Proceedmgs of the 7th Iniernaiional Workmg Conference on Stored- product ProtectuJn - Volllme 1

Development and non-preference of Sitophilus zeamais on, stackburnt' maize

D P Giga and J Canhao1

Abstract

Maize stackburn has emerged as a significant threat to foodsecunty m sub-Saharan Africa m the last decade resulting msigruficant losses commercially stored maize Stackburntmaize IS charactensed by a moderate to severe reddish-brown discoloration of the kernel and germ of maizeresulting from heat build-up m the mtenor of polypropylenebag stacks Stackburn causes a reduction m the nutritionalvalue of the gramA comparative study of the development of the maize

weevil, Suophslue zeamms, on normal and stack burntmaize showed significant differences m progeny emergence,mean duration of development and mdices of susceptibilitybetween the two maize types An index of susceptibility of12 34± 0 14 was recorded for normal maize while that forstackburnt maize was 9 29 ± 0 10In free-choice tests weevils strongly selected normal

maize, mcludmg adults which had been conditioned onstackburnt maize

Introduction

In Zimbabwe, large quantities of white maize (more than1 2 rmlhon tonnes, annually) are successfully storedoutdoors m modular 5000 tonne stacks These are built on adunnage of poles and are covered with tarpaulms Wovenpolypropylene sacks have replaced Jute sacks (m the late1980s) and are now used for maIze storageDunng recent years, comcIdmg With the mtroductlOn of

woven polypropylene sacks, maIze throughout the centre ofsome bagstacks have been found to be dIscoloured from alight tan through to a dark red-brown colour after storage(Tyler, 1992) The discoloratIon, termed stackburn,results m the affected maize bemg downgraded Withattendant fmanClal losses Stackburn has also occurredfrequently m food aId maize m several countrIes mcludmgAngola, Mozambique, Swazl1andand Zambia (Phl1hps andDonaldson, 1994)

1Department of Crop Science, Umverslty of Zimbabwe

The charactenstic discoloration of the germ and pencarprepresents a form of heat damage and IS recogmsed as afeature of 'dry gram heatmg ' as distinct from the wellknown 'damp gram heatmg ' (Tyler, 1992) Expenmentaldata from the Natural Resources Institute (NRI) has shownthat the discoloration results from moisture and temperaturedependent chemical reactions, part of which may be non-enzymic brownmg CPhillrps and Donaldson, 1994)Increased discoloration IS charactensed by a reduction mlysine and a conversion of sucrose to reducing sugars WhIChmay subsequently act as reactants m further chemicalcomplexes Stackburmng of maize causes a reduction m thenutntive value for all classes of livestock and can bepredicted to do so for humans as well Pamgrahi et al(1996) In the case of insects, SltophLlus zeamms andTribolium. casitmeum development times, survival of Flprogeny and multiphcation rates were adversely affected onstackburnt maize when compared to normal maize (Giga,1995, Tanyongana and Nyanongo 1995)Expenmental stack data from Zimbabwe has shown that

stackburn progression IS charactensed by rapid internalheating of polypropylene bagstacks to a maximum of 40"(and maintenance of temperatures at this level, With onlymmor fluctuations, throughout storage (Conway, 1996)The causes of the rapid built up of mternal stacktemperatures are unclear, though insects have beenImplicated (Kennedy and Devereau, 1994, Conway, 1996)Kennedy and Devereau (1994) also observed a temperaturelimit of 40°C m stacks followed by coohng after fumigatIonsto varymg degrees m expenmental stacks The lethaltemperature for SztophLlus spp and Tnbolwm spp , themsects WhICh were found m greatest numbers m thesamples, are reported by vanous authors (Howe, 1962a,Reddy, 1950) as 35 - 40°CHowe (l962b) proposed a process by which msect heatmg

m gram may progress An msect mfestatlOn m the gramrespIres and produces heat, causmg a nse m temperaturewhich m turn mcreases the reproductIve rate and metabohcrate of the msects The mfestatIon thus grows at anacceleratmg rate, releasmg heat at an acceleratmg rate andresultmg m greater temperature nses ThIS processcontmues untIl the temperature becomes unfavourable forthe msects and they either mIgrate or dieWhether the msects migrate back mto stackburnt gram

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Proceedmgs of the 7th Internatw'HaL Worktng Conference on Stored-product Protectwn - VoLume 1

once cooled ISnot known The objectives of this study wereI) to deternune whether S zeanUU8 adults, m laboratorychoice tests, showed any preference for stackburnt maizeand 11) to compare the development of the msect onstackburnt and normal maize

