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Seasonal Fluctuations in Rainfall, Food and Abundance of Tropical InsectsAuthor(s): Henk WoldaSource: Journal of Animal Ecology, Vol. 47, No. 2 (Jun., 1978), pp. 369-381Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/3789 .

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Journalof Animal Ecology (1978), 47, 369-381

SEASONAL FLUCTUATIONS IN RAINFALL, FOOD ANDABUNDANCE OF TROPICAL INSECTS

BY HENK WOLDASmithsonian TropicalResearchInstitute, P.O. Box 2072 Balboa, CanalZone

SUMMARY

(1) Light trapdata, coveringover 3 years,arediscussedfor some groupsof insectsin thelowland tropical monsoon forest on Barro Colorado Island. The groups concerned areHomoptera, Tettigoniidae and Mantidae.

(2) The data arecompared with information on rainfallandleaf production. It is shown

that, especially for those groups of Homoptera that have nymphs feeding on the foliage,there is a good relation between leaf production and the demography of the speciesconcerned.

(3) In all cases the data are discussed in general terms although data per species areavailable.

(4) In some cases anomalies in the rainfall pattern, such as an inch or so of rain in earlyMarch, the mid dry season, has a strong positive effect on the populations throughincreasing leaf production, in other cases it may actually have a negative effect.

INTRODUCTION

Several studies have provided evidence showing that tropical insects undergo seasonalchanges in abundance, at least for those parts of the tropics where wet and dry seasonsalternate (Davis 1945; Dobzhansky & Pavan 1950; Owen 1969; Emmel & Leck 1970;Gibbs & Leston 1970;Robinson & Robinson 1970;Fogden 1972;Owen & Chanter 1972;Janzen 1973a, b; Ricklefs 1975;Frith 1975;Janzen & Pond 1975;Bigger 1976;Buskirk&Buskirk 1976;Wolda 1977a)although not all authors agree (Elton 1975). In areas with apronounced dryseason, such as most of Panama, theabundanceof most insects in thedryseason is relatively low. In areas with a very mild dry season, such as Sarawak, insectsdecrease in abundance during the drier season and increase during the wetter season(Fogden 1972). Some

spidersin New Guinea show the same seasonal

pattern (Robinson,Lubin & Robinson 1974). On the other hand, Janzen (1973a, b) finds for an area with asimilarlymild dry season in Costa Rica, that a sweepnet sample of insects taken in the dryseason contains more insects than sucha sample from thewet season, at least in secondaryvegetation.

It seems likely that the seasonal presence of insect species is synchronized with aseasonalpresenceof its food, if food availability variesseasonally. For foliage feedersthismeans that they should usually be present when new leaves are produced. The apparentabundance of foliage in a forest does not necessarily imply an abundance of food forherbivores, as already pointed out by Voute (1957). Plants have evolved defensemechanisms against herbivorywhile herbivoreshave evolved means to cope with some of

these defenses. As a result of this coevolution, mature leaves often have toxins, are toughand, the best defense of all, have low contents of usable nutrients.Young leaves in many

0021-8790/78/0600-0369$02.00 ? 1978 Blackwell Scientific Publications

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Rainfall, food and tropicalinsects

cases lack such effectivedefenses.Their toxin levels arelower, theyareless tough and have

a higher nutrientcontent than mature leaves (Feeny 1970). This is also truefor speciesof

trees on Barro Colorado Island, the area to be discussed in the presentpaper (C. Milton,

personal communication). Fogden (1972)finds that the seasonal

cycleof

caterpillarsand

orthopteroid insects is well correlated with that of leaf production, He also finds that

parasiticHymenoptera increase during the caterpillar season. Janzen & Schoener (1968)find in the dry season that differencesbetween areasin the abundance of insects correlate

with differencesin the production of leaves and shoots.

During the season when most leaves aremature, the insects may depend entirelyon the

few new leaves available (Feeny 1970; Rockwood 1974) and one might expect slight

changes in leaf production, especially duringthe off-season, to have a major effect on the

demography of insect populations. The data to be discussed in the present paper supportthis hypothesis.

