Late season food level, cannibalism, and oviposition in adult mantids (Orthoptera: Mantidae) : sources of variability in a field experiment

6 Pages·1991·2.8 MB·

by  W F Fagan

PROC. ENTOMOL. SOC. WASH.
93(4), 1991, pp. 956-961
LATE SEASON FOOD LEVEL, CANNIBALISM, AND OVIPOSITION IN
ADULT MANTIDS (ORTHOPTERA: MANTIDAE): SOURCES OF
VARIABILITY IN A FIELD EXPERIMENT
W. F. Pagan and L. E. Hurd'
Ecology Program, School ofLife Sciences, University ofDelaware, Newark, Delaware
19716.
—
Abstract. Feeding experiments in the field with mantids typically have yielded results
in which statistical variation could not be attributed to variability in performance of
specific individuals. Adult female Tenodera aridifolia sinensis Saussure, which were in-
dividually marked and confined to replicated enclosures in the field, were subjected to
two feeding levels and starvation control. All groups lost weight during the two week
experiment; however, this loss was from oviposition in well-fed mantids, from decline in
body mass among those which starved. Amount ofcannibalism was unrelated to feeding
level; however, only cannibals managed to gain body mass in starved and low food level
groups. More and slightly heavier oothecae were produced in the high food level group
than in the other two; however, cannibals oviposited fewer oothecae than non-cannibals.
Variabilityin performanceofindividualswithin andbetweentreatmentgroupsapparently
depended on nutritional historypriorto this study aswell as to experimental feeding level
herein.
Key Words: Mantodea, Mantidae, Tenodera, food limitation, cannibalism, predator fit-
ness, predators, experimental variability
Adult female Tenodera aridifoliasinensis therewasconsiderable variabilityingain or
Saussure (Mantodea: Mantidae) in the loss of body mass within these field-col-
northeastern United States must cope with lected experimental cohorts. The sources for
a diminishing food supply late in the grow- this variability could not readily be ascer-
ing season, during oogenesis (Eisenberg et tained at the time ofthese experiments be-
al. 1981). Food limitation at this critical causeindividual mantidswere not followed
time ofthe season can reduce fitness by de- overtime. Further, sinceboth ofthesestud-
creasingbodymass, which inturndecreases ies were open field experiments, actual lev-
theenergyavailableforeggproduction. The els ofprey availability could not be deter-
extentoffood limitation variesamonghab- mined: field-collectedgroupswerecompared
itats and years, and it can be alleviated if to cohorts fed ad libitum in the laboratory.
the mantid fortuitously is perched upon an Sincemantidscan feedataratewellbeyond
inflorescence which attracts flower foragers the level which produces further gains in
(Hurd 1989). body mass (Hurd 1991), it is probable that
Although the results of the two studies thenumbersofpreyconsumedbytheselab-
cited above support conclusions therein, oratory cohorts were greater than would
translate into enhanced fitness. Thus we
' Correspondence author. cannot infer that mantids from the field
VOLUME 93, NUMBER 4 957
which gained body mass at the same rate as tids under field environmental conditions,
those fed ad libitum in the laboratory nec- and at the same time to control food level
essarily ate the same number ofprey. It is and follow individuals during the course of
also possible that some mantids which the experiment.
gained biomass in the field resorted to can- On 24 September, 24 adult female T. a.
nibalismintheabsence ofalternate prey. In sinensiswere collected and weighed. On 25
the laboratory well-fed mantid nymphs September these mantids were randomly
rarely ate each other, whereas starved placed in the enclosures, one pair to each
nymphs exhibited cannibalism rates in ex- cage. Each enclosurethenwas randomlyas-
cess of 20% (Hurd and Eisenberg 1984). signedtooneofthreetreatmentgroups(four
Therefore it is possible that the tendency pertreatment)whichdifferedinthenumber
toward cannibalism in adults is related to ofprey to be added: 1) H = high food level,
hunger level. in which 10 insect prey were added twice
The present study was designed to relate weekly, 2) L = low food level, in which five
foodlevel,bodymass,oviposition,andcan- insects were added twice weekly, and 3) Z
nibalism in individual adult female T. a. = zero food level, in which no prey were
sinensisinthefieldattheendofthegrowing added. Experimental preylevelswere in the
season. We monitored individual perfor- range suggested by Bartley (1983). Mantids
mance for these variables in order to deter- were color coded with nail polish applied
mine the sources of statistical variability to the dorsal surface of the prothorax, so
which typically have attended experiments that individuals could be identified from
with field-collectedanimalswhoseearly life each pair. Prey were collected from the pas-
history is not known. ture with a sweep net, and represented the
availableinsectbiomasspresentatthattime
Materials and Methods
of year: primarily honey bees with an oc-
The study site was a pasture on the Ex- casional cricket or grasshopper. Prey were
perimental Farm ofthe University ofDel- distributedarbitrarilyamongenclosuresac-
aware, Newark, Delaware. There were res- cording to the above feeding regimes.
