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African black beetle
Text only version of Farmnote 398 - PDF Available
Figure 1 African black beetle adult damage to rye grass pasture....results in yellowing of plants.
Figure 2 African black beetle adult root pruning damage to rye grass seedlings....compared to undamaged seedlings.
Figure 3 African black beetle adults....
Figure 4 Immature life stages of African black beetle—eggs, three larval instars and pupae.
Figure 5 The horizontal opening on the tip of the abdomen of African black beetle larvae is a distinguishing feature...
African black beetle, Heteronychus arator, is a southern African insect that was first reported in Western Australia in the 1930s. It is now widespread in the south-western coastal region of WA.
The adult beetle is the main pest stage. The beetles are of considerable economic importance because they attack a wide range of plants:
- pasture, particularly newly-sown ryegrass (See Figures 1 and 2) and perennial grasses
- barley, triticale and wheat crops, but not oats
- irrigated and dryland summer forage crops such as millet and maize
- a wide range of vegetable crops, most importantly potatoes
- grapevines and olives
- ornamental plants and newly-planted trees such as blue gums.
Larvae of this beetle damage turf and also underground crops, notably potato tubers.
Description and life cycle
Newly emerged adult African black beetles are brown and fairly soft but soon harden and darken to a glossy black with the sexes distinguishable by the appearance of the front legs (Figure 3). Adults are 12 to 14 mm long, the females being slightly larger than males. These beetles are strong fliers even though they feed under the soil surface on plant roots.
The eggs are white, oval and about 1.8 mm long (see Figure 4). They are laid singly in the soil just under the vegetation mat, and can be seen clearly with the naked eye. They may be confused with the eggs of other beetles, snails and slugs.
The larva (grub) is a typical C-shaped cockchafer with 3 pairs of legs on the thorax. There are three larval stages, or instars, between which the larvae moult (see Figure 4). While larval body size increases steadily, each instar’s head capsule is very similar, being an average of 1.4, 2.5 and 4.0 mm wide in successive instars.
Larvae are creamy-white except for the brown head-capsule and the hind segments, which appear dark where the contents of the gut show through the body wall. From the second instar stage these larvae are easily identifiable as they are the largest and most common of the cockchafer group in pastures in the coastal areas.
African black beetle larvae can be distinguished from other species with the naked eye, as their anal opening and associated spines are horizontal. Other species have a vertical opening as the figure below clearly shows.
Most larvae feed just below the surface of pasture within the main root mat. Wet conditions during the egg and first instar larval stages are fatal but as the larvae grow, their ability to cope with high moisture levels increases.
When they have finished feeding, the larvae, which are then about 25 mm long, clear their gut, become more cream in colour and steadily shorten and become more rounded as they change into the pupal stage.
The pupa or resting stage is at first pale yellow but changes to a reddish brown as it approaches the adult stage (see Figure 4).
Figure 6 The generalised seasonal pattern of abundance of the different stages of the life cycle of African black beetle in southern WA.
African black beetle has one generation per year. Adults are present during winter but any larvae seen at that time are late developers from the previous summer and do not survive. These beetles mate in spring, which coincides with crawling activity on the soil surface at night, and lay eggs under the soil surface. See figure 6.
Adult numbers decline in spring as the old generation dies off. Larvae develop during summer. However, there is high natural mortality due to the dry WA summer conditions so many do not become adults. This is unlike the eastern states where summer rainfall is greater and the survival rate of larvae is higher.
As with all insects, the rate of growth depends on temperature. Because of higher temperatures in northern parts of the beetle’s distribution such as around Gingin, development stages such as mating, egg-laying and abundance of larvae peak about six weeks earlier than in the southernmost areas around Albany.
African black beetle is primarily a grass-favouring, pasture-dwelling insect. As such, most of its pest status results from planting crops such as cereals after infested pastures and from the beetle's wide distribution in the higher rainfall, coastal and horticultural areas of the southwest of WA.
Because of the beetle’s strong flight activity, crops may become infested when the beetles disperse.