PROCEDURESThe data on insect abundance to be discussed were collected by means of two lighttrapsinside the forest on Barro Colorado Island (BCI), Panama Canal Zone. One trap was in

thecanopy, the other some 2 m above groundlevel. They were set up by N. Smythe as partof the Smithsonian Environmental Science Program (ESP) and have been in operationsince June 1973,each producingone sample of insects perday. As I have discussed before

(Wolda 1977b), there are good reasons to believe that, at least for many species, the

catches in thelighttraps do providereliable information on realchanges in population size

for that part of that population that flies, i.e. the adults.

The insects to be discussed here are mostly Homoptera, all of them sapsuckers, but

variable in where they get the sap from, especially in the nymphal stages. Othergroups tobe mentioned arekatydids (Tettigoniidae, mostly leaf eaters)and mantids (predators). In

all thesecases the data are available per species, but in thepresentpaperI will concentrate

on higher taxa. As a resultof variable weatherconditions, the numberscaught varyfrom

day to day and because of the depressingeffect of the full moon (cf. Wolda 1977a)thereis

a variation with a period of about 4 weeks which reflects changes in trappabilityof the

insects rather than real changes in the populations. For these reasons the data are

presentedper period of 4 weeks, of which there are thirteen per year.Rainfall data are from tables produced by the Meteorological and Hydrographical

Branch of the Panama Canal Company, which operates a rain gauge on BCI, and from

unpublished data of Smythe and Windsor.Leaf flush data are available sinceAugust 1974.The sampling programmewas setup byR. Foster, as part of the ESP, and the actual sampling was done by Bonifacio de Leon.

Because of trees dying and others being added to the programme, the actual number of

trees included in the census varied somewhat. At the moment there are 372 individual

trees being sampled everyweek, excluding the palms, belonging to 182species.This is not

a random sample since common species are underrepresentedwhile rare species are

overrepresented. Nevertheless, it is felt that the data do give a fairly good impression of

leaf flush in the forest in general. Leaf flush was scored in each tree as 'complete', i.e. the

entire tree was producing new leaves, as 'partial'with new leaves on only some branches,or as 'absent'. The presence of many lianas and epiphytes makes scoring often very

difficult and the 'partial' category especiallyshould be treatedwith caution. In the presentpaper I will only be using the number of trees scored as in complete flush, expressedas a

percentageof all trees sampled.

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An impression f theagingof leavescan be obtainedby comparingheproductionofnew leaveswith itterfall.Thedata on litterfallwerealso obtained hrough he ESP.

RESULTS

Beforediscussing nyresults,a wordof caution s in order.The conclusions o be givenarebasedon correlations etweenvariables bservednthe forest or a periodof almost4

yearsor, as is thecasefor leafflush,for not even 3 years.It will be shown that therearedifferences etween heseyearsand anattemptwill be made oexplain uchdifferencesnthe insectsby referringo corresponding ifferencesn othervariables.Sucha procedurecanbe rather rickywithso few datapoints.However,as willbeshown, hecorrelations obediscusseddo makesense.Theconclusions obegivenshouldbe considered ypothesesto be testedwiththe data to be collected n the nextfewyears.

Rainfall, leaf production

The relevantdataarepresentedn Fig. 1. Almostall of the annualrainfall allsfrom

May to Decemberwithonly occasionalshowersduring he dryseason.Good mid-dryseasonshowersoccurredn week10,in earlyMarch,both in 1974and 1975.No rainto

speakof fellduring he 1976dryseasonand several oodshowersoccurred uring hedryseasonof 1977.The 1976rainyseason startedearly,but is the drieston record.It alsoendedearly(in week47, lateNovember)and thefollowingdryseason asteduntilafterweek19(midMay).

Leafproduction eems o berather trongly elatedwith rainfall.The'normal'picturefor a forestsuchasthaton BCIseems o be a major eaf flushat thebeginning f therainy

30 -

E

2-

ca

-J 0

60-

20-V2- MW ,

1973 1974 1975 1976 1977

FIG. 1. Rainfall, leaf fall and leaf production in the forest on Barro Colorado Island.