identpopulationsofthreespeciesofmantids Each time food was added, all enclosures
in this field: T. a. sinensis, its congener T. first were checked for oothecae, which were
angiistipennis Saussure, and Mantis reli- removedandweighed. Since morethan one
giosa Linnaeus. ootheca was never found in any cage at the
Twelve 1-m^enclosuresweresetupinthe sametime, the deflated condition ofthe ab-
field in September 1990. Enclosures were domen following recent oviposition made
constructed ofLumite Saran netting (12 x iteasy to tell which mantid in each pairwas
12 strands/cm^) fitted on PVC frames with responsible. Mantids were weighed at the
the bottom side left open to theground and end of each week, at which time any can-
vegetation within the cage. Cages were sta- nibalismwhichhadoccurredwasnoted. The
pledtothegroundandtightlyfittedto 7-cm- experiment was terminated on 9 October,
high squares of aluminum sheeting which since prey were by then too scarce to con-
penetrated the soil to a depth of3^ cm to tinue, and the possibility of a killing frost
prevent migration ofarthropods under the was relatively great.
mesh. The top of each enclosure was zip-
Results
pered on three sides to allow access. Prior
to thisexperimenteach enclosure wasemp- Mean body mass declined in all three
tied of arthropods through a combination treatment groups from 25 September to 9
ofD-Vac suction, pittraps, andhandsearch. October (Table 1). Final mass in treatment
These enclosures allowed us to keep man- Z was 25% lower than initial mass; H and
958 PROCEEDINGSOFTHEENTOMOLOGICALSOCIETY OFWASHINGTON
"%
two cannibals, numbers la and 7b (see
change," Table 1). Treatment L produced
mixed results in this regard: only two in-
dividuals, a cannibal (2b) and a non-can-
nibal (3a), gained weight during the first
week, and one cannibal (11a) lost weight.
During the second week 1 la gained weight,
but cannibal 4a lost. There were no canni-
bals duringthe first week in H, and halfthe
25 SEPT 2 OCT 9 OCT
individualsgainedbodymass. Cannibals6a
DATE
and 1Ob made substantial gains the second
Fig. 1. Change in body mass ofadult female Ten- week, while non-cannibal 8b was the only
odera aridifoliasinensis in field enclosures over time.
Figuresrepresentrange(verticallines),mean(horizon- other individual to gain.
tal lines), and 95% confidence limits (boxes) for LSD Mantids in all three treatments ovipos-
ranges. Treatments: Z (unfed) = open boxes; L (low ited, butnofemaleproducedmorethan one
food level) = hatched boxes; H (high food level) = ootheca(Table 1). Fiveoothecaewerefound
shaded boxes. in H, and three each in L and Z. Among
mantidswhich surviveduntil theendofthe
L mantids lost 19% and 16%, respectively, experiment, only one failed to oviposit in
overthe same period. As a result, there was treatment H, and three each in L and Z.
no significantdifferencein mean body mass There was no obvious tendency for any
among treatment groups at the end of the treatment group to oviposit earlier or later
experiment. However, since each ootheca than the others. Ootheca mass was not sta-
produced during the experiment could be tistically different among treatment groups,
attributed to an individual, we could par- although the two heaviest were found in
titionoutthelossofmassduetooviposition treatment H. Cannibals did not generally
from loss ofbody mass due to simple star- produced larger oothecae than non-canni-
vation.Whenweightsofoothecaewereadd- bals, and in fact produced fewer oothecae
ed back to the weights of mantids respon- than non-cannibals in all three treatment
sibleforthem,thetreatmentgroupsdiverged groups.
