PertanikaJ. Trap. Agric. Sci. 16(2): 81-86(1993) ISS : 0126-6128 Universiti Pertanian Malaysia Press

The Relationship between Population Fluctuations of Helopeltis theivoraWaterhouse, Availability of Cocoa Pods and Rainfall Pattern

RITA M HAMAD' and CHU G GAIT FEE2IDepartment of Plant Protection, Faculty of Agriculture

43400 UPM, Serdang, Selangor Darul Ehsan, Malaysia.2Ebor Research,Locked Bag No. 7202, Batu Tiga,

40706 Shah Alam, Selangor Darul Ehsan, Malaysia

Keywords: Population fluctuations, Helopeltis theivora, cocoa pods, rainfall

ABSTRAK

Kajian mengenai faktorfaktor yang mempengaruhi Jluktuasi populasi danpada Helopeltis theivora yangberhubungan dengan sumber makanan dan curah hujan telah dilakukan. Hasil kajian menunjukkan bahawaJluktuasi populasi dipengaruhi oleh curah hujan dan jumlah buah-buah koko yang ada dimana terdapatkorelasi positif antara mereka. Hasil kajian juga menunjukkan bahawa pengurangan yield yang nyata didapatibersamaan dengan kenaikan populasi mind. Jumlah cherelles dan buah koko yang didapati pada satu pokokakan menentukan daya tankannya terhadap infestasi mirid, disebabkan lebih banyak cherelles dan buah kokodidapati pada pokok yang diinfestasi oleh mind apabila dibanding dengan yang tidak diinfestasi.

ABSTRACT

Investigation on factors affecting the population fluctuations ofHelopeltis theivora in relation to food supplyand rainfall in the field was conducted. Results showed that population fluctuations seem to be dictated by rainfalland numbers of available pods as shown by positive correlations between them. The results also showed asignificant yield decrease with increased mind populations. The numbers of cherelles and pods on a tree determineit attractiveness, since significantly more cherelles and pods were found on infested trees than on uninfested trees.

INTRODUCTION

Helopeltis theivora formerly known as Helopeltistheobromae Miller (Stonedahl 1991) is the majorpest of cocoa in Peninsular Malaysia. Feeding bythe active stages causes lesions on shoots andpods of all ages including the newly formedcherelles. There is some disagreement over fac-tors responsible for easonal fluctuations in H.theivora. In Malaysia, pod production peaks twicea year. The relative scarcity of pods at othertimes of the year can influence the populationdynamics of the species; Azhar (1986) empha-sised pod shortage a a limiting factor to popu-lation increase. While the fluctuation in podnumbers probably plays an important role in theecology of H. theivora and of other mirids, thisrole is likely to be moderated or even masked byother factors such as weather. The importanceof cocoa pods as a source of food and a anoviposition site for H. theivora has been stressedby many workers (Miller 1941; Tan 1974). Co-

coa pods are the main source of food for devel-opment and playa vital role in the reproductiveuccess of H. theivora (Rita and Khoo 1983).

Young shoots are not as good as pods for nym-phal instars, but can sustain them when pods arecarce. However, pods are critical to adults for

optimum survival and reproduction (Alias et al.1988). The feeding and oviposition preferencefor pods may be attributed to the suitability oftheir food value and to their texture for egglaying (Rita 1992). Tan (1974) observed that anincrease in the population of H. theivora coin-cided with both increasing numbers of pods andincreasing rainfall. A decrease in the numbers ofH. theivora follO\ ed a decrea e in rainfall eventhough the pod numbers remained high. Betrem(1941) also observed that there was a decreaseof H. antonii Sign. numbers following a dryeason. Roepke (1916) also observed that hot

and humid weather conditions with sufficientsunshine and intermittent, but not too heavy,

RITA M HAMAD AND CH G GAlT FEE

rainfall were optimum conditions for H. antonii.In West Africa, Sahbergella singularis Hag!.

and Distantiella theobroma (Dist.) showed well-marked annual population fluctuations whichcoincided with the main rainy seasons to influ-ence mirid population patterns (Marchart 1969).In Indonesia, H. antonii populations decreasedfollowing a dry season (Roepke 1916). The im-pOl-tance of cocoa pods and vegetative tis ues asfood has been stressed for S. singularis and D.theobroma (Williams 1954).

Food upply and rainfall no doubt contrib-ute to the changes in the pattern of miridpopulations. Studies were conducted in the fieldto further investigate factors which affect thepopulation fluctuations of H. theivora to gain abetter understanding of the mirid's ecology.These investigations covered a three-year studyof population fluctuations in one plantation.

