Table Of ContentJ.ENTOMOLSOC.BRIT.COLUMBIA,94,DECEMBER, 1997 31
Mortality in eggs of pear psylla (Homoptera: Psyllidae)
caused by fenoxycarb in combination with a water drench
DAVID R. HORTON* and DEBRA A. BROERS
USDA,ARS,5230KONNOWACPASSROAD,WAPATO,WA 98951
ABSTRACT
Applicationsoftheinsectgrowthregulatorfenoxycarb inpearbeforethe appearanceof
foliage kills the eggs ofpear psylla, Cacopsylla pyricola (Foerster), laid on newly
expandedfoliage4weeks afterthe application. Wehypothesizedthatwaterintheform
ofrainfall or overhead irrigation transports the productfrom sprayed wood onto new
foliageinsufficientamountsthateggs onfoliage arekilled. We tested this hypothesis
bycoveringfenoxycarb-sprayedlimbswithwaterproofplastic, removing the covers 4
weekslater, immediatelydrenchinghalfofthelimbswithwater, andthencomparingegg
hatchondrenchedandnon-drenchedlimbs. Wealsomonitoredegghatchonuncovered
limbs. Egg mortality 4 weeks after the application was two-fold higher on limbs
drenchedwithwaterthanoncovered,non-drenchedlimbs (52% vs26%). Mortalityon
limbswithoutfenoxycarbwaslessthan 10%. Eggsdepositedonfenoxycarb-treatedbut
uncoveredlimbsalso showedhigh rates ofmortality (33-44%), whichmayhave been
duetorainfall inmid-Apriltransportingfenoxycarb ontofoliage.
Keywords: pearpsylla, insectgrowthregulators, YakimaWashington
INTRODUCTION
Theinsectgrowthregulatorfenoxycarb iseffective againstpearpsylla,Cacopsyllaj?yricola
(Foerster), animportantpestofpearsinthePacificNorthwestandpartsofEurope(Solomonand
Fitzgerald 1987,McMullen 1990,Higbeeetal. 1995). Currently, itisbeingusedagainstpsylla
(under the trade name 'Comply') during the prebloom period. Horton (1996) showed that an
application offenoxycarb at the delayed dormant stage in pear (bud scalesjust beginning to
spread)reducedthehatchofeggsdepositedonnewlyexpandedfoliage4 weeks afterthespray.
Eggmortalitywas about40% on sprayed trees compared to less than 5% on unsprayed trees
(Horton 1996). Laboratory studies show thateven highly dilute solutions offenoxycarb cause
extensiveeggmortalityinpsylla(McMullen 1990), andonepossibleexplanationfortheegg-kill
onunsprayedfoliage isthatfenoxycarb istransportedfrom sprayed wood to foliage by rainfall
oroverhead irrigation. In this study, we looked at the effects ofwater on egg-kill by covering
fenoxycarb-sprayedlimbswithwater-proofcovers, anddeterminingeggmortalityoncoveredand
uncoveredfoliageeitherwithorwithoutawaterdrench.
MATERIALS AND METHODS
Experimentswereconductedatanexperimental orchardnear Yakima, WA, in spring 1996
and 1997. Fivetreatmentswerecompared: (1)control (nofenoxycarb, limb leftuncovered), (2)
fenoxycarb (limbleftuncovered); (3)control (nofenoxycarb,limbcovered);(4)fenoxycarb(limb
covered); (5)fenoxycarb (limb covered, coverremoved after4 weeks and limb drenched with
water). Fenoxycarb (0.15 g [AI] perliterofwater)wasappliedwithahandsprayeratthedelayed
' Towhom all correspondence shouldbe addressed
32 J.ENTOMOLSOC.BRIT.COLUMBIA,94,DECEMBER, 1997
dormantstage (19Mar 1996; 21 Mar 1997). Control limbswere sprayedwithwater. Inboth
years, 5 limbs per treatment (each at mid-canopy level) were randomly selected from
approximately 20 trees. Treatments were then randomized among limbs. For the most part,
limbswereondifferenttrees;however, afewtreesinbothyearshadtwolimbsselected,inwhich
casetheywere chosenfromopposite sides ofthe tree. Each limb was 0.75-1.0 m in length.