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Rainfall, food and tropical insects

season in May-June, with very little production during the later part of the rainy seasonand during the dry season. This is the general impression obtained in previous years and

very similar to what Frankie, Baker & Opler (1974) report for a climatologically similar,

although slightly drier, site in Costa Rica. Old leaves fall mainly during the dry season,

especially in Januaryand February.As can be seen in Fig. 1,apart from themajor peak inleaf flush in May-June, there are minor peaks in October and in January.

The actual picture deviates in some important aspects from this generalized 'normal'

year. The rain at week 10in 1974 was followed by many treesand shrubsproducinga flushof new leaves. Unfortunately I have only casual notes about this as the samplingprogramme had not startedyet. The mid dry season rain of 1975and the firstrains of the

rainy seasons of 1975 and 1976 were followed by an increase in leaf production. Thisincreasestartsless than a week after therainand reaches apeak some 2 to 3 weeks after therain. It is interesting to see that the rains of Januaryand February 1977 did not have an

appreciableeffect on leaf production. It seemsthata completely dry period of some length

is requiredfor a good shower to have any effect.It is conceivable that the subjective appreciation of what constitutes a 'complete' and

what a 'partial' leaf flush changes with time. One might think that the large difference

between 1975 and 1976 could be explained by a change in scoring and would thus be an

artifact. I do not think this is so, at least it cannot be the entireexplanation. First of all, thesame observer collected all the data and he is an experiencedwoodsman. Secondly, even if

one takescomplete and partial flushestogether, 1976 does not even reach half the level of

1975. In fact, the partialand complete flushes scored in 1976combined reach a consider-

ably lower level than the complete scores alone in 1975. Thirdly, the leaves produced in

1976 fell in the littertrapsin early 1977 and litterfall then was quite a bit less than duringthe

dryseason of 1976.

Undoubtedly,the difference between the two

years is,at least

partly, real.The minor peaks in leaf flush in October 1974 and January 1975 are much higher than

the corresponding peaks in the other years, for some reason. Production in late August-

early September 1974 was extremely low. Leaf production then went up sharply, corre-

lated with, and may be as a resultof, extensive rains after a drierperiod.The next, narrow,

peak in leaf flush in week 47 (late November) also coincides with major rains after some

weeks with little rain.

It thus seems that relatively minor variations in rainfall can cause quite importantvariations in leaf production and, therefore, in food for herbivores. In so far as the

availability of food for a particular herbivore is correlated with the presence and/or

quantity of young leaves in general, one would predict that the numbers would increasesome 2-3 weeks after the mid dry season rains in 1974, i.e. there should be many more in

period 4 than in the previous threeperiods. One would also expect such insects to be moreabundant during the late wet season of 1974 and the dry season of 1975, than they are

during the similarperiods in other years. The expected effect of therainsinmid dryseasonin 1975is difficult to formulate. Ifabove a certain minimum level of leaf flush the numbersare no longer governed by the actual amount of new leaves present,which does not seem

unlikely, the rains in March 1975 may not have any effect at all.These expectations hold especially for those specieswhich depend both as nymphs and

adults on new leaves for food. For many fulgoroids this is not true.For these, such as the

Derbidae, where the nymphs feed on fungi in rotting wood, the effect of the rain may bequite different. These species seem to be near-absentin the dry season. Those species thatuse rainas a clue to emergefrom diapauseor to startreproducing, might find the situation

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in Marchquite unsuitablefor reproductionas thereis little good food for thenymphs. For

such species the rains in the dry season might have a negative effect on their abundance.

Homoptera

For severalmajor groups within the Homoptera, the number of individualscaught perperiodof 4 weeks is plotted in Figs 2 and 3. Some relevant statisticsaregiven in Tables 1,2

and 3. These are the mean number of individuals per week for the first part of the dryseason (Table 1), for the end of the dry season (Table 2) and for the early rainy season

(Table 3). The statistics are, in a sense, questionable. In order to get sufficient degreesof

freedom to make the standard errorsmeaningful, I had to take mean numbers per week

rather than per 4 week period. This means that the variances not only include random

error, but also systematic errors such as effects of the lunar cycle. Any real changes in

numbers during the period in question also show as an increased variance, making the

demonstration of significant differences between means more difficult, however, it also

means that one may confidently accept differencesthat turn out to be significantstatisti-cally in spite of this.

