INTEGRATED CONTROL OF SWEET POTATO WEEVILCYLAS FORM/CARllis FA'BRiciU~s-- ~

ByM. S. PAlANI SWAMI

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THESIS

submitted in partialfulfilment of therequirementfor the degree

DOCTOROF PHILOSOPHY

Faculty of AgricultureKeralaAgriculturalUniversity

DEPARTMENT OF ENTOMOLOGY- ---. <'-COLLEGE OF AGRICULTURE

VELLAYANI - TRIVANDRUM

1987

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ABSTRACT

The incidence of sweet potato weevil Cylas formicarius

Fabricius (Coleoptera: Curculionidae), and intensity of damage

done to sweet potato (Ipomoea batatas(L~) in Kerala were assessed

in an elaborate survey aaopting a purposive and stratified random

sampling technique.

A series of experiments were conducted in the Instructional

Farm, College of Agriculture, Vellayani, during 1984-'86, for

standardising the different methods of control for integrating

the same in a viable technology. The incidence of the pest and

intensity of damage in the survey and in different experiments

were assessed in terms of percentage of crown infestation, crown

damage grade index (DGI), percentage of tuber infestation, tuber

DGI and number of weevils obtained from samples of infested tubers

stored for 30 days.

The survey revealed that sweet potato weevil has got

established in all the sweet potato growing areas of Kasaragod,

Malappuram; Palghat, Tric hur , Quilon and Trivandrum Districts

located in the three agro-climatic regions of the State, indicating

the extreme adaptability of t~e insect to verying agro-climatic

situations. Though the pest was not found serious in low/wet

lands~ it was found as a major limiting factor in the production

of sweet potato tubers in the upland cultivation. Among the

Districts there was no significant variations in the intensity of

pest incidence. But within each District there was wide varia-

tions in the occurrence of the pest and in the intensity of

damage noted. But no contiguous areas could be identified within

the District having higher or lower intensity of damage. The pest

was less in alluvial soils and early harvest (105-110 days after

planting) reduced the damage. In genera~, crop rotation had little

effect on the incidence of the pest. The parasitoids (Rhaconotus

sp. and Bracon sp.) and a predator (Drapitis sp.) were observed in

all the tracts having sweet potato crop. Ipomoea purpuria and

I. biloba were the alternate hosts found in all the Districts,

while I. paundurata was seen in Malappuram and Palghat Districts

and I. hispida was noted in Kasaragod, Quilon and Trivandrum

Districts only. Amaranthus viridis and Vigna sinensis were found

harbouring the adults of C. formicarius in the field. The yie ld

loss caused by the pest was estimated as Rs. 96.04 lakhs annually

. and of this Palgt<1t, Malappuram and Kasaragod Districts incurred

53.77,19.91 and 19.38 per cent loss respectivelywhile the

remaining Districts together accounted for 6.95 per cent only.

In a screening experiment having 40 entries, a local cultivar

Selopia was located as 'moderately resistant' and it was signifi--cantly superior to all other entries. The varieties H 268 and

76 OF 217 were 'ocderately resistant' and 'less resistant respect-

ively. All the t~ee were on par with reference to yield. Entries

with red skin and yellow or orange flesh, higher latex content and

deeply set tubers ~ere found less susceptible. Definite correla-

tions between checical constituents and resistance were not observed

in the studies. Antibiosis was observed as a factor contributing

relative resistance to the weevil.

Data on the incidence of G. formicarius at different

growth stages of sweet potato showed that the crown was more

susceptible from 42 to 77 DAB. The number of eggs on tubers

were high between 50 and 70 DAB. Immature stages of the pest in

tubers did not reach the peak even at harvest (105 DAP).

Reridging done as a cultural practice, reduced the pest

incidence significantly and the optimum period was between 50 and

80 DAP.

Application of Eupatorium or Clerodendron leaves @ 3 t ha-1

at 30 DAB was effective in reducing the weevil damage in field.

Among locally available organic cakes and wood ash tried

at varying doses and growth stages of the crop mahwa cake @-1 -1

1000 kg ha or neem cake @ 1500 kg ha applied at 50 DAP reduced

the weevil incidence significantly.

In a series of experiments the methods for trapping the

adults of C. formicarius in field, as a control measure, were

standardised. It was observed that (i) fresh sweet potato tubers

of 6 cm diameter cut to 100 g piece was the best bait mat~rial

for the trap, (ii) the peak collection of the weevils in the trap

was between 16.00 hand 22.00 h, (iii) setting up of the trap at

16.00 h and collecting and destroying the weevils from the bait

at 06.00 h the next day were found effective and convenient,

(iv) optimum spacing for setting the traps in field was found as

5 ill (v) four trappings done at 10 day intervals between 50 and

80 DAP were found to be optimum for reducing the weevil population

in field.

A field experiment showed that endosulfan, fenthion or/

fenitrothion 0.05 per cent emulsion used as soil drench at 50 and

80 DAP were very effective against the pest and their residues in

tuber at harvest were non-detectable. The treatments did not

affect the soil microflora significantly~

Drenchings done at different growth stages of the crop

showed that the applications at 50 and 80 DAP w~re effective.

Tobacco decoction, though less toxic, was also effective in

controlling the pest.

The toxicity of endosulfan, fenthion, -fenitrothion or tobacco

decoction applied in soil, bioassayed using adults of C.formicarius

as test insect, remained high (above 50 per cent) for four weeks

and that of tobacco decoction for two weeks.

An integrated pest control technology, consisting of -

removal of alternate hosts, selection of pest free planting

materials, mulching with Eupatorium or Clerodendron leaves @

3 t ha-1 at 30 DAF, a reridging at 50 DAF, trapping the weevils.

at 10 day intervals between 50 and 80 DAP and one soil drenching

with endoEulfan, ienthion or fenitrothion 0.05 per centat 65 D.AP,early

harvesting (105-110 DAP) and proper disposal of crop residues at

harvest, was found to be the most effective against C.formicarius

and the natural enemies were least affected in the treatment.

It also gave the beEt cost-benefit ratio. It was followed by

the treatment in WQich all the above cultural practices were

adopted with the ejdition of a reridging at 80 DAP and deletion

of insecticidal drenching at 65 DAP. The third treatment

consisting of chemical method (soil drenchings at 50 and 80 DAP)

alone also was quite effective and economically viable. T~

cost-benefit ratio was least in this treatment. The integrated

technology if adopted extensively for a few cropping seasons in

succession will reduce the insect population to negligible levels

in .the agro-ecosystem.

The coefficient of variations of the data obtained in the

various experiments showed that the assessment of the damage

grade indices of the tubers and number of adults emerging from

tuber samples collected from treatments and stored for one month,

were more reliable methods for estimating the incidence of

C. formicarius.

.