CEC Research | https://doi.org/10.21973/N3166Z Fall 2020 1/8

FactorsaffectingacornpredationandinfestationinthreeCaliforniaoaks

NicoletteBalmaceda1,JovanaDurovic2,SierraMontes1,KylerPlouffe3

1UniversityofCalifornia,Merced;2UniversityofCalifornia,Riverside;3UniversityofCalifornia,SantaBarbara.

Oak woodlands are in decline due to failed oak recruitment in California. Oakrecruitmentisheavilyimpactedbyvertebrateherbivoryandinsectinfestationofacorns.Oakshavedevelopedchemicaldefensessuchastanninstoprotectacornsfromattack.Studies have explored infestation differences in red oaks but have not comparedinfestation between red oaks and white oaks. We explored how Cydia latiferreana(filbertworm) infestation, Curculio occidentalis (filbert weevil) infestation, and Sciurusgriseusnigripes(westerngraysquirrel)seedpredationdifferedamongtwowhiteoaks,Quercusdouglasii(blueoak),Q.lobata(valleyoak),andaredoak,Q.agrifolia(liveoak)in a Central California oak woodland. We collected 1352 acorns and recorded thepresence and abundance of both boring insects. Additionally, we performed anexperiment to determinewhich acorns squirrels preferred to eat.We found that liveoak acorns had the highest presence of boring insects. Filbertworms were morecommonlyfoundinliveoak,whilefilbertweevilswerefoundequallyinallthreespecies.In live oaks when filbertworms were present, weevils were less likely to be present.Insectsoverall infested largeracornswithineachspecies.Squirrelspreferredblueandvalley acorns over live oak acorns. Our findings can provide insight into how thepreferences of herbivorous vertebratesmay be impacting oak recruitment as well asindirectlyinfluencingacorninfestinginsects.Keywords: insect infestation, filbert weevils, filbertworms, acorn predation, oakrecruitment

INTRODUCTION

Oak woodlands are failing to regeneratethroughout California (Kueppers et al.2005). Regeneration is contingent on oakrecruitment, theprocessofacorndispersaland germination into saplings. Issues thatcontribute to failed oak recruitment areintensified competition, fire suppression,increasedclimaticvariability,andherbivory(Kueppers et al. 2005, MacDougal et al.2010). Acorn germination is a critical yet

vulnerable step of oak recruitment. Thereare immense losses of acorns prior togermination due to many different factorssuch as burning in wildfires, insectinfestation, and consumption (Olson andBoyce1971).Vertebrate herbivory has been shown to

contribute to mortality of acorn crops(CrawleyandLong1995).Itwasfoundthattreeswithlowacorncropssufferedahigherpercent mortality than trees with largeacorn crops. Oaks have several seed

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predatorsandhavedevelopedmechanismstodefendthemselvessuchasseedmasting.Seedmasting is when oaks produce largeracorncropsthantheirannualaverage.Mastyearsoccurindifferentcyclesdependingonthe oak species (Sork 1993). It ishypothesized that masting is used tooverwhelm vertebrate predators with anabundance of food, allowing some acornstoescapeconsumptionandgerminate.Inadditiontomasting,oakshaveevolved

varying tannin concentrations to deteracorn predation (Smallwood et al. 2015).Higher tannin concentrations have beenshown to decrease acorn digestibility formany acorn predators such as squirrels.High tannin concentrations have beenshown to cause negative physiologicaleffects on vertebrate consumers, such asinjurytogastrointestinaltractsandliverandkidney failure (Shimada and Saitoh 2003).Although high tannin concentrationsnegatively affect acorn predators, theybenefit oaks by allowing late-seasongermination success (Wood 2005).However, acorns that last longer may bemoresubjectedtoinsectinfestation(KoenigandFaeth1998).Oaks also exhibit defense mechanisms

against insect infestations. Weevilinfestations have been shown to decreasegermination of acorns by 60% (Lombardoand McCarthy 2009). Some oaks canproduce larger seeds, alter theirgermination schedules, and vary theirtanninconcentrationstoallowgerminationdespite infestation (Xiao et al. 2007).Although tannins are generally used todiscourage infestations, the high tanninacorns may offer protection againstherbivores. It has been found that insectswillliveonfruitsthatdiscourageherbivoresto avoid accidental consumption by fruit

herbivores (Herrera 1984). Despite hightannin contents, weevils have been foundto infest different species of high tannincontentoaksathighrates(Xiaoetal.2007).This suggests theymay choosehigh tanninacorns to avoid consumption by oakherbivores. However, studies have notcomparedweevil infestation between hightanninandlowtannincontentoaks.IntheoakwoodlandsofcentralCalifornia

