionaltaxaandassayfortoxicitythosewhichpossesssimilarnon-adhesive
trichomesecretions.
a<::knc)wli;dgi;mi:nts
IthankJohnJMellenandCindyGallowayforusefulsuggestionsduring
thecourseofthisstudy,HughLieckforphotography,andCarolAltmanfor
preparation ofthe manuscript.
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