TABLE. Meannumberof individuals erweek for the first12weeksof eachyear,withstandardrror,orsomegroupsofHomoptera aughtnthe ight rapson Barro

Colorado sland

1974 1975 1976 1977Cicadoidea 1 33 + 196 1 42+ 0-53 3.50+ 1 79 1 25+0.43Membracoidea 2.00+0-39 * 483+1 04 * 1.83+0-34 1 50+0-67

Cercopoidea 0.50+0 15 * 183 +052 2.42+ 093 0-92+0.23

Psylloidea 7.50+ 196 * 64-42+26-29 * 3.17+0.88 1 58?0.58Cicadellidae 1175+ 2.06 ** 4542+ 8.84 ** 13-67+181 ** 4-33? 128

Delphacidae 2-08+058 3.67+1.51 3.17+ 161 0.58+0.23Fulgoroidea- Delph.) 23-67+5.64 *** 9308+ 10-84 ** 5025+ 593 ** 20-17+ 835Achilidae 1142+ 2-65 ** 2608 + 377 ** 12.- +209 6.17+ 2.72Cixiidae 4.83+148 *** 1650+2-25 15-25+1.83 *** 1.50+0.72Derbidae 2-50+085 2-83+ 032 5.25+ 187 * 0-50+023

Tropiduchidae 0 ** 1.42+ 048 0.83+ 0.30 0.50+ 029Flatidae 3.33+ 120 *** 32-33+5 55 ** 13-58+240 8.58+365

Dictyopharidae 0-42+0-15 *** 6.58+159 * 2.17+0.47 1-58+0-92

Signifcant ifferences etween uccessive earsare ndicated y *(P<0-05), **(P<0-01) or ***

(P<0 001).

Before discussing the statistics I will, briefly, mention some salient points of Figs 2 and

3. The cicadas are highly seasonal. There are three common dry season species, all ofwhich are rare in the traps. Of the nine rainy season species some are found for the first

time in April, others in May. Some disappear in August, others stay on until October.

After that no cicadas are caught, or heard, until the beginning of the dry season. The

nymphs are subterraneanand could, conceivably, be affected by oddities in the rainfall

patterneither through changes in soil moisture or throughchanges in activity of the plantroots on which they feed.

The thirteenspeciesof spittlebugs (Cercopoidea) thatwerecaught, four of themin largeor very large numbers, are all rainy season species with only some individuals extendinginto the dry season. They usually do not appear in the trap until early June, well after the

rains have startedin early May. Severalof these species, especially the common ones, havesubterraneannymphs.

The Psyllids have two peaks in abundance, one late in the rainy season and one early in

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400-

a)-o

-o 200-

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600-

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1973 1974 1975 1976 1977

FIG.2. Seasonal distribution, in numberof individualsper4 weeks, of four groups of Homopterain a set of two light traps on BCI.

1000-

0 0

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z-a0

1000'0 oLa)

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1973 1974 1975 1976 1977FIG. 3. Seasonal distribution, in number of individuals per4 weeks of threegroups of Homopterain a set of two light traps in BCI.

374

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thedryseason.Of thenineteen pecies cored, ourpeak nthedryseason.Thenymphsof

psyllidsaremostlyout on thefoliageor in gallson thefoliageandonemightexpect hemto be rather ensitive o changes n theavailability f newleaves.

Thereareseveralpecies

of membracidsnvolved,

all of whichshouldbe classifiedas

rainyseasonspecies,althoughsome also occurduringpartof thedryseason.They,andtheirnymphs, eed on leavesand on new shoots and onemightexpect hem o respond o

changes n the seasonalpatternof productionof new leavesand shoots.Thedelphacidswereexcluded rom thetotalfulgoroidsbecausemost of thespecies ly

veryerratically. he occurrence f macropterousndividuals f somedelphacid pecies nthe trap s almostcomparableo that of alatesof antscaughtduring heirmating light.On a particular ay,or on a few successivedays, manymaybecaught,whileduring henextfew weeksonemaynotget anyat all.However, heyareallrainyseasonspecies,witha majorpeakat thebeginningof therainyseason(Fig. 3).