over time: starved mantids in treatment Z
lost weight, those in H gained, and those in Discussion
L were intermediate (Fig. 1). By the end of A comparison ofFig. 1 with Table 1 sug-
the experiment these differences were sta- gests that although the decline in weight
tistically significant (.Fj u = 4.53, P= 0.03). among mantids in treatment Z was due
There were two cannibals in both treat- mainly to loss ofbody mass through star-
ments H and Z, and three in L (Table 1). vation, weight decline in well fed mantids
Thereforethetendencytowardcannibalism could be attributed primarily to oviposi-
wasnotrelatedtofoodlevel. However,non- tion. The fact that treatments could not be
cannibals lost an average ofapproximately differentiatedsimplyonthebasisofchanges
18% ofinitial body mass (exclusive ofovi- in raw mantid weights means that it would
position loss) in treatment Z over the two be difficult to assess the food quality of a
weeks, 16% in L; cannibals gained < 1% in habitat simply by sampling individuals at
Z and 4% in L. Both cannibals and non- different times while oviposition is occur-
cannibalsgained mass in treatment H: 18% ring. Such comparisonswouldbevalidonly
and 10%, respectively. prior to the onset ofoviposition, as in Ei-
During both weeks ofthe experiment, all senberg et al. (1981) and Hurd (1989).
individuals in Z lost body mass except the Inanycase,ovipositionwasanimportant
VOLUME 93, NUMBER 4 959
Table 1. Live weights(g) o"f%adult female T. a. sinensis, and weights ofoothecae inexperimentalenclosures
atthreedifferentfoodlevels. change"denotesthe percentageofweightchangebetweenconsecutivesample
dateswhichwasnotaccountedforbyoviposition: weightofoothecawasaddedtosubsequentbodyweight,this
sumsubtractedwasfrompreviousbodyweight,thenthedifferencewasdividedbythatpreviousweight(x100%).
Ootheca weights are positioned to indicate time interval during which oviposition occurred. "Mantid no."
denotesrandomlyassignedenclosurenumberandindividual(aorb)ofeachpairassignedtoeachenclosure. C
= death by cannibalism sincelastsampledate.
960 PROCEEDINGS OFTHEENTOMOLOGICAL SOCIETY OFWASHINGTON
Surprisingly, cannibalism was neither re- amongindividualsinnatureundoubtedlyis
lated to food level nor generally beneficial the rule. Therefore, although the average of
in terms ofegg production for mantids in approximately 3 beesprovided/mantid/day
thisexperiment. However,judgingfrom live was sufficient to alleviate food limitation in
H
weight data, it did contribute significantly treatment relative to the other treat-
tothedietsintreatmentsLandZ: cannibals ments, an individual of any pair may ac-
in thesegroups were the only individuals to tuallyhaveeaten0-6bees/day. Evenamong
exhibit substantial gains in body mass both individually caged mantids kept under
weeks of the study. This was also true of identical conditions in the laboratory, feed-
treatment H during the second week, but ing rates can be quite variable (Hurd and
not during the first when there was no can- Rathet 1986).
nibalism. The potential benefit of canni- The remaining variation in performance
balism to starved mantids in treatment Z is amongindividuals in thisexperimentcould
clearly a direct contribution ofbiomass by have been a function of activity level; i.e.
the victim; in the fed groups, cannibalism even among unfed, non-cannibalistic man-
also could have alleviated competition for tidsintreatmentZwhereactualfeedingrates
alternate prey (Hurd and Eisenberg 1990b). were all zero, some individuals may have
Infact, acannibal suchas 2b(Table 1) from lost more body mass than others because
treatmentLeffectivelydoubleditsavailable their activity levels and consequent ener-
prey, elevating its food level to that of an getic demands were higher. This may have
individual in one ofthe pairs in treatment been most important in the starved cohort,
H. This is therefore another source ofvari- since food limitation can induce dispersal
ability in the data, which may reflect an behavior in these mantids, at least as ju-
advantage ofcannibalism forthese mantids veniles (Hurd and Eisenberg 1984).