Materials and Methods

Samples of stackburnt and normal maize were collected fromthe Gram Marketing Board Depot m Mvurwi for theexpenments The maize was stored m a cold room and pnorto the start of the experiments, subsamples were Withdrawnand frozen for two weeks to dismfest the gram Thereafter,the gram was equihbrated to 70% r h and 28°CExpenments were conducted m sealed plastic boxes placed man incubator set at the expenmental temperature Thehumidity was mamtamed using sodium chlonde solutions

Development and indices of susceptibihty

Fifty grams of maize were placed m small glass Jars foreach maize type and infested With 15 pairs (15 males and 15females) of newly emerged (approximately l-week old)adults The insects were left to OViPOSiton the gram for oneweek under the expenmental conditions and after threeweeks dally observations were made noting the dailyemergences By recording the daily emergences the meandevelopment times of the emerged insects from the time ofinfestation were calculated An index of susceptibihty forthe two maize types was calculated usmg the formula(Doble, 1974)Index = lin no progeny I/mean development time /,100The expenment was replicated ten times

Choice experiments: Preference/non- preferencetests

A senes of choice tests were conducted to determmewhether there are any differences m the selectIOnof normaland stackburnt maize by weevils that had received differentcondltIomng treatments The followmg tests wereperformedI) white vs stackburn maize usmg unconditioned weevIls,11) yellowvs stackburnmaIZe usmg unconditionedweeVils,m) white vs stackburn maIZeusmg weevils conditioned on

white maize,IV) white vs stackburn maIZeusmg weevIls conditioned on

stackburn maize,IV) white vs stackburn maize usmg a mixture (1 1 ratio)

of weevIls conditioned on white and stackburn maizeAll tests were conducted at 28°C m sealed plastic boxes

(35cm /, 14cm ;< Bcm) usmg maize eqUilIbrated at 70%r h and 28°C The weevIls were conditioned on theappropnate maIZe types under the expenmental conditionsfor three days before use m the tests

In each case, one petndish (9cm diameter) filled Withnormal maize and another With stackburnt maize were placedat opposite ends of the plastic boxes to which fifty weevilswere introduced m the centre The boxes were then sealedand Immediately placed m darkness m an incubator Thenumbers of weevils inside the petndishes contammg the twotypes of maize and outside the dishes were counted 1, 2, 4,8 and 24 hours after mtroduction Five replicates of eachtreatment were set up With the expenment being repeatedtwice Thus there were ten replicates for each treatment

Results and Discussion

Development and Indices of Susceptibility

The mean development penod (days), mean number ofadults emerged and the mean susceptibihty mdices areshown m Table 1 Compansons of data using Student t-testsshowed srgruficant differences (p < 0 01) between the twomaize types for all the parameters measured Developmenttimes of weevils were sigruhcantly longer on stackburntmaize With significantly fewer insects developing on thismaize Similarly the index of susceptibihty of stackburnmaize (9 29) was significantly lower than normal maize(12 34) These data mdicate that stackburnt maize ISnutritionally' mfenor ' for weevil development and survivalOther workers (Tanyongana and Nyanongo, 1996, Alloteyand Odamtten, 1996) have shown multiplication rates ofseveral species of storage insects to be significantly lower onstackburnt maize compared to normal maize Whether tlusdifference was due to mcreased development times onstackburn maize or reduced oviposition m the more "brittle'stackburn maize IS not known Smce the stackburnt maizewas equihbrated to the same moisture content as the normalmaize m the current study, the lower emergence rate,delayed development and lower susceptibility index may beattnbuted to some other factor (s ) besides the moisturecontent

Table 1. Mean development penod, emergence and mdlcesof susceptibilIty of Sttophllus zeanuItS on normaland stackburnt maize

MaIZe Devpt penod No adults SusceptibilItyType (days) emerged mdexNormal 36 3±0 33 89 0±3 4 12 34±0 14Stackburnt 39 9±0 28 38 6± 1 1 9 29±0 10t-values 4 02 14 08 17 73DF 18 18 18

Preference/non-preference tests

Figures 1 - 5 shows the numbers of weevIls found onstackburnt and normal maize With time The figures clearlyshow a preference by the weevils for the normal undamaged

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Proceedmgs of the 7th Internatumal Worhng Conference on. St01'eCl-p1'OductProtection - Volume 1

maize Chi-square tests for all the experiments. Irrespectiveof the condrtionmg of the insects showed significantdeviations from the 1 1 ratio Initially there was a smalldifference in the number of msects recorded on both maizetypes but With time sigruficantly more weevils were

recorded on the normal maize compared to the stackburntmaize Conditiomng adults on stackburnt maize did notsigmficantly affect their behaviour when given a choice ofthe two types of maize All insects showed a strongpreference for the normal, undamaged maize

40 - \

35

30

25:ie:l 20ec:I::Ii!