Therestof thefulgoroidsandthecicadellids rerelativelyowinabundance uring he

dryseasonand reach heirmaximum roundJulyandAugust.Nymphsof cicadellids reusuallyout on thefoliage,butpossiblymainlyon shrubsandherbs,plants hatwerenotincludednthecensus or leafproduction.Fulgoroidnymphshavevarious eedinghabits,whichI willmentionbelow.

It is expected hat,in 1974,the numberof individualsmightbehigher n weeks 13-16

(period4) than in weeks 1-12 (periods1-3), becauseof the effects of the rainsthatoccurredduringearlyMarchof thatyear.This is confirmedby the analysis or several

groupsof Homoptera Table2). For membracids, icadellids, ulgoroidsand eventhe

delphacids,hemeannumber f individuals erweek ncreasedignificantlyfter herain,in spite of the dry weatherduringthat period,presumablybecause of the increased

availability undocumented)f food.Inthecercopidshechange sadecrease, ut Idoubtthat this small differencehas anyrealmeaning, n spiteof the statistics.In thecicadastherewas anoverseven-foldncrease,but thiswas not significant.Theincrease sentirelyduetoa newspeciesof Taphura hichappearedarlier n 1974 han notheryears.On the

psyllidsthe rain and correspondingeaf flush had no effect, presumablybecauseitoccurred owards heendof theirseasonalpeak.

TABLE2. Meannumberof individualsperweek,withstandard rror, or thefourthperiod,weeks13-16,of eachyear, or somegroupsofHomoptera aught

inthelight trapson BarroColorado sland

1974 1975 1976

Cicadoidea 8.33+ 4.54 2.50+ 0.96 0.50+0.20Membracoidea 12.75+ 1.97*** 8.75+ 1.70 6.75 +2.56

Cercopoidea 0 ** 0 0.25 +0.25

Psylloidea 7.75+ 2-93 4.50+ 1-89* 4.50+1.19Cicadellidae 44.0 + 8-22** 21 75+ 3-77* 14-75+4-59Delphacidae 5-25+ 1.11* 0.50+ 0.50 3-50+ 104

Fulgoroidea (-Delph.) 84-25+ 2135* 8275 + 21-05 50-25+ 686Achilidae 54.0 +11.29** 34-75+ 9.66 9.50+1.04Cixiidae 20-50+ 4.57** 27-25+11-26 16.0 +2-31Derbidae 3.50+ 1.66 1.75+ 0.85 0-50+0.50*

Tropiduchidae 0.50+ 0.29 0.50+ 0.50 1.0 +0-41Flatidae 25-50+ 9.99* 25-25+ 5.81 20.0 +4-56

Dictyopharidae 3.0 + 2.04 4.75+ 1.80 2.25+0 85

Significant differences with the previous 12 weeks (Table 1) are indicated after eachnumber with *(P<0-05), **(P<001) or ***(P<0001).

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376

8C

6C

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2C

200

IiIIIIIIIIIIIIIIIIIIIIIII . I I I! ' I I

1973 1974 1975 1976 1977

FIG.4. Seasonal distribution, in number of individuals per 4 weeks, of threefamilies of Fulgoroi-dea in a set of two light traps on BCI.