Quercus douglasii (blue oak), Q.lobata(valley oak), andQ. agrifolia (live oak) co-occur.Blueandvalleyoakaremembersofthewhite oak clade; and live oak is in theredoakclade.Redoakshavehighertanninconcentrations within their acorns thanwhite oak species (Pyare et al. 1993). Inaddition to having differing tanninconcentrations, the acorns of these oaksalso have varying nutritional profiles(Koenig and Benedict 2002). The live oakacornshavehigherlipidcontentsthanblueandvalleyoakacorns.ThetwoinsectsthatcommonlyinfesttheseoaksaretheCurculiooccidentalis (filbert weevil), hereafterreferred to as weevil, and the Cydialatiferreana (filbertworm). In June toSeptember, the weevils infest acorns byovipositing eggs beneath the shell ofacorns. The weevils hatch from the eggsanddevelopforaboutamonth,consumingtheacornmeatandproducingfrass(Plumb1979). When the acorn falls, the weevilschewholes in the acorn shell and emerge.Theythenburrowundergroundwheretheydevelop into adult weevils. In June toOctober, the filbertworms deposit theireggsonthesurfaceoftheacornshells.Thelarvaehatchineighttotendaysandburrowinto the acorn where they consume theacornmeat and produce silk covered frassfor two to four weeks. The filbertwormsemerge when the acorns fall and pupate

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underground. Weevils and filbertwormsmaybefoundco-occurringwithinthesameacornofeachofourfocaloakspecies.In this study we explored how insect

acorn infestation proportions differedamong oak species and acorn sizes.Specifically, we tested if weevils andfilbertworm abundance varied among oakspecies and acorn sizes. We hypothesizedlive oaks would overall have the highestpresence of both insect species, blue oakwould have intermediate insect presence,and valley would have the lowest insectpresence. We reasoned this would occurbecauseliveoakhasthehighesttanninandlipid content, blue oak has intermediatetanninandlipidcontent,andvalleyoakhasthe lowest tannin and lipid content.Furthermore, we predicted that westerngray squirrels (Sciurus griseus nigripes)wouldprefervalleyandblueoakacorns tolive oak acorns because live oak acornshavehighertannincontent.Weadditionallypredicted that the proportion of insectinfestationwouldbehigherinlargeracornsbecause there would be more availablefood resources. Finally, we predicted thatcohabitation between weevils andfilbertwormswouldbemorecommoninthelargerblueandvalleyacornsbecausetherewouldbemorefoodandspaceavailable.

METHODS

2.1StudySite

This study was conducted at HastingsNatural History Reservation, a 2,500-acreUC Natural Reserve in Monterey County,California. The climate during summersreaches temperatures over 38 degreesCelsius, while the winters have occasionalsnowfall. The reserve primarily consists of

south-facing slopes that are covered inchaparral,oakwoodlands,andsavanna.Alldata was collected during the week ofNovember5–11,2020.

2.2ObservationalStudy

Wecollectedatotalof1352acornsfromthreedifferentoakspecies(Quercuslobata,Q. agrifolia, Q. douglasii). We sampled 15treesforeachoakspecies,totaling45trees.Acorns were sampled by haphazardlychoosingacorn-bearingtreesofeachofthethree oak species. We then randomlycollected 30 acorns from the groundbeneatheachtree.We recorded the length and width of

eachacornandnotedthenumberofinsectholes visible on the acorn shell. Usingshears and tweezers, we dissected eachacorn to examine evidence of insectpresence. Insect presence was defined asthe presence of frass or physical insectlarvae.Ifphysicalinsectlarvaewereabsent,we accounted for the fact that they mayhave already emerged by counting thepresence of frass as insect presence. Ifwesawlargerfrasswithsilk,weclassifieditasfilbertworm presence. Additionally, if wesaw densely packed frass without silk weclassifieditasweevilpresence.

2.3ExperimentalStudy

Todeterminesquirrelpreferenceforacornsof different oak species, we performed achoice trial experiment. We gathered 36acorns from a blue oak, 36 acorns from avalley oak, and 39 acorns from a live oak.Wechosetwodifferent locationsbasedonproximity to squirrel colonies.We conductedfour trials inwhichwehaphazardlysetoutpilesofacornsofall threeoakspecies.We

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used camera traps to record the order inwhichtheacornsweretaken.