15

10

5

01hr

Fig. l.

30

I__ norma'i--slbum

2hr 4hr 8hr

TIme

Mean number of weevils on normal and stackburnt maize using unconditioned weevils

25

20

...11

~~e::I 15c __ slburnc01CIl::Ii!

10

sr----------------_-li0-1-----------_----------+--------------------'

1hr 2hr 4hr 8hr 24hr

Time

Fig. 2. Mean number of weevils on normal yellow and stackburnt maize using unconditioned weevils

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Proceedmqs of the 7th lniernaiumal Warkmg Conference Oil Stored-produci Protection - Volume 1

45 rI

40 t1

35

30

..GI,g 25E~cc.. 20GI::!:

II

15 tI

10

5It

joL_1hr

r=+= norrnalll::!-S/bumJ

2hr 4hr 8hr 24hr

Time

Fig. 3. Mean number of weevils on normal and stackbumt maize condinoned on stackbumt maize

40

35

30

25

.8E~ 20cc::E

15

10

5

0lhr

Fig. 4.

I_.::+-normal___ slbum

2hr 4hr

Time

8hr 24hr

Mean number of weevils on normal and stackbumt maize conditioned on normal marze

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Proceedmqs of the 7th International Wmkmg Conference on Stored-product Proiectum - Volume 1

35

30

25~eE! 20cIII

~15

10

~- normal..... sJbum~-~

5

o1hr

--_._--+-2hr

_---I

4hr

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Fig. 5. Mean number of weevils on normal and stackbumt maize conditioned on both types of maize

References

Allotey, J and Odamtten, G T 1995 Insectmultiphcation, food preference, contammant Asperqillusspecies and damage caused by Cryptolestes ferruqineu« ,Rhyzopertha domuuca , and Suophilue zeamms tonormal and stackburnt maize m Ghana In Odamtten GT and Clerk G C (eds ) Investigation on maizestackburn maize m sub-Saharan Afnca Proceedmgs of aworkshop 30 October - 3 November, 1995, Universityof GhanaConway, J A (ed ) 1996 Third Annual Report on ECDGXIIproject P 156Dobie, P 1974 The laboratory assessment of the mherentsusceptibility of maize varieties to post-harvest mfestationby Siiopiuius zeamms Journal of stored ProductsResearch, 10, 183 - 197Giga , D P 1995 Development of Siioplulu« zeamalS onstackburnt and normal maize In Odamtten G T andClerk G C (eds ) Investigation on maize stackburnmaize m sub-Saharan Afnca Proceedmgs of a workshop30 October - 3 November, 1995, Uruversity of Ghana, 53-57

Howe, R W 1962a Observations on the rate of growth anddisruption of moultmg m the larvae and pupae ofTribouuni castaneum (Herbst) (ColeopteraTenebnomdae) at sub-optimal temperatures Entomologia

Expenmentahs et Apphcata, 5, 211- 222Howe, R W 1962b A study of the heatmg of stored gramcaused by msects Annals of Applied Biology, 50, 137-157Kennedy, Land Devereau, A D 1994 Observations onlarge-scale outdoor maize storage m jute and wovenpolypropylene sacks m Zimbabwe In Highley, E,Wnght, E J , Banks, H J and Champ, B R (eds )Proceedmgs of the 6th International Workmg Conferenceon Stored-product Protection 290 - 295Parugrahi, S , Bestwick, LA, Davies, R and Wood, CD 1996 The nutritrve value of stackburned yellow maizefor livestock tests m 1n mtro and in broiler chicksBritish Journal of Nutntion, 76, 97 -108Phillips, S I and Donaldson, T J 1994 Report No 5·ECDGXII Research Project Workshop, Harare, Zimbabwe,NRI Report R2140 (S) Chatham, UK Natural Resources

InstituteReddy, D B 1950 Ecological studies of the rice weevilJournal of Economic Entomology, 42, 203 - 206Tanyongana , Rand Nyanongo , R C 1995 A companson ofmsect development on normal and stackburnt maize InOdamtten G T and Clerk G C (eds ), Investigation onmaize stackburn maize m sub-Saharan Africa Proceedmgsof a workshop 30 October - 3 November, 1995,Urnversrty of Ghana, 24 - 26Tyler, P S 1992 Heat and discoloration of bagged maizeWorld Gram, September, 14 -16

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