a)C)-0

I-

~0

ii

20-

0

C) 100-

0

i 0a

i3 0

J-r9....r^ T]_^ T rF

1973 1974 1975 1976 1977

FIG.5. Seasonal distribution, in number of individuals per 4 weeks, of three families of Fulgoroi-dea in a set of two light traps on BCI.

~~~~~~~~~~~~~~~~~ . r. . . . . n

t II

. . . . . . . I . . . . . . ..-

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For the families within thefulgoroids the resultsvary. The increase is significantonly intheAchilidae, Cixiidaeand Flatidae (Figs 4 and 5). Most, if not all, speciesof derbids have

nymphs that feed on fungi in rottingwood and one would not expect them to respond to aflush of new leaves or a

temporarymoisteningof

logs. Manyachilids and cixiids

mayhave

similar feeding habits although some nymphs may be out on the foliage. In actual fact,

only some species showed a clearcut relation with the rains in early March 1974. In theCixiidae it is only two common species that show this response, viz. Pintalia erecta Metc.and Muirolonia metallica Fowl. Among the Achilidae the response is only clear inmembers of the genus Catonia and a few others. In most species of the families that dohave their nymphs feeding on young leaves, viz. Tropiduchidae, Flatidae and Dictyo-pharidae, the response is there. It is highly significantin the Flatidae, and it is a six-fold,albeit nonsignificant, increase in the Dictyopharidae. The Tropiduchidae are not com-mon enough in the trap to expect any significant results.

TABLE. Meannumberof individuals erweek,withstandard rror, or periods5through9 (weeks17to 36),for eachyear,for somegroupsof Homoptera aught n

thelighttrapson BarroColorado sland

1974 1975 1976

Cicadoidea 53.0 + 1097 31-65+ 9.63 26-25+ 6.08Membracoidea 13-85+ 2.43 13-95+ 1.92 19-70+ 3.52Cercopoidea 14-20+ 4.46 28-70+ 7.84 23-85+ 6.65Psylloidea 3.40+ 0.74 3.45+ 0.93 7.50+ 3.33Cicadellidea 140-95 27.0 201 79+ 17-33 141-25 24-92Delphacidae 99-30+ 57-80 85-42+ 22-50 45-35+ 20-87Fulgoroidea -Delphac.) 15690+ 24-61** 322-10+4203 25310+ 36-08

Achilidae 60-05+ 9.93* 116-50+17-86 81 20 + 1195Cixiidae 39-75+ 8.57* 10560+ 23-16 77-50+ 11-86Derbidae 7.0 + 1.73* 14-45+ 2.45 29-05+ 5-40Tropiduchidae 0.65+ 0.23** 2.25+ 0.46* 1.0 + 0.41Flatidae 36-75+ 5.81 48-80+ 5.85 42-40+ 6-57Dictyopharidae 7.55+ 1.57 14-45+ 3.46 14-05+ 3.05

Significant ifferencesetween onsecutive earsare ndicated y *(P< 005) or **(P<001).

It is also expected, because of the relatively high level of leaf production, that thenumber of individuals during the early part of the dry season should be much higher in1975 than in the other years. As shown in Table 1, the numbersare significantlyhigher in

1975 than in 1974 during the first three periods. It is not found in cicadas, in the erraticdelphacidsand in the derbids,whose nymphs do not feed on young leaves. In the achilidsand cixiids only some species, the same as the ones mentioned above, are responsibleforthe relationship shown. In the cercopids the increase is significant, but I doubt that it has

anything to do with the leaf flush. In the threefamilies with nymphs on the foliage (Fig. 5),the increase is spectacular. In fact, the mean number of individuals per week during thefirst 12 weeks of 1975 was about the same as duringthe maximum in the preceedingrainyseason (Table 3). It seems that the increased production of new leaves was sufficient to

prevent a decrease in abundance during the dry season, whatever the weather. In early1976 the numbers are lower again in many groups, as one would expect with the lowerlevel of leaf production then. However, thereare a few notes of caution here, showing that

one should not expect one environmental factor to determinechanges in numbers all byitself,without any help from other factors. For one thing, thedecreasedid not occur in theCixiidae. Partly this is due to other species being involved, but this is certainly not the

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entire story. The numberspresent during some partof the yearareprobably a function ofthe circumstances during that period and of the numberspresentat the beginning of that

period. During the peak of the rainy season in 1975 (Table 3) the number of cixiids wasmuch higher than in 1974 and a spill-over of individuals into the dry season may have

caused the higher numbers during early 1976. This may also be the explanation for the

relatively high levels during that period of flatids and dictyopharids.The rainduringearly March 1975 in no instance had a significanteffect on the numbers.