Limbswere sprayedjusttothepointofdrip (50-100ml offenoxycarb orwaterperlimb). For
theuncoveredcontrols(treatment(1)), 10 and 15 fruitingclusterswererandomlyselectedfrom
approximately 10unsprayedtreesin 1996 and 1997,respectively.
Treatedandcontrollimbsthatweretobeprotectedhadaninverted,bowl-shapedcover(50
1 in volume) constructed of 10 mil Mylar plastic. The cover was placed so thatthe limb was
completely protectedfrom precipitation. Holes were punched along the bottom edge ofeach
cover,andtwinewasusedtotiethebottomofthecovertothelimb. Irrigationwasdelayedboth
yearsuntil afterthe study.
Thewaterdrenchwasappliedbyremovingthecover4weeksafterthespray (20April 1996;
21 April 1997) andthendrenchingthelimb withwaterusingahandsprayer(waterapplieduntil
runoff). Limbswerecutfromthetreesimmediately afterthedrenchandtakentothe laboratory
tomonitoregghatch. Limbsfromtheother4treatmentswerealsocollectedatthistime. Control
samplesweredrenchedwithwaterbeforeremovingthemfromthetrees.
Tomonitoregghatch, 3-10bloomclusters (dependinguponeggdensities)weretakenfrom
eachlimb. Thechppedendofeachclusterwasputinwatertokeepthefoliageturgid. Several
egg-ladenleavesoneachclusterweremarkedwithasmallspotoftypewritercorrectionfluid, and
theeggswerecounted. Clusterswerethenkeptat23° C to allowtheeggstohatch. After8-10
days,unhatchedeggswerecounted. The 3-10 flowerclustersfromeachlimbwere assumedto
beasinglesample, andthedatawerepooledfromclustersononelimb. Therefore, samplesizes
were 10 limbsforeachtreatment(5 limbs in 1996 +5 limbs in 1997) exceptfortheuncovered
control, inwhichw = 25 (10 clustersin 1996 + 15 clusters in 1997).
Effectsoftreatmentonthepercentageofeggsfailingtohatchwereestimatedwithatwo-way
(yearxtreatment) analysisofvariance (ANOVA). Percentageswere arcsintransformedbefore
analysis. Non-transformedmeansaregiven. Specifictreatmentcomparisonswereextractedwith
pre-plannedsingledfcontrasts.
RESULTS
Therewere largedifferences amongtreatmentsinpercentageofeggsfailingtohatchinboth
years (Fig. 1). Pattemsweresimilarbetweenyears, asindicatedbynonsignificantyearandyear
X treatment effects (Table 1). Egg mortality was very high in several of the fenoxycarb
treatments,reaching50%inthewaterdrenchtreatmentand33-44% onlimbsthatweresprayed
but left uncovered (Fig. 1). The increase in egg mortality caused by fenoxycarb was highly
significant (Table 1; contrast (1)). Egg mortality was two-fold higher on fenoxycarb-sprayed
branchesthatweredrenchedwithwaterthanonlimbsthatwerecoveredbutnotdrenched(Fig.
1; Table 1: contrast (2); mean [averaged over years] percentage kill = 52.9% [limb covered;
drenched] and26.3% [limb covered; no drench]). Contrast (3) tests whethereggmortality on
fenoxycarb-treatedlimbsdifferedbetweenuncoveredlimbs (i.e., thoseexposedtorainfall) and
covered limbs in the absence ofa water drench. There is a marginally significant indication
(Table 1: contrast(3) [p =0.076]) thateggmortalitywashigherontheuncoveredlimbs (mean
[averaged overyears] percentage egg kill = 38.7%) than on the non-drenched, covered limbs
(meanpercentageeggkill =26.3%).
J.ENTOMOLSOC.BRIT.COLUMBIA,94,DECEMBER, 1997 33
Control Fenoxy. Control Fenoxy. Fenoxy. &
Drench
Figure 1. Mean (±SE) percentage egg mortality on control (no fenoxycarb) and fenoxycarb-
treatedlimbs. Blackbars: noplastic covers;cross-hatchedbars: limbswithplastic covers. See
Table 1 forresultsofANOVA. n= 5 replicatesperbar,exceptforuncoveredcontrolsinwhich
10 (1996) and 15 (1997).
Table 1.
Analysisofvariance andcontrasts summarizingthe effects offenoxycarb and waterdrench on
hatchofeggsdepositedbypearpsylla(meanssummarizedinFigure 1). Dataarcsintransformed
foranalysis.