METHODS

The tudy was conducted over a period from July1986 to October 1989 at Seafield Estate, ShahAlam on a block of 49.8 hectares of Sabah mixedhybrid cocoa intercropped with coconuts. In thefirst four months the area was subject to tworounds of insecticide spraying (Lindane 250 g ailha) which killed off mo t of the H. theivora popu-lation; subsequently no insecticides were applied.A central plot of 15 rows x 15 trees was used in thestudy. The numbers of H. theivora on each tree inthe plot were as essed at monthly intervals by themethod known as 'counting to hand height'(Williams 1954). In this method, the trees are

invidually numbered, and the numbers of miridsfound infesting up to a height of about six feetare counted but not removed. Altogether, assess-ments were made on thirty-nine sampling occa-sions. At each count the numbers and sizes ofcocoa cherelles and pods on each tree were re-corded: sizes were divided into cherelles 10 emlong), small pods (10-12 em long), medium sizedpods (12.1-14 em long) and large pods (>14 emlong). Local rainfall and crop yield data (dry beanweight) derived from the 49.8 hectares block wererecorded. Subsequently, data from the 15x15 cen-tral plot were computed based on the relationshipbetween the numbers of H. theivora, the numbersof cocoa pods and yield as based on 'per 100trees'. As ociation and tests of significance todetermine correlations between the numbers ofH. theivora per 100 tree, the numbers of cocoapods and crop yield per 100 trees, and the rainfallwere statistically compared using Spearman's rankcorrelations (Siegel and Castellan 1988). The datafor the first 10 months were not included in theanalyses to allow for the mirids to recover fromthe insecticide treatments.

Additional data were obtained for each sam-pling on the numbers of cherelles and pods onthe infested trees, and on the uninfested treessurrounding the infested trees, a well as on thenumbers of nymphs and adults of H. theivora onthe infested trees. These were statistically ana-lysed and the differences between individualmeans were tested using Duncan's MultipleRange Test. (Table 1).

TABLE 1Number of pods and H. theivora per uninfested and infested tree from

May 1987 to September 1989

Mean number of pods per tree/per sampling

Mean number of H. theivoraper tree/per ampling

Cherelle Mediumand and

small largepods pods

ninfested tree 1.3 0.2b 1.7 O.2b

Infested tree 4.4 0.6a 5.1 O.4a

Total

3.1 0.3b

9.6 0.8a

ymph

o

1.5 1.6

Adults

o

0.8 0.5

Total

o

2.4 1.7

Note: AboUl 3% of the uninfested trees are without pods.Any two means within the column followed by the same letter are not significantly different at 5% level basedon Duncan Multiple Range Test

82 PERTANIKAJ. TROP. AGRIe. Sel. VOL. 16 NO.2, 1993

RELATTO TSHIP BETWEE POP LATO FL CTUATTO lS OF H. THENORA V\ATERHO SE

RESULTS

Fig. 1 shows the relationship between the num-bers of H. theivora per 100 trees, the numbers ofpods, the crop yield per 100 trees and therainfall. In the first 10 months of sampling therewere fewer H. theivora compared to subsequentperiod . This was because it took time for the H.theivora population to build up after insecticidetreatment was stopped. Fig. 1 shows that, afterthe fir t ten months, the population fluctuatedannually with two peaks, in June-July and De-cember-January.

2000

Rainfall occurred throughout the year, butwith harp increases in April and May of eachyear. The wettest months usually occur towardsthe end of each year, usually from October toDecember, and may extend into January. Thepopulation peaks of H. theivora in December-January coincided with months of heaviest rain-fall, although a one month gap was observed inthe June-July peak. Some cherelles and podswere available throughout the year with mostafter the onset of the rainy season, especiallybetween October and January. A flow chart ofcorrelations is shown in Fig. 2.

increased numbers of pods

increased H. theivora(r=0.61)**

decrea ed cropyield (r=-0.55) **

increa ed H. theivora(r=0.34)*

/increasing rainfall

Fig. 2: Diagram showing relationship between H. theivorapopulation with availability ojJood (total numbersoj pod), rainJall and crop yield

Hg. 1: Relationship between numbers oj H. theivora,numbers oj pods, crop yield per 100 trees and therainJall pattern between July 1986 to September1989.

