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

[摘要] English: Black maize beetle (Heteronychus arator Fabricius) (Coleoptera: Scarabaeidae) iseconomically the most important coleopteran pest that attacks the subterranean partof maize seedlings in South Africa. The sporadic nature of black maize beetleoutbreaks led to the need for developing an early warning system. This requiredimproved knowledge on the ecology of this pest. Black maize beetles are nocturnaland are attracted to artificial light. Ninety nine modified Robinson light traps wereplaced in quarter degree grids (30km x 30km) throughout the eastern part of themaize production area. Weekly captures for three consecutive months from Februaryof 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 beetleswere placed in breeding containers to harvest eggs for ecological studies. L1-larvaewith visible blackened hind guts were placed singly into 7cm diameter pots prefilledwith varying organic matter concentrations (10% intervals) for both Avalon andHutton soil forms. Other L1-larvae were also subjected to varying moistureconcentrations (10% intervals) in both soil forms (Hutton and Avalon) premixed with50% organic matter. Data were recorded and analysed. Spatial variation indicatedthat 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. Thelong term temporal variation indicated that epidemic outbreaks recur in the samearea with 32 year intervals. The next expected outbreak will be during 2041. Shortterm temporal variation indicated that populations tend to recur at five year intervalsbut with varying intensities. Through the use of monthly weather variables as well asblack maize beetle captures of the previous year (February to end April) a predictionmodel was developed. This prediction model was able to explain 62.93% of thenumber 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. Theprediction model developed for black maize beetles indicated that average solarradiation contributed to 26.99% of the total prediction model while minimumtemperature contributed another 15.96%. Both variables are related to heat andcontributed in total to 42.95% of the 62.93% that is predicted by the model. Larvaldevelopment in two soil forms (Hutton and Avalon) differed significantly, where thefavourable 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 collectedin the area known for black maize beetle outbreaks had a larger moisture rangesuitable for larval development than other areas known for Avalon soil forms. Highlysignificant black maize beetle larval mass differences were also recorded withvariation in organic matter content. The higher the organic matter content the greaterthe mass gain of the black maize beetle larvae, pupae and pre-adults as well as thetime 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 mattercontent 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. Thisindicated that black maize beetle larvae feeding exclusively on organic matter areable to reach pre-adults and that no-till practice especially in the known distributionarea of black maize beetles may lead to a significant increase in black maize beetlenumbers. Results indicated that with Hutton soil an 80% organic matter content isneeded for larvae to reach maturity unless black maize beetle larvae are able to feedon living plant material such as plant roots. It took approximately five days longer forbeetles to emerge from a Hutton soil form with an 80% organic matter contentcompared to the same soil with 100% organic matter content. This indicated that thelower the organic matter content in soil, the longer it will take for black maize beetlesto become pre-adults unless larvae are able to feed on living plant material. Bydetermining organic matter content in the soil and measuring soil moisture levels amore effective prediction model for black maize beetles can be developed.

[发布机构] University of the Free State