2.4StatisticalAnalysis

For all statistical analyses we used JMPPro14.Weusedwidthasaproxyforsizeinour analyses because length and widthwerecorrelated(N=1350,R2 =0.437,p<0.0001). To test the effects of oak speciesand acorn width on insect presence weusedaχ2test.Toanalyzetheeffectsofoakspecies, acorn width, and weevil presenceonfilbertwormpresenceweusedaχ2test.Additionally,weanalyzedtheeffectsofoakspecies, acorn width, and filbertwormpresenceonweevilpresenceusingaχ2test.To test whether squirrels preferred acornsfrom particular oak species, we used arankedWilcoxontest.

RESULTS

In total, we surveyed 1352 acorns in 45different trees and found 24.5% infestedwith insect parasites (Figure 1.). Theproportion of insect presence was highestinliveoakacorns(Table1;Figure2.).Acrossall oak species, wider acorns had moreinsectspresent (Table1; Figure3.). Inblueandvalleyoaks,weevilsweremorelikelytobepresentwhenfilbertwormswerepresent(Table 1; Figure 4.). In live oaks, weevilswere less likely to be present whenfilbertwormswerepresent(Table1;Fig.4.).

Table 1.Summaryof χ2 statistics for threemodelstesting effects on insect presence, filbertwormpresence, and weevil presence. The first modelincludes theeffectsofoak species andacornwidthon insect presence. The secondmodel includes theeffects of oak species, acorn width, and weevilpresence on filbertwormpresence. The finalmodelincludestheeffectsofoakspecies,acornwidth,andfilbertwormpresence onweevil presence. Asterisksindicatesignificanttreatmenteffects(*P<0.05;**P< 0.001; ***P < 0.0001) (ninsect = 1349, nfilbertworm =1352,nweevil=1352).

Figure 1. Map of trees sampled. Trees weresampled at Hastings Natural History Reservation.Circlediameterrepresentstherelativeproportionofacorns with insect (Curculio occidentalis and Cydialatiferreana)presence.Colorsrepresenttreespecieswhereblueisblueoak(Quercusdouglasii),brownisvalley oak (Q. lobata), and green is live oak (Q.agrifolia). The smallest circle represents 3% of theacorns having insect presence while the largestrepresents75%oftheacornshavinginsectpresence.

75% 3%

Percent Insect Presence

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Filbertwormsoccurred in64%of the331acorns with insect presence. Filbertwormshadthehighestproportionofinfestationinlive oak acorns (Table 1; Figure 2.). Acornwidthhaddifferenteffectsonthepresenceoffilbertwormsacrossoakspecies(Table1;Figure5.).Invalleyoaks,wideracornsweremore likely to have filbertworms (Table 1;Figure5.).Forblueandliveoaks,therewasno effect of acorn width on filbertwormpresence(Table1;Figure5.).Weevils were present in 35% of the 331

acornsthathadinsectpresence.Therewasno difference in weevil presence betweentheoakspecies(Table1;Figure2.).Ineachspeciesofoak,weevilpresencewashigherinacornswithlargerwidth(Table1;Figure6.).Squirrels tended to prefer blue oak

acorns, followedbyvalleyoakacorns,thenliveoakacorns(nblue=36,nvalley=36,nlive=39,χ2=33.50,P<0.0001).

Figure 2. Number of acorns with various insectpresence in different oak species. Overall insectpresence (filbertworms plus weevils) andfilbertwormpresencewerehighestinliveoakacornsat Hastings Natural History Reservation. Weevilpresence did not differ among oak species. Colorsrepresent tree species where blue is blue oak(Quercusdouglasii),brown is valleyoak (Q. lobata),and green is live oak (Q. agrifolia) (ninsect = 1349,nfilbertworm = 1352, nweevil = 1352, d.f.insect = 2,d.f.filbertworm=2,d.f.weevil=2,χ

2insect=77.19,χ

2filbertworm

= 20.93, χ2weevil = 2.11, pinsect < 0.0001, pfilbertworm0.05).

Figure 3. The effect of acorn width on insectpresence in three oak species. Overall insectpresence(weevilsandfilbertworms)occursinwideracorns for all three oak species atHastingsNaturalHistory Reservation. Colors represent the presenceorabsenceofinsectswheregreyisabsentandblackis present. Error bars represent + 1 S.E. (n = 1349,d.f.=2,χ2=14.78,p<0.001).

Figure 4. Effect of filbertworm presence on thepercentageofacornswithweevilpresence. Inblueoaks(Quercusdouglasii)andvalleyoaks(Q.lobata),weevil (Curculio occidentalis) presence is higher inacorns with filbertworm (Cydia latiferreana)presenceatHastingsNaturalHistoryReservation. Inlive oaks (Q. agrifolia), weevil presence is lowerwhenfilbertwormsarepresent.Colorsrepresentthepresence or absence of filbertwormswhere grey isabsentandblack ispresent (n=1352,d.f. =2, χ2=17.52,p<0.001).