The only two significant cases are decreases(Table 2), in psyllids, indicating the decreaseat the end of their season, and in cicadellids.

It is conceivable that species which, in a given year, can start building up their

population at a relatively early date, reach higher levels of abundance later in the yearthan they do in other years.One might expect, on this basis, that in 1974 numbers of those

groupswhich responded positively to theearly Marchrains, to be higherthan in 1973. For

early 1973 there are no insectdata, but weather data suggestno reason why insects should

h ve had an earlystart then, as they had in 1974.Here only theperiodof week 23 to 36 canbe compared, as data for period 17 to 36 (Table 3) are not available for 1973. The onlysignificant differences found are in the Flatidae (from 17-5+ 2-6 in 1973 to 30-5+ 4-5 in

1974; P<0-05) and in the Derbidae (from 291 +6-5 to 12.2+3-3; P<005). In the

Dictopharidae there was also an increase, from 6-4 to 8-2 per week) but this was not

statistically significant. In 1975 conditions, again, were favourable early in the year andnumbers increasedagain, significantly so in severalgroups (Table 3). In the Dictyophari-dae the increasewas not significant, but the level reachedin 1975 was significantly higherthan that in 1973. It thus seems that, at least sometimes, an early start of the season mayhave a favourable effect on the numbers reached later on.

The Derbidae merit specialdiscussion.They decreased from 1973 to 1974and increasedafter that. As explainedabove, they are not expected to respond to the production of newleaves. However, they may respond to rain by emerging from theirhiding places in orderto reproduce. If dry weather reproduction is unsuccessful, mid dry season rains couldhave a negative effect on the numbers. The data support this hypothesis. From 1973 to1974 numbers decreased significantly(see above), presumablybecause of the early Marchrains in 1974. In 1975 those rainsagain occurred,but were less intense, and may not havehad such a strong effect, and numbers increased somewhat. In 1976 dry season rains didnot occur and the number of derbids reached a high level, about the same as that in 1973.

Othergroups of insects

Data are available, or are becoming available, for a number of other groups of insects.Of these I will only brieflydiscuss here the Tettigoniidae (katydids), as representativesofleaf eating herbivores,andmantids as a group of predators.The data areplotted in Fig. 6.

Among the species of katydids caught only two are predatory and represented onlywith a total of four individuals.All the others are, probably, mostly folivorous. As Fig. 6

shows, the effect of the mid dry season rains, if any, was negative. An explanation will

have to wait until one knows more about the biology of the species involved. One might

expect a positive response to increased leaf production during certain times of the year,but this was clearly not the case.

Mantids are predatorsand theirrelationship with leaf production, if any, is expecte-'to

be only indirect through increasedabundance of theirprey. I do not know enough abouttheirdiet to be able to measuredirectly the abundance of theirpreyitems. However, dataon general flying insect abundance are available from a set of three traps at a location

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called he'weir',at some200mfrom thetrapsdiscussednthispaper Wolda1977a).Theweirtrapsproduce hetotaldryweightof insectssmaller han 5 mmand the numbers finsects arger han 5 mm.The attercategorymay nclude hemorecommonprey temsofthemantids.Theseasonof the

mantidsapparentlysfromperiod5to 10, .e. fromweek17to 40. For thisperiod he dataon generalnsectabundance,.e. insects lyingatnightand

capturedn thetraps, sgiven n Table4, togetherwiththe totalnumber fmantids aughtduring hatperiod.Thesedatasuggest hattheabundance f mantids orrelateswiththeabundanceof their prey. However,the suggestedcorrelation,which may very well

represent realphenomenon, ouldbespurious.Thereareonlythreedatapointsand themeasureused forpreyabundances, at best,not very good.

400-

o 200-

-I

- 200-

1973 1974 1975 1976 1977

FIG.6. Seasonal distribution, in numberof individuals per 4 weeks, of two groups of Orthopterain a set of two light traps on BCI.

TABLE4. Relationbetween eneralnsectabundance, smeasured y lighttraps,andtheabundance f Mantoidea,alsomeasuredby lighttraps,during heperiod rom