The triggers for flight activity are not known. Most flight activity occurs in late summer-autumn, which coincides with the new generation of adults. Some flight activity also occurs in spring, but it is minor compared to the surface crawling. Largescale flights are sporadic and may be localized within a district, making them difficult to predict. The occurrence of large flight events may be monitored with use of lights (shed/street) or an insect light trap.
African black beetle is not a major pest of established annual pastures typical of most of the south-west, despite the high density of beetles often found in them. This is because the pasture plants have died, with the onset of the warm, dry summer, by the time the beetle larvae are large and most voracious. Also, if the established pasture has been allowed to set seed, the resultant high seedling density can cope with moderate infestations of adult beetles.
However, African black beetle adults have been a problem during establishment of ryegrass pastures at the break of the growing season in autumn. If such areas of relatively low seedling density are infested with low to moderate numbers of adult beetles before the onset of cooler weather, many plants can be lost. The damage may be exacerbated by the habit of adult beetles feeding along seeder drill runs.
The situation may be different in perennial or irrigated pastures and turf. Visible damage results from very heavy infestations of larvae, more than about 80 per square metre. Healthy, well-fertilised and well-watered pastures and turfs are much more tolerant of African black beetle attack than those under stress.
African black beetle is difficult to control. The soil acts as a protective blanket and also makes it difficult to judge pest abundance. In susceptible regions of WA which is primarily in the higher rainfall areas adjacent to the south west and southern coasts, it is advisable to sample before planting. A density above 3 adult beetles per square metre should be considered a threat to susceptible crops. A rough estimate of beetle numbers can be gauged by using a spade and examining soil in squares the length of the blade of the spade, for example for a 15cm square spade sample, 44 such samples is a square metre.
ControlIf beetles exceed the threshold there are a number of control techniques to consider.
Chlorpyrifos is the most commonly used insecticide against African black beetle. If possible, incorporate the chemical to a depth of 15 cm immediately, using a rotary hoe to ensure thorough mixing. This maximises direct contact of the insecticide with the insects, which is essential for control.
Relying on the beetles to crawl through treated soil is not effective. Use the higher registered rates for heavy infestations. This is particularly effective in reducing stem damage, which kills plants.
Also consider applying this insecticide prior to a light rain event, to wash the insecticide from the soil surface to a depth of 15 cm, as this is where the beetles and larvae occur.
- does not persist in soil more than one week after application at a dose sufficient to kill newly arriving beetles,
- is not effective as pre-plant applications against beetle infestations later in the life of a crop,
- is not effective after only one application in high-risk situations and the need for a follow-up application should be considered.
An alternative to topical chemical applications are systemic insecticide seed dressings that have been shown in trials to protect newly sown pasture from damage by African black beetle adults.
Synthetic pyrethroids and insecticides from other activity groups apart from chlorpyrifos are registered for use on turf also for controlling this pest.
Cultural control techniques
The following are some cultural control techniques that could be considered:
prior to planting, keep the area as bare fallow for as long as is feasible, especially in terms of susceptibility to wind and water erosion,
consider removing kikuyu grasses well prior to seeding as this grass is a favoured food plant for black beetle and can sustain high beetle populations.
Other cultural control methods are not suitable for broadacre cropping but may be used in other crops:
Larvae of black beetle are susceptible to the commercially available entomopathogenic nematode Heterorhabditis bacteriophora. This is a relatively expensive option but is suitable for high value crops. High soil temperature and high soil moisture levels are required for the nematodes to be effective.Physical barriers
In vineyards, grow guards if buried can physically exclude adults from ringbarking young vines, which are susceptible for up to two to four years from planting, depending on the vigour of the vines.
The size of planting material for blue gum plantations, vineyards and olive groves makes them amenable to using plastic mesh sleeves. These sleeves make it impossible for the adult beetles to feed on the stem of the plants at ground level.Mounding
If damage to the stem of grapevines near ground level has occurred, mounding plant rows has enabled such plants to recover. This method may also be applicable to protecting blue gums and olive plants from damage by the beetle.
Planting into black plastic mulch on slightly raised angled beds may help reduce the numbers of adult beetles that are able to walk to the base of plants.
This is the Text only version of Farmnote 398 - full PDF Available
Page amended: December 2009