Effects of male to female ratio and number of females per nesting tunnel on sex ratio and number of progeny of the alfalfa leafcutter bee Megachile rotundata (Hymenoptera: Megachilidae)

4 Pages·1994·2.1 MB·English

by  D F Mayer

J.Entomol.Soc.Brit.Columbia91,December. 1994 43
Effects ofmale to female ratio and number of
females per nesting tunnel on sex ratio and number of
progeny ofthe alfalfa leafcutter beeMegachile rotundata
(Hymenoptera Megachilidae)
:
D.F.MAYER
DEPARTMENTOFENTOMOLOGY,WASHINGTONSTATEUNIVERSITY,lAREC
PROSSER,WA99350
ABSTRACT
Thispaperreportstheresultsoftwo3-yearstudiesontheeffectsof: 1)theratioofmalestofe-
malesintheparentalgenerationonpercentfemaleprogeny;and2)theratiooffemalestonest-
ing tunnels on percent female progeny. In study 1): there were no significant differences be-
tween treatments inpercent females intheprogeny ortotal numberofnestingcellsproduced
eachyear. The averagepercentagesoffemales were 22% in 1990, 27% in 1991, and 28% in
1992. Instudy 2): therewere significantly fewerfemaleprogeny with2 femalespertunnel in
1991,and inallyears, significantlyfewerprogeny whenthefemaleswerecrowded.Thetotal
numberofcellsproducedwasinverselyrelatedtocrowdingwithratiosoffemalespertunnelof
1:2, 1:1 and2:1.
Keywords:alfalfaleafcutterbee,Megachilerotundata,alfalfaseed
INTRODUCTION
Thealfalfaleafcutterbee,Megachilerotundata(R),istheprimarypollinatorofcommercial
alfalfaseed in the Pacific Northwestofthe United States and southwestern Canada (Richards
1984;Mayeretal. 1990).AlfalfaseedgrowersintheU.S.oftenpurchaseleafcutterbeesbythe
gallonfromproducersinCanada.Agallonofbeescontainsabout 10,000cells,althoughtheper-
cent females in a gallon may vary considerably. Consumers (alfalfa seed growers) prefer the
highestpossiblepercentfemalesbecausefemales, notmales,pollinatethe seedcrop.
VariablesexratioshavebeenreportedforM. rotundata(Osgood 1964,StephenandOsgood
1965, Waters 1969, Maki and Moffett 1986). Basal cells in the nesting tunnel tend tocontain
femalebees,whereasapicalcellscontainmales (Rothschild 1979).
Some studies show that tunnel diameter affects sex ratios (Stephen and Osgood 1965). In
wider tunnels the percentage offemales produced is greater than in narrower tunnels of the
samedepth. Klostermeyeretal. (1973) showedthatthe percentoffemales increasedprogres-
mm
sively in4, 4.8 and6.2 diameterholes. However, Rothschild (1979) foundnodifferences
mm
inpercentfemalesusing6.35 and7.14 diameterholes.
Gerberand Klostermeyer(1970) showed sex determinationtobe avoluntary actforM. ro-
tundataconcluding thatthe stimulus which induces the bee tolay fertilized, orfemale eggs is
associated with tunnel depth. LaterGerberand Klostermeyer(1972) showed that altering the
depthatwhichcellswere completedchanged theprobability thatacell wouldreceive afertil-
ized egg. Depths of4, 8 and 16 cm showed a significant association with sex ratios although
therewasnone witha 12cm tunnel.Rothschild (1979) alsofound noassociationbetweensex
and tunnel with an 11.7cm tunnel. StephenandOsgood (1965) found nocorrelationbetween
sexratioandtunneldepthsof5.0,7.5and 15.0cm.JayandMohr(1987)foundsexratios(male
tofemales) from 1.6:1 to3:1 despite the use oftunnels ofstandard length and diameter. They
also concluded that there were no significant increases in females produced in replacement
nests. Tepedinoetal. (1994)inaone-yearstudy showed that intertunnel distancehadasignif-
icanteffectonthepercentageoffemaleprogeny.
TepedinoandParker(1988)foundthatthesexratioofemergentsecond-generationbeeswas
stronglybiasedtowardfemalesandthesexratioofdiapausingbeeswasstronglybiasedtoward
males: only 35.5% ofsecond generationbees were males compared with 61.7% ofthose that
44 J.Entomol.Soc.Brit.Columbia91,December, 1994
entereddiapause.ParkerandTepedinohadreportedlimitedevidenceofalternationofsexratio
in 1982.
Gosek et al. (1988) reported percentages offemales in different breeding lines from 0 to
87%,withameanof48%; inwildpopulationsitwas45%.Anover-abundanceofmalesledto
asmallnumberoffemalesinthenextgeneration(one-quartertoone-fifthofnormal).Thehigh-
estproportionoffemales (50% abovenormal) wasobtainedwhenthe parental generationhad
onlyalownumberofmales.Theyconcludedthattheoptimalnumberofmalesinisolatedmat-
ingcagesisabout 15%ofthepopulation.
GerberandKlostermeyer(1972)andRothschild(1979)concludedthatfemalesactasthough
they know beforehand the numberofcells they will provisionpertunnel. They could 'know'
thenumberofeggsthey willfertilize.
Ireporthereresultsof2 studies: 1) theeffectoftheratioofmalestofemales intheparental
