Subramanyam, Bhadriraju

K-State Research Exchange

Starches and Insect ControlStarches affect red flour beetle development and reproduction.Natural starch is an abundant nutrient carbohydrate (C6H10O5)n,found mainly in the seeds, fruits, tubers, roots, and stem pith ofplants, notably in corn, potatoes, wheat, and rice, and varyingwidely in appearance according to sourcebut commonly prepared as a white, amor-phous, tasteless powder. Natural starchusually consists of 25% amylose (linearstarch polymers) and 75% amylopectin(branched starch polymers).Resistant starches (RS) includestarches that are resistant to enzymatichydrolysis in the small intestines of hu-mans. RS are classified as RS1, RS2, RS3,and RS4.• RS1 refers to resistant starch that isencased physically by whole grains.• RS2 is a granular resistant starch.• RS3 refers to non-granular, retro-graded, or crystalline resistant starch.• RS4 is a manufactured resistantstarch.Whole grains can deliver RS1, greenbananas deliver RS2, and RS3 is found inready-to-eat breakfast cereals, breadcrusts, coocked and cooled potatoes, andcooked and cooled pasta. RS4 is manu-factured from various sources, includingwheat, potato, and tapioca, and is avail-able from a variety of ingredient suppli-ers.Red Flour BeetlesThe literature has very little informa-tion on the effects of various starches onred flour beetle development and repro-duction. One study published in 1966showed 100% mortality of larvae of thered flour beetle, a major pest of flour mills,when they were fed potato starch.In the laboratory, my graduate student,Xue Meng, conducted a series of experi-ments to see:1. How well larvae develop on six dif-ferent starches with various amylose con-tents.2. Whether or not adult flour beetlesare attracted to starches.3. If flour beetles lay eggs on starches.Development StudiesThe six types of starches used includedwaxy corn starch (2% amy-lose), corn starch (25% amy-lose), wheat starch (25%amylose), cross-linked wheatstarch (70% total dietary fi-ber), high-amylose starch(70% amylose), and potatostarch (12-20% amylose).The development onstarches was compared withdevelopment on flour aloneand flour plus 5% by weightof brewer’s yeast diet. Thelatter diet is used in the labo-ratory for mass-rearing thisspecies.About 90% of the starch particle sizeswere below 15 to 68 microns, whereas80% of the flour particle sizes were be-low 132 microns. Therefore, starch par-ticle sizes were smaller than that of theflour particles, and it is possible for lar-vae or adults of red flour beetle to con-sume them.About 5g each of starch, flour, andflour plus yeast (5% by weight) weretransferred individually into 30-ml plas-tic condiment cups. Fifty red flour beetleeggs were added into each cup. Threecups were sampled every three days. Lar-vae from cups were recovered.Measurements were made on the fol-lowing aspects: length of the larvae, widthof the head capsule (to indicate stage oflarval development – instars), weight oflarvae, and proportion of thetotal larvae that were alive atthe observed time periods.On starches, developmentof larvae was adversely af-fected, and larvae failed todevelop beyond second in-stars. Additionally, survival onflour and flour plus yeast wasfar superior to survival onstarches. (See the illustrationon page 53.)The worst larval survivalwas on potato starch, followedby wheat starch and cornstarch. Potato starch, whichbelongs to the RS2 type, is re-sistant to enzyme digestion due to lowsurface-to-volume ratio of the granulesand cross-linking between molecules pro-vided by enriched phosphorus contentin these compact granules.The cross-linked wheat starch and70% amylose corn starch are resistantto enzyme degradation, becaue theirtightly packed structure reduces enzymeaccess to the starch chains. The devel-opment results are consistent with thisfinding.The adverse effects of starches ongrowth of larvae compared to flour orflour plus yeast suggest that the starcheslack certain essential nutrients for nor-mal development. What these constitu-ents are at this point is unclear, but wespeculate that it could be protein compo-nents that these starches lack.Adult Preference TestsIn 30cm-diameter and 8cm-high are-nas, 2.5g of a starch and 2.5g of flour wereplaced at the north and south ends. Fiftyunsexed adults of the red flour beetle werereleased at the center of the arena. Allarenas were checked at 36, 48, 60, and72 hours after adult introduction to de-termine the number of adults retained instarch and flour.These studies showed that 48-80% of52  First Quarter 2010 MILLING  JOURNALPest ManagementDr. BhadrirajuSubramanyam_________________________________On starches, development oflarvae was adversely affected,and larvae failed to develop be-yond second instars. Addition-ally, survival on flour and flourplus yeast was far superior tosurvival on starches._________________________________52_53_Subi_Column.pmd 2/18/2010, 11:43 AM2the released adults were retained in starchand flour in each arena.The results also showed that the num-ber of adults retained in starches did notvary significantly from those in flour, sug-gesting that starches were as attractive asflour to the adults.Egg-Laying TestsA pair of newly-emerged adults of flourbeetles were placed on 2.5g each of the sixstarches, flour, and flour plus yeast diet in30-ml plastic condiment cups. Every threedays, the adults were sifted form the starchor flour and moved to fresh starch or flourdiets. There were four such transfers. Thenumber of eggs laid was counted everythree days ending on day 15.These results showed that on average,red flour beetle laid 97 and 109 eggs perfemale on flour and flour plus yeast, re-spectively, whereas on starches, the aver-age number of eggs laid ranged from 0.1to three eggs per female. The low numberof eggs laid on starches suggests that trig-gers for egg laying are absent in starches.A simple test in the laboratory showedthat adults placed on a sieve above flourdo not lay eggs and need to be in the flourMILLING  JOURNAL First Quarter 2010  53to lay eggs. This suggested that feedingmay stimulate egg laying.In additional tests, we observed thatred flour beetles placed on flour and thentransferred to starches laid eggs. Simi-larly,  beetles that failed to lay eggs onstarches when placed on flour laid eggsduring the next three days, reinforcingthat feeding on suitable substrates (flour)is necessary for egg laying.Currently, tests are underway with dif-ferent percentages of protein admixedwith wheat starch to determine whetherprotein plays an important role in devel-opment, survival, and oviposition of thered flour beetle.Bhadriraju Subramanyam (Subi) is aprofessor in the Department of Grain Sci-ence and Industry at Kansas State Uni-versity, Manhattan. He can be reached at785-532-4092 or sbhadrir@k-state.edu.Illustration courtesy of the Kansas State University Department of Grain Science and Industry.Effect of Starch vs. Flour on Larval Development_________________________________Currently, tests are underwaywith different percentages ofprotein admixed with wheatstarch to determine whetherprotein plays an important rolein development, survival, andoviposition of the red flourbeetle._________________________________52_53_Subi_Column.pmd 2/18/2010, 11:44 AM3

Starches and insect control

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Starches and insect control

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