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

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.