'86 '87

c. No. of pods

'88

Monthly sampling

'811

Fig. 2 indicates that food in the form ofnumbers of pods available in the field and theincreased rainfall were associated with an in-crease in the H. theivora population since posi-tive correlations were obtained between theseparameters. But the H. theivora population ap-peared to reduce the total crop yield at the sametime since a negative correlation was obtainedwhen decrea ed yield of harvested crop andincrease in the H. theivora population were com-pared.

The numbers of H. theivora were calculatedin terms of the numbers per pod, the meannumbers per infested tree and the percentagesof infested tree (Fig. 3). At the populationpeaks, the mean numbers of H. theivora perinfested tree varied from 3 to 10 and the per-centages of trees infested during the June-Julypeak were between 10-20% but increased toabout 40% in the December-January peak. It was

PERTANlKAJ. TRap. AGRIe. SCI. VOL. 16 0.2,1993 83

RITA WHA.\1AD AND CHUNG GAIT FEE

"

100-

PE""CE'Oto

RELATIOt SHIP BETWEE POPULATO FLUCT ATIOr S OF H. THEIVORA WATERHO SE

pattern when mirid species rely on them forfood (En n.vistle 1972). The H. theivora popula-tion was not directly affected by the availabilityof pods, or rainfall alone, but by an interactionof rainfall availability of food as suggested byTan (1974). Our results also strongly suggestthat H. theivora decreases yield, since a signifi-cant overall yield decrease wa observed withincreased mirid populations.

Patches, or pockets of H. theivora, have beenobserved, where individual trees harbour rela-tively large populations of H. theivora. The re-sults show that the numbers of cherelles andpods on a tree determine its attractiveness, espe-cially as significan tly more cherelles and podswere found on infested trees than on uninfestedtrees. Aggregation also occurs with West Mri-can mirid such as D. theobroma and S. singularis.This happens either in open areas of cocoawhere there are many young fans and chuponsin incomplete parts of an otherwise closed canopy(Youdeowei 1965; Lodos 1969; Johnson 1971;Entwistle 1985). In H. theivora, a heavy localisedattack on pods of certain trees is the result ofheavy egg-laying and nymphal feeding by H.theivora related especiall to cherelle and podabundance at the time of infestation (Rita 1992).In this study, no evidence was obtained thatparticular trees have cherelles or pods that areresistan t to or deter the pest. Sometimes a treewith many pods remained unattacked for severalmonths; but this was probably because it was notfound, since, once detected, it was heavily at-tacked as were other trees (Rita 1992). Miridaggregation as well as the ability to build uprapidly to assume destructive proportions wasevident in this study in the January 1989 peak(Fig. 1). This rapid build- up necessitates regularmonitoring which can provide a basis for chemi-cal control (Youdeowei and Toxopeus 1983; Ho1986; Wills 1986).

In Malay ia, census ystems for H. theivorahave been formulated such as the early warningsystem (EWS) and plot and threshold responsesystem (Wills 1986; Wood and Chung 1989).Results obtained in this study could help inimproving the census systems by incorporatingan additional criterion involving selection ofcertain suitable sampled trees. These sampledtrees chosen in the census system, for example,might be those bearing large numbers ofcherelles and small to medium sized pods asthese will attract possible infestations. Knowl-

edge of the population fluctuation of H. theivoracould help in reducing the amounts of insecti-cide used. Populations of H. theivora have beenshown to have two annual peaks coinciding withthe wet seasons; insecticide spraying would nor-mally be done preferably during these peakperiods which coincide with peak of cherelleproduction. Insecticide spraying in wet seasonsi less effective and more difficult to organise,but it has been shown that spraying during peakperiods is effective in reducing the mirid popu-lation (Chung and Wood 1989). However, dam-age can occur earlier at the cherelle stage; there-fore to prevent loss, it would be better to spraybefore H. theivora population increases. Miridsalso feed on shoots causing dieback and affectvegetative growth which eventually cause canopydieback (Alias et al. 1988; Chung and Wood1989). For a long-term strategy, it would bebetter to have a monthl census and to controlHelopeltis activity above 10 - 15 % threshold assuggested by Wood and Chung (1989). Thisstrategy could be used in population manage-ment to ensure healthy cocoa growth and goodyield.

ACKNOWLEDGEMENTS

The authors are grateful for the financial sup-port provided by IRPA No: 1-07-05-18. Facilitiesto conduct the experiment provided by SeafieldEstate, Shah Alam are gratefully acknowledged.

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(Received 5 February 1993)

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