Dusty Surface Beetles

Pests of Field Crops in Southern Africa

DUSTY SURFACE BEETLE

(Coleoptera: Tenebrionidae)

Þ

Þ

Þ

Þ

Þ

Text Box: A small, unobtrusive beetle, which is often present in large numbers, the dusty surface beetle, Gonocephalum simplex, is sometimes capable of doing considerable damage to germinating crops. The beetle is brownish-black in colour, usually coated with a layer of dust, and is roughly rectangular and flattened in shape. It is about 10 mm long and 5 mm in width, and often feigns death when disturbed. The hard forewings, or wing-covers, are ridged longitudinally, and while membranous wings are present, the beetle has seldom been seen to fly. Eggs are creamy white and oval, being less than 1 mm long, and the larvae are elongate and cylindrical, with three pairs of legs on the thorax. They are straw-coloured, shiny and tough-skinned, reaching about 15 mm in length when fully grown. They are called “false wireworms” because they closely resemble the insects known as "wireworms" from colder climates, but which belong to a different beetle family (Elateridae). TOP

Text Box: The pest, in either beetle or false wireworm form, has been recorded as a sporadic pest of maize, wheat and other cereals, tobacco, cotton, sunflower, coffee, groundnuts, strawberries and potatoes. The beetle is often present within crops, vegetable beds and gardens in great numbers without doing any apparent damage. TOP

Text Box: Two types of damage by the beetle have been recorded. Young transplants, such as coffee or tobacco, or newly germinated crops of sunflower, cotton, or even maize, sometimes have their stem bases eaten into or partly ring barked. The beetles are also responsible for digging up and eating into the germ of dry-planted seed, particularly maize, thus reducing the plant stands. Serious losses caused by either type of damage are most likely under drought conditions, or, in the second case mentioned, whenever there is a long dry spell after planting. A third type of damage is caused by the larvae (false wireworms) feeding on roots in the soil. There have been cases of severe, although very sporadic, damage by the larvae of this species in wheat particularly, necessitating replanting of large patches within a field. The false wireworms seem to eat either into the actual seed, or they consume the developing roots and shoots. Wheat plants that have been damaged wilt and dry off about three weeks after planting. Early-planted summer crops (i.e. before the rains appear) such as cotton, groundnuts, early maize, paprika and tobacco may be damaged by quite large larvae which have not yet pupated. The damage is to the stems just below soil level, and has the effect of causing gaps in plant stands. Other species of false wireworm than the one described here may be responsible for similar damage. TOP

Text Box: G. simplex has one generation a year. The majority of eggs are laid singly in the soil from the end of the wet season (April onwards) and since the beetles are relatively long-lived, they may continue to lay in the early part of the dry season if conditions are suitable for them. The eggs take one to two weeks to hatch. The larvae live and feed in the soil from April right through the dry winter months, and then form a loose earthen cell in which to pupate. Peak emergence of the beetles occurs in about December, during the main rains, explaining why October-planted crops can suffer damage by the larval stages. Eggs that are laid late in the season give rise to adults that emerge later, so that there is considerable overlap of generations. The beetles tend to spend the daylight hours in mulch or amongst clods of soil, coming out to feed at night. As the season becomes drier, they may be found concentrated in irrigated fields, or under clumps of crop residue. It is probable that in drought conditions many beetles die before they are ready to lay eggs. TOP

Text Box: The presence of trash on the soil surface harbours the beetles, and it appears that certain crops, such as soyabeans and groundnuts, seem to favour their presence. The beetles also prefer the heavier red soils to sandy soils, and an infestation often remains and builds up in a particular spot from season to season, since they do not move very far. Damage by the beetles themselves can be expected particularly in maize that is planted after the first week in December, in dry ground. The beetles do not usually affect earlier plantings, since few will have emerged before this time, but these may be subject to serious stand-reducing damage by the larvae. Under conditions such as these, then, it would be wise to control the adults before damage can occur, and since they are very poorly controlled by insecticidal sprays, this has to be done either by seed dressings or the application of baits. The most commonly used, and probably safest, of the home-made baits, is made up of maize meal, sugar and trichlorfon. As with cutworm control, this should be moistened to a crumbly texture and spread as late in the day as possible, using at least 20 kg/ha. The addition of sugar to the bait greatly enhances it, making it even more attractive to dusty surface beetles than to cutworm. In South Africa, ready-made quinalfos bait is available. The commonly used seed dressing for maize is imidachloprid, and will give adequate protection from damage to the seeds, but may not eliminate an infestation of the beetles from a field. The baiting method can also be used effectively in controlling the beetles in coffee nurseries. When wheat is to be grown on red soils with a history of damage by the larval stage, particularly after soyabeans or groundnuts, it is advisable to control the beetles before egg-laying begins by the end of April. February would probably be the best month for this operation, before full canopy develops. Many beetles will be killed at the end of the season if the crop trash is gathered into windrows and burned, since they will shelter here as the season dries. However, this is usually too late to prevent egg-laying, and therefore damage by the false wireworms to the subsequent wheat crop. TOP

Dusty Surface Beetles: look carefully for the two in the middle.

False wireworm