Source df MS F p>F
Year 1 0.060 1.86 0.18
Treatment 4 0.807 25.23 <0.001
YearxTreatment 4 0.017 0.52 0.72
Error 55 0.032
Contrasts:
(1)Controlversusfenoxycarb' 1 2.69 84.06 <0.001
(2)Coveredlimb: fenoxycarbversus 1 0.43 13.34 <0.001
fenoxycarb+ drench^
(3)Fenoxycarb: covered limb(nodrench) 1 0.10 3.27 0.076
versusuncoveredlimb^
' Contrastcomparesthemeanofthetwocontroltreatments(7.4%mortality)tothemeanofthethree
fenoxycarbtreatments(39.2%)(averagedoveryear).
^Contrastcomparesmiddlecross-hatchedbartocross-hatched baron right(Figure 1:averagedoveryear).
^Contrastcomparesblack baronrighttomiddlecross-hatched bar(Figure I.averaged overyear).
DISCUSSION
By applyingfenoxycarb atthedelayeddormant stageandthen again 4 weeks later, growers
obtain goodcontrol ofprebloompsyllapopulations (Hilton andWestigard 1994; Horton 1996).
34 J.Entomolsoc.Brit.Columbia,94,December, 1997
However,thedoubleapplicationisveryexpensive, andmanygrowersgenerallyeliminateeither
thefirstorsecondspray; questionsremainaboutthebesttimingforasingleapplication (Hilton
and Westigard 1994; Horton 1996). Horton (1996 and unpublished; see also Hilton and
Westigard 1994) suggestedthat a single delayeddormant application, at least in certainyears,
provides a level ofcontrol similar to that oftwo applications. One factor contributing to the
effectivenessofthedelayeddormantspraymustbethehighratesofeggmortalitythatoccurwell
into April (Fig. 1; andHorton 1996), despitethefactthatthese late-spring eggs are deposited
abnostexclusivelyonunsprayedfoliage.
Rates ofeggmortality were two-fold higher on covered, drenched limbs than on covered,
non-drenched limbs (Fig. 1). The most logical explanation for this observation is that water
transportedfenoxycarb from sprayed wood onto unsprayedfoliage; residue analysis wouldbe
necessary to confirm this explanation. There was a weak (p = 0.076) suggestion in the
fenoxycarb treatmentsthateggmortalitywashigheronuncoveredlimbsthanonnon-drenched
coveredlimbs (Table 1: contrast(3)), whichmayindicatethatprecipitation also transportsthe
fenoxycarb. Overhead irrigation couldnothavebeenresponsible for the egg mortality on the
uncoveredlimbs,becausethefirstirrigationinbothyearsoccurredaftercompletionofthe study.
Bothyears had intervals ofmeasurable rainfall between 10 April and the day that limbs were
removed,including0.36 cmofrainthatfellthedaybeforelimbremoval in 1997 (fromNational
ClimaticDataCenterpublicationsforcityofYakima).
Someoftheeggmortalityinthefenoxycarb treatmentscannotbe attributedtothe actionof
water, because eggs that were deposited on covered limbs also showed increased levels of
mortalityeven inthe absence ofa water drench (Fig. 1: compare middle cross-hatched bar to
controls). Eggs laid on unsprayed surfaces by females that previously contacted fenoxycarb-
treatedsurfaceshave lowerhatchrates(Higbeeetal. 1995), andthismightexplainthe20-30%
ratesofeggmortalitythatoccurredonthecovered,non-drenchedlimbs.
hi summary, our results reconfirm the observation that a single application offenoxycarb
before spreadofbudscales causesmortality in eggs ofpearpsyllawell into April (seeHorton
1996). Resultsofthedrenchingexperimentsuggestthatgrowerswhorelyon asingle sprayto
manage psylla may improve control byjudicious use ofoverhead irrigation. Furthermore, a
single spraymightprovetobelesseffective in averydryspringthanin awetter spring, which
mayexplain someofthevariation amongyears incontrol thattheseniorauthorhasnoted.
ACKNOWLEDGMENTS
ThisresearchwassupportedinpartbytheWashingtonTreeFruitResearch Commissionand
the WinterPearControl Committee. Wethank Carrol Calkins andBradHigbeefor comments
onthemanuscript.
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