Black Maize Beetle
LIFE CYCLE
Eggs are white, oval in shape and measure approximately 1.8mm long. They are laid singly at a soil depth of 1 to 5cm. Females lay between 12 to 20 eggs in total.
Eggs hatch after approximately 20 days. There are three larval instars. Larvae are creamy white except for the brown head capsule and hind segments of the abdomen, which appear dark gray/blue green, where the contents of the gut (plant roots and soil) show through the body wall. The head capsule is smooth, measuring 1.5 mm, 2.4 mm, and 4.0 mm at each respective instar. The third instar is approximately 25 mm long when fully developed. The larvae look very similar to many other beetle larvae, including dung beetles, and are therefore better distinguished by their activity. Most other beetles do not attack plants as larvae, so where root pruning can be observed, this is a good indication that black maize beetle larvae are present.
Black maize beetle larvae are soil-dwelling and resemble white 'curl grubs’, with three pairs of easily visible legs on the thorax.
The larvae, when fully grown, enter a short-lived pupal stage, which measures approximately 15 mm long and is typically coleopteran in form (cylindrical shape), initially pale yellow, but becoming reddish-brown nearer to the time of emergence.
Adults are 12 to 15 mm long, shiny black dorsally and reddish-brown ventrally. The females are slightly larger than males. Males and females are readily differentiated by the shape of the foreleg tarsus
There are two main flight peaks during the year in South Africa, late spring to early summer (October – December) and late summer to early autumn (February – April). The second is a much larger peak. These flight peaks are important in cereal crops, as the adult beetle can cause significant damage to the plants. For turf the most important stage is just before these flight peaks when the larvae are at their most mature stage.
DAMAGE TO TURFGRASS
The adult is considered the main pest stage, attacking a wide variety of plants. However, larvae can cause damage to turf. Two of the most prominent grasses which are favoured by these beetles are ryegrass and kikuyu. Black maize beetle larvae prune or completely severe grass roots close to the crown of the plant. Where there are a large number of larvae these results in patchy turf that can be rolled back or pulled out due to the lack of root system. This is exacerbated during drought conditions as drought stressed turf stops producing new roots.
The beetles chew into the stems of the growing plants just below the soil surface, causing rapid wilt of the growing leaves and death of the plant. The damaged area of the stem has a frayed (shredded) appearance, which distinguishes it from the damage caused by cutworms. The fraying is caused by the beetles consuming the soft tissues but leaving the fibrous material. The larvae prefer to feed on organic matter in the soil but may cause some root damage.
CONDITIONS
Black maize beetles favour sandy soils, which are high in organic matter. They thrive at temperatures of 20-25°C, and stop feeding at soil temperatures below 15°C. Higher than usual spring and summer temperatures therefore result in a higher than normal survival rate of the larvae. Larval survival is also increased by drier than normal summer conditions. Drought conditions are therefore conducive to black maize beetle infestations. Successive droughts especially lead to black maize beetle outbreaks.
MONITORING
The best time to monitor beetle numbers is in February and March, which will help to determine the risk of black maize beetle damage. In order to assess the density of beetles carry out the following:
- In ten representative spots dig a cube of soil 20cm wide and 10-15cm deep
- Break up each cube, counting how many black maize beetle larvae can be found
- Add the total number of larvae found in the ten cubes, and then multiply this amount by 2.5. This will give you the larval abundance per square meter.
When the number of larvae exceeds 15 per square meter, they likely have already, or will cause damage. In dry years, damage will be apparent at even lower densities.