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Figure5.Theeffectofacornwidthon filbertwormpresence in three oak species.At Hastings NaturalHistoryReservation,filbertworm(Cydialatiferreana)presence occurs in wider acorns for valley oak(Quercus lobata). Filbertworm presence occurs insmaller acorns for blue oak (Q. douglasii) and liveoak (Q. agrifolia). Colors represent the presence orabsence of filbertworms where grey is absent andblack is present. Error bars represent + 1 S.E. (n =1352,d.f.=2,χ2=9.54,p<0.05).

Figure 6. The effect of acorn width on weevilpresence in three oak species. At Hastings NaturalHistory Reservation, weevil (Curculio occidentalis)presence occurs in wider acorns in all three oakspecies.Greybarsrepresentabsenceofweevilsandblack bars represent weevil presence. Error barsrepresent+1S.E. (n=1352,d.f. =2, χ2=9.17,p<0.05).

DISCUSSION

Insectlarvaeweremostcommonlyfoundin live oak acorns. We believe this isbecause they have the highest tannincontent (Koenig and Faeth 1998). Wepostulate the higher tannin concentrationmay defend the larvae from herbivorouspredatorssuchassquirrels.Squirrelsdonotlike to consume high tannin acorns due tobitterness (Pyare et al. 1993). We believeinsects may have adapted to infest highertannincontentacornstoavoidbeingeatenby herbivores. This is corroborated byHerrera (1984)who found that frugivorousinsectsavoidripefruitsthataremorelikelyto be eaten by herbivores. This idea isfurther supported by our finding thatsquirrels chose to collect valley and blueoak acorns over live oak acorns.Additionally, it has been found that lipidsarecrucialforcornearworm(Heliothiszea)larvaedevelopment(Schiffetal.1988).Webelieve that filbertworms andweevilsmayhavesimilarneedsandthusprefer liveoakacorns because they have higher lipidcontent (Koenig and Faeth 1998). Even iffilbertworms and weevils are deterred byhigh tannins in acorns, larval lipid needsmay counteract the deterrence caused bynegative effects of tannins. However, thiscanonlybeexaminedthroughfurtherstudy.Interestingly, filbertwormswere found in

greaterproportionsinliveoaks,butweevilswere found in almost equal proportionsacross oak species. This pattern may beexplained by the species’ differing lifehistories.Within an acorn growing season,only one generation of weevils areoviposited, while filbertworms havemultiplegenerations(Kooetal.2003).Duetothefilbertwormhavingaquickerlifecycletheygrowand consumeacorns faster, and

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thus they may be outcompeting weevils.Furtherstudiescouldexploreifthispatternis due to temporal variation since there iscontradicting research showing thatweevilpresence is higher than filbertwormpresence(Lewis1991).Overall, the insects we observed were

infesting smaller acorns (live oak) at ahigher rate than larger acorns (blue andvalley oak). However, within each oakspecies the insect abundance is higher inthe larger acorns. This could be becausebiggeracornshavemoreavailable food forlarvae.Webelieve that infestationmaybegreaterinliveoakacornssincetheyprovidethemostprotection fromacornherbivoresbecauseofthehightannincontent(KoenigandFaeth1998).We found complex interactions between

plant defense mechanisms, the indirecteffect of oak herbivores, and insectinfestation. We believe that vertebrateherbivory by squirrels has causedfilbertwormsandweevilstoadapttoinfestacorns with higher tannin contents. Anadditional driver may be that high tanninacorns, like the live oak, have high lipidcontents that are beneficial to larvaldevelopment.Sincewefoundthatliveoakshad a much greater infestation than anyother oak species we examined, theymaybe more at risk of failed oak recruitmentbecause weevil infestations decrease thegermination ability of acorns by 60%(Lombardo and McCarthy 2009). Oaksproducingacornswithhightanninsmaybeputting themselves at risk of increasedinsect infestation. Having a clearerunderstanding of infestation cycles andbeing able to predictwhen andwhat oakswill become infested can allow us toimplement measures to preventinfestations.Ourfindingscanalsohelpwith

understanding how the preferences ofherbivorous vertebrates may be impactingoak recruitment as well as indirectlyinfluencingacorninfestinginsects.

ACKNOWLEDGMENTS

This work was performed at HastingsNatural History Reservation,doi:10.21973/N33Q0G.Manythanks to Jenfor hosting us and providing our researchsite.Wewish to thank our instructors Timand Krikor for their mentorship andguidance. Finally, we would like to thankJohn Parsons, MC and Jake for theiroverwhelmingsupport.

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