Early warning system for the black maize beetle (Heteronychus arator Fabricius) in a major maize producing region of South Africa

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Date
2015-05
Authors
De Klerk, Nicolene
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University of the Free State
Abstract
English: Black maize beetle (Heteronychus arator Fabricius) (Coleoptera: Scarabaeidae) is economically the most important coleopteran pest that attacks the subterranean part of maize seedlings in South Africa. The sporadic nature of black maize beetle outbreaks led to the need for developing an early warning system. This required improved knowledge on the ecology of this pest. Black maize beetles are nocturnal and are attracted to artificial light. Ninety nine modified Robinson light traps were placed in quarter degree grids (30km x 30km) throughout the eastern part of the maize production area. Weekly captures for three consecutive months from February of every year were preserved in 70% alcohol and counted. The flight pattern of H. arator was in this way monitored for 11 consecutive years. Some captured beetles were placed in breeding containers to harvest eggs for ecological studies. L1-larvae with visible blackened hind guts were placed singly into 7cm diameter pots prefilled with varying organic matter concentrations (10% intervals) for both Avalon and Hutton soil forms. Other L1-larvae were also subjected to varying moisture concentrations (10% intervals) in both soil forms (Hutton and Avalon) premixed with 50% organic matter. Data were recorded and analysed. Spatial variation indicated that black maize beetles in most years are limited to a small area (540 000ha) located on the borders of the Free State, Mpumalanga and Gauteng provinces. The long term temporal variation indicated that epidemic outbreaks recur in the same area with 32 year intervals. The next expected outbreak will be during 2041. Short term temporal variation indicated that populations tend to recur at five year intervals but with varying intensities. Through the use of monthly weather variables as well as black maize beetle captures of the previous year (February to end April) a prediction model was developed. This prediction model was able to explain 62.93% of the number of black maize beetles expected to fly during February to end of April. However, the weather station must be within 10km from the sample area. The prediction model developed for black maize beetles indicated that average solar radiation contributed to 26.99% of the total prediction model while minimum temperature contributed another 15.96%. Both variables are related to heat and contributed in total to 42.95% of the 62.93% that is predicted by the model. Larval development in two soil forms (Hutton and Avalon) differed significantly, where the favourable moisture content for Hutton soil ranged from 40% to 80%, whereas in Avalon soil it was limited to a range of 60% to 70%. This indicated that soil collected in the area known for black maize beetle outbreaks had a larger moisture range suitable for larval development than other areas known for Avalon soil forms. Highly significant black maize beetle larval mass differences were also recorded with variation in organic matter content. The higher the organic matter content the greater the mass gain of the black maize beetle larvae, pupae and pre-adults as well as the time it took to the pre-adult stage. In Hutton soil, all larvae reached maturity at 80% to 90% organic matter content. However, with Avalon soil with an organic matter content of 90% only 40% of the larvae managed to develop into pre-adults. At 100% organic matter content all larvae managed to become the pre-adult stage. This indicated that black maize beetle larvae feeding exclusively on organic matter are able to reach pre-adults and that no-till practice especially in the known distribution area of black maize beetles may lead to a significant increase in black maize beetle numbers. Results indicated that with Hutton soil an 80% organic matter content is needed for larvae to reach maturity unless black maize beetle larvae are able to feed on living plant material such as plant roots. It took approximately five days longer for beetles to emerge from a Hutton soil form with an 80% organic matter content compared to the same soil with 100% organic matter content. This indicated that the lower the organic matter content in soil, the longer it will take for black maize beetles to become pre-adults unless larvae are able to feed on living plant material. By determining organic matter content in the soil and measuring soil moisture levels a more effective prediction model for black maize beetles can be developed.