week17to week40,whenmantidshad theirpeak nabundance

Dryweightof insects Numberof insects NumberofYear smaller han 5 mm(g) larger han5 mm mantids

1974 16-98 3647 3381975 16-04 2368 1821976 39-01 5244 422

DISCUSSION

It has beenshownthat somegroupsof insectsmaybe ratherstrictlycontrolled n their

seasonalityand theirabundanceby theavailability f food in a tropical orest,and thateven smallaberrationsn the rainfallpatternmayhavea strongeffecton theavailabilityof food, its seasonalityand quantity.It has becomeobvious that the generalquestionaboutseasonal hangesntheabundance f insects na tropical oresthasto berephrasedandmademoreprecise.Questionsaboutdistributionn theforest,kindsof insectsanddifferencesbetweenyearshave to be clarified.Data from only one yearcan be verymisleading, s is readilydemonstratedor some nsectgroupsasinFig. 5.Taking heyear

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from June 1973 to June 1974,one finds a pronounced decreaseinnumbersthroughout the

dry season. Taking theyearfrom June 1974 to June 1975,one concludes that these insectsare virtually non-seasonal in their abundance. Comparing the dry season with the rainyseason by taking one sample in each, can be very

misleading.A sample in early

Aprilcan

have more insects than those from October if the April sample is collected after a mid dryseason rain and the October sample is late in the rainy season with little or no new leaves

produced. The changes during a season are as important as differences between seasons.Janzen (1973b) suggests that sapsucking insects may be more sensitive to changes in

availability of young leaves and shoots than folivores. The present data show that here

too one has to be more precise. For some sapsuckers he is right, for others he is not.Whetherall folivores are as the tettigoniids discussed in the present paper I do not knowuntil data on such other groups become available.

ACKNOWLEDGMENTSThanks are due to all those who contributed in collecting data for the EnvironmentalScienceProgramof the Smithsonian Institution, or who helped setting it up, suchas Nick

Smythe, Robin Foster, Don Windsor and, especially, since he did most of the actual

collecting of data on leaf production and litterfall, and since he operated thelight traps, toBonifacio de Leon. I gratefully acknowledge the help in sorting the insects given by

Miguel Estribi and Saturnino Martinez.

REFERENCES

Bigger, M. (1976). Oscillations of tropical insect populations. Nature, London,259, 207-9.Buskirk, R. E. & Buskirk, W. H. (1976). Changes in arthropod abundance in a highland Costa Rican forest.American MidlandNaturalist, 95, 288-98.

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Ecological Monographs,15, 243-95.

Dobzhansky,T. &Pavan,C. (1950). Local and seasonal variations in relativefrequenciesof species of Drosophilain Brazil. Journalof Animal Ecology, 19, 1-14.

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caterpillars. Ecology, 51, 565-81

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Gibbs, D. G. & Leston, D. (1970). Insect phenology in a forest cocoa-farm locality in West Africa. JournalofAppliedEcology, 7, 519-48.

Janzen, D. H. (1973a). Sweep samples of tropical foliage insects: Description of study sites, with data on speciesabundances and size distributions. Ecology, 54, 659-86.

Janzen, D. H. (1973b). Sweep samples of tropical foliage insects: Effects of seasons, vegetation types, elevation,time of day and insularity. Ecology, 54, 687-708.

Janzen, D. H. & Pond, C. M. (1975). A comparison, by sweep sampling, of the arthropod fauna of secondaryvegetation in Michigan, England and Costa Rica. Transactionsof the Royal Entomological Society ofLondon,127, 33-50.

Janzen,D. H. &Schoener,Th. W. (1968). Differences in insect abundance and diversitybetween wetterand driersites during a tropical dry season. Ecology, 49, 96-110.

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(Araneae: Araneidae) in Panama: a year's census data. Zoological Journal LinneanSociety, 49, 345-57.Robinson, M. H., Lubin, Y. D. & Robinson B. (1974). Phenology, natural history and species diversity of

web-building spiders on three transects at Wau, New Guinea. Pacific Insects, 16, 117-63.

Rockwood, L. L. (1974). Seasonal changes in the susceptibility of Crescentia alata leaves to the flea beetle

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(Received22 March 1977)

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