in nature at the end ofthe growing season Allofthesesourcesofvariabilitynotonly
by reducing competition for increasingly reflect differences in response to environ-
limitedprey. SinceoothecaeofT. a. sinensis mental conditions by individual mantids,
are spatiallyaggregated(Eisenbergand Hurd but they also may be responsible for vari-
1990), it seems likely that adult females en- ationinimpactonthepreycommunity. This
counter each other in nature at this time. could be exhibited in the amount ofdirect
Thechange inbody massotherthan from reduction ofprey, or even in the direction
oviposition (% change. Table 1) was gen- of impact. These predators are capable of
erally more variable among fed mantids in indirect effects such as enhancing survivor-
treatments LandH than inZ, even between ship ofone prey species by interfering with
members of a pair in the same enclosure. the impact ofother predators on that prey
Although in Z the only food was the other species (Hurd and Eisenberg 1990a).
member of the pair of mantids, food pro- Acknowledgments
vided to H and L enclosures probably was
ThisworkwassupportedbyaScienceand
not evenly divided between the occupants
thereof. Forinstance, 9aintreatmentH was Engineering Fellowship to the first author
andbyanHonorsProgram ResearchGrant,
able to produce the second largest ootheca
in the experiment as well as to gain body both from the University ofDelaware. This
massduringthe first week, while 9bdid not is contribution #151 from the Ecology Pro-
gram, University ofDelaware.
oviposit and lost body mass. Thus differ-
ences in actual feeding rates within, as well Literature Cited
as between, cages likely was another im- Bartley,J.A. 1983. Preyselectionandcapturebythe
portant source of variability in this exper- Chinesemantid.Ph.D.Dissertation.Universityof
iment. Uneven apportionment ofresources Delaware, Newark, Delaware, USA.
VOLUME NUMBER
93, 4 961
Eisenberg,R. M.andL.E. Hurd. 1977. Anecological Hurd,L.E.andR. M.Eisenberg. 1984. Experimental
studyoftheemergencecharacteristicsforeggcases density manipulations ofthe predator Tenodera
oftheChinesemanlis(Tenoderaaridifoliasinensis sinensis (Orthoptera: Mantidae) in an old-field
Saussure).AmericanMidlandNaturalist97:478- community. I. Mortality, development and dis-
482. persal ofjuvenile mantids. Journal Animal Ecol-
. 1990. Eggdispersionintwospeciesofpraying ogy 53: 269-281.
mantids (Mantodea: Mantidae). Proceedings En- . 1989a. A mid-summer comparison ofsizes
tomological Society Washington 92: 808-810. and growth rates among nymphs of three sym-
Eisenberg, R. M., L. E. Hurd,andJ.A. Hartley. 1981. patric mantids(Mantodea: Mantidae) in twoold-
Ecologicalconsequencesoffoodlimitationforadult field habitats. Proceedings Entomological Society
mantids {Tenodera sinensis Saussure). American Washington 91: 51-54.
Midland Naturalist 106: 209-218. . 1989b. Temporal distribution of hatching
Hurd, L. E. 1988. Consequences of divergent egg times in three sympatric mantids (Mantodea:
phenology to predation and coexistence in two Mantidae) with implications for niche separation
sympatric,congenericmantids(Orthoptera: Man- and coexistence. Proceedings Entomological So-
tidae). Oecologia 76: 547-550. ciety Washington 91: 55-58.
. 1989. Theimportanceoflate season flowers . 1990a. Arthropod community responses to
to the fitness ofan insect predator, Tenodera si- manipulation ofa bitrophic predatorguild. Ecol-
nensisSaussure(Orthoptera: Mantidae),ananold ogy 71: 2107-2114.
fieldcommunity.Entomologist(London) 108:223- . 1990b. Experimentally synchronized phe-
228. nology and interspecific competition in mantids.
. 1991. Growthefficiencyinjuvenilemantids: American Midland Naturalist 124: 390-394.
Absence ofselection for optimization in a food- Hurd, L. E. and L H. Rathet. 1986. Functional re-
limited environment. Proceedings Entomological sponse and success injuvenile mantids. Ecology
Society Washington (in press). 67: 163-167.