Afrikaans: Swartmieliekewer (Heteronychus arator Fabricius) (Coleoptera: Scarabaeidae) is ekonomies die belangrikste kewerplaag wat die ondergrondse dele van mieliesaailinge in Suid Afrika aanval. Die sporadiese aard van swartmieliekewer uitbraak het die ontwikkeling van ‘n tydige waarskuwing stelsel genoodsaak. Om dit te kon doen moes ‘n deeglike ekologiese studie van die plaag gedoen word. Swartmieliekewers vlieg gedurende die nag en word daarom deur kunsmatige lig aangelok. Nege-en-negentig gemodifiseerde Robinson ligvalle was in kwart graad blokke (30km x 30km) geplaas dwarsdeur die oostelike deel van die mielieproduksie area. Weeklikse vangste vir drie aaneenlopende maande vanaf Februarie van elke jaar was gepreserveer in 70% alkohol en getel. Die vlugpatroon van H. arator was op hierdie wyse vir 11 agtereenvolgende jare gemonitor. Data was aangeteken en geanaliseer. Sommige gevangde kewers was in broeihouers geplaas om eiers te oes vir die ekologiese studie. L1-larwes met sigbaar verdonkerde agterlywe was enkel in 7cm deursnee potte wat vooraf gevul was met variëerende organiese inhoud konsentrasies (10% intervalle) vir beide Avalon en Hutton grondvorms geplaas. Ander L1-larwes was ook blootgestel aan variëerende vog konsentrasies (10% intervalle) in beide grondvorms (Hutton en Avalon) wat voorafgemeng was met 50% organiese materiaal. Ruimtelike variasie het aangetoon dat swartmieliekewers in meeste jare beperk was tot ‘n klein area (540 000ha) wat voorkom op die grense van die Vrystaat, Mpumalanga en Gauteng provinsies. Die langtermyn temporale variasie van swartmieliekewer populasies het getoon dat epidemiese uitbrake in dieselfde area uitbreek met 32 jaar intervalle. Die volgende verwagte uitbraak sal gedurende 2041 wees. Korttermyn temporale variasie het aangetoon dat populasietoenames met vyf jaar intervalle voorkom, maar met verskillende intensiteite. Deur die gebruik van maandelikse weersveranderlikes asook swartmieliekewer vangstes van die vorige jaar (Februarie tot einde April) is ‘n voorspellingsmodel ontwikkel. Die voorspellingsmodel kon tot 62.93% van die aantal swartmieliekewers wat verwag word om te vlieg gedurende Februarie tot einde April van die volgende jaar voorspel. Die weerstasie moet wel binne 10km van die monster area wees. Die voorspellingsmodel wat ontwikkel is vir swartmieliekewers het getoon dat gemiddelde sonlig uitstraling tot 26.99% van die totale voorspellingsmodel bygedra het, terwyl minimum temperatuur ‘n verdere 15.96% bygedra het. Dus, beide veranderlikes is verwant aan hitte en dit het in totaal tot 42.95% van die 62.93% bygedra. Larwale ontwikkeling in twee grondvorms (Hutton and Avalon) het wesenlik verskil waar die gunstige voginhoud vir Hutton grond gestrek het van 40% tot 80% terwyl in Avalon grond dit beperk was van 60% tot 70%. Dit het aangetoon dat grond wat van die area wat bekend is vir swartmieliekewer uitbrake, ‘n groter vogreeks geskik vir larwale ontwikkeling het in vergelyke met ander areas bekend vir Avalon grond vorms. Hoogs betekenisvolle swartmieliekewer larwale massa verskille was aangeteken met verskillende organiese inhoud. Hoe hoër die organiese inhoud hoe groter die massa toename van die larwes, papies en volwassenes asook die tyd wat dit geneem het tot volwassenheid. In Hutton grond het alle larwes volwassenheid bereik by 80% tot 90% organiese inhoud. In Avalon grond met ‘n organiese inhoud van 90% het egter slegs 40% van die larwes volwassenheid bereik. By 100% organiese inhoud het alle larwes volwassenheid bereik. Dit het aangetoon dat swartmieliekewer larwes wat uitsluitlik net op organiese materiaal voed, volwassenheid bereik en dat geenbewerkingspraktyke veral in die area bekend vir swartmieliekewer voorkoms tot ‘n wesenlike toename in getalle sal bydra. Resultate het aangetoon dat Hutton grond met 80% organiese inhoud nodig is vir larwale ontwikkeling om volwassenheid te bereik, behalwe as larwes op lewende plantmateriaal soos plant wortels, kan voed. Dit het ongeveer vyf dae langer geneem vir kewers om te ontpop in ‘n Hutton grondvorm met ‘n 80% organiese inhoud in vergelyking met dieselfde grond met ‘n 100% organiese inhoud. Dit het aangetoon dat hoe laer die organiese inhoud in die grond hoe langer sal dit neem vir swartmieliekewers om volwassenheid te bereik behalwe as larwes lewende plantmateriaal kan vreet. Deur die organiese inhoud van grond asook die vogvlakke in grond vas te stel kan ‘n meer effektiewe voorspellingsmodel vir swartmieliekewers ontwikkel word.
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Keywords
Dissertation (M.Sc. (Zoology and Entomology))--University of the Free State, 2015, Corn -- Diseases and pests -- South Africa, Beetles -- South Africa, Scarabaeidae -- South Africa
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