generationonpercentfemale progeny; and 2) the effectofthe ratiooffemalestonestingtun-
nelsonpercentfemaleprogeny.
MATERIALS AND METHODS
Each year, new nestblocks were prepared by taping laminate wood pieces (1cm x 13cm x
12cm) togetherwith strapping tape toform small blocks with 104nesting tunnels, andcover-
ingthebackofeachblockwithaluminumfoil.Tunnelswere5mmindiameterand 12cmdeep.
One nestblockwasplaced ineachcage.
Eachyear,loosebeecells wereobtainedfromMrPollinationServices inCanadaduringthe
winterand storedat 3° Cforabout36weeks. Inthe spring, thecellswereremovedfrom stor-
ageandincubatedat28-29°C. Adultsthatemergedafterabout 19-21 dayswereallowedtofly
Table1
Effectoftheratioofmaletofemale leafcutterbeesonpercentfemaleprogenyandtotalcellsproduced.
Prosser,WA.
1990 1991 1992
% % %
Males:Females Females Total Females Total Females Total
6:1 24a 684a 28a 576a 32a 589a
3:1 19a 683a 24a 664a 30a 661a
2:1 26a 726a 31a 660a 29a 694a
1:5 18a 606a 29a 867a 21a 588a
Means within acolumn followed by the same letterare not significantly different at the/7=0.05 level,
Newman-Keulsstudentizedrangetest.
Table2
Effectoftheratiooffemaleleafcutterbeestotunnelsonpercentfemaleprogenyandtotalcellsproduced.
Prosser,WA.
1990 1991 1992
Females:Tunnels %Females Total %Females Total %Females Total
1:2 18a 566a 33a 421a 30a 505a
1:1 23a 496a 30a 303b 34a 483a
2:1 22a 181b 19b 300b 28a 286b
Means within a column followed by the same letterare not significantly different at thep=0.05 level,
Newman-Keulsstudentizedrangetest.
.
J.Entomol.Soc.Brit.Columbia91,December, 1994 45
inthe laboratory and malesand females were countedandcollected intoseparate vialsoffthe
windows.Theadultswerethenreleasedintothecagescontainingbloomingalfalfaandthenest
blocks.
Forthemale-to-femaleratiostudies, 16cages(6x6x 1.8m)andforthefemales-per-tunnel
studies, 12 similar cages were erected over different plots ofblooming alfalfa each year at
Prosser,WA.Formale-to-femaleratiostudies,80femaleswereputineachcageandthenmales
addedtoobtaina6:1 ratioin 4cages,3:1 in4cages,2:1 in4cages,and 1:5 in4cages.Forfe-
male-per-tunnelstudies,52femaleswereputin4cages(1 female:2tunnels), 104femaleswere
putin4cages(1 female:1 tunnel)and208females in4cages(2females:1 tunnel). Maleswere
putinthecagesatthe sametime and inequal numbertofemales(1:1). In 1990,bees were put
inthecageson30July; in 1991 on5July; andin 1992 15July.
Thebeesforaged andconstructedcells inthe nestblocksduringeach season. Atthe endof
thenestingseason(August)thebeecellswereextractedfromthelaminateboards,countedand
putintocoldstorageat3°C.Afterachillingperiodofabout36weeks,thecellswereincubated
at28-29°C,theadultsrearedoutand sexedtodeterminesexratios.
Thedatawereanalyzedasarandomizedcompleteblockdesignbyanalysisofvariance,with
Newman-Keulsstudentizedrangetestformeanseparations(Lund 1989).
RESULTS ANDDISCUSSION
MaletoFemaleRatio
Therewerenosignificantdifferencesbetweentreatmentsinthepercentfemalesintheprog-
enyinanyyear(Table 1),andnosignificantdifferencesbetweentreatmentsinthetotalnumber
ofcellsproducedeachyear.There werenosignificantdifferences inthemeanpercentfemales
indifferentyears(22% in 1990,27% in 1991, and28% in 1992).
IncontrasttoGoseketal. (1988),Ifoundthattheratioofmales tofemaleshadnoeffecton
percentfemalesintheprogeny.Thisdifferencecouldconceivablybeattributedtodifferenttest
conditions,methods,orevendifferencesbetweenracesofthealfalfaleafcutterbee.
FemalesperTbnnel
In 1991, there was a significantly lowerpercentage offemale progeny produced with 2 fe-
malespertunnel(Table2).Inall3years,thereweresignificandyfewerprogenyproducedwhen
thebees werecrowded (2:1 ratio) than whenuncrowded (1:2ratio). The total numberofcells
producedwasinverselyrelatedtocrowdingwithratiosoffemalespertunnelof1:2, 1:1 and2:1
Large-scaleresearch isneededtoconfirmtheseeffects inthe field.
CONCLUSION
Thepercentageoffemaleprogeny inanypopulationofalfalfaleafcutterbeesappearstode-
pend on several, interrelated, factors. It was not affected by different ratios ofmale to female
parentsorthenumberoffemalesreleasedintolargecageswithafixednumberofwoodentun-
nels. Larger numbers offemales made significantly fewer cells. The results were consistent
over three years except that in one year the percentage offemales was reduced in the most
crowdedcondition.
ACKNOWLEDGMENTS
IthanktheWashingtonAlfalfaSeedCommissionandtheUSDA/ARSforpartialfundingof
thisresearchandJ.D.LundenandM.R.Jassofortheirhelp.
46 J.Entomol.Soc.Brit.Columbia91,December, 1994
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