Whitegrubs

Pests of Field Crops in Southern Africa

WHITEGRUBS AND CHAFER BEETLES

(Coleoptera: Scarabaeidae)

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Text Box: Whitegrubs are the larval forms of scarabaeid beetles. They are nearly always found dwelling in the soil and often feeding on the roots of plants. They are not to be confused with "cutworms" which are the larval forms, or caterpillars, of certain moths. A common damaging species is the larva of a beetle known as Eulepida mashona, or as "mandere" in the Shona culture. Some details on this species will follow. The "chafer" beetles or “rose beetles” that commonly cause damage to leaves during the wet season also belong to the scarabaeid family, but their larvae, while answering to the same description as will be given here, are smaller and much less damaging in the juvenile stages than this species. The grubs of Eulepida mashona are C-shaped and dirty white in colour, with a dark, reddish-brown head. When they have been feeding, the rear end of the body usually takes on a greyish appearance, due to the quantities of soil they consume. They have three pairs of legs and large biting mandibles. Pupae are found in earthen cells usually a little deeper in the soil. They are inactive, creamy white creatures, which are intermediate stages between grub and adult. The beetles are about 3 cm long, reddish-brown in colour and, when newly emerged from the soil, have a covering of yellow bristles. They are commonly found swarming around “Msasa” trees at dusk during the first rains. TOP

Text Box: Whitegrubs feed on the roots of a very wide range of plants. Some that have been recorded are pasture grasses, napier fodder, maize, tobacco, wattle saplings, potatoes, tomatoes, asparagus, sunnhemp, groundnuts, strawberries, pineapples and many vegetable and garden plants, including lawns. They are a sporadic pest of maize on sandy soils. The beetles eat the foliage of Msasa trees (Brachystegia spiciformis) and Mnondo trees (Julbernardia globiflora), but they are short-lived and do little damage. A variety of chafer beetles (Adoretus, Anomala and Schizonycha spp.) can cause foliar damage in the adult stage to grapevines, maize, soyabeans, deciduous fruit, roses and many other ornamentals. TOP

Text Box: The natural food of whitegrubs is almost certainly decaying plant material in the soil, and here they would be regarded as an essential contributor in the recycling of organic materials. Indeed some species, such as the large "rhinoceros beetle" larvae (Oryctes sp), live exclusively in compost and manure heaps. However, when humus is in short supply and large areas of crops are planted, they happily feed on plant roots. The feeding of the grubs in lawns or pastures causes yellowing patches where the grass eventually dies, and may even be rolled back like a mat due to the complete destruction of the root system. Other plants simply wilt and die, such as groundnuts and strawberries, or, if they are tall, they lodge, such as maize and napier fodder. The grubs also bore into potato tubers, apart from eating through roots. They can usually be found in the vicinity of the root system, so that it is not difficult to identify the cause of the problem. The feeding of various chafer beetles occurs at night and takes on a characteristic form. They tend to make running "zig-zag" holes within the blade of the leaves, not at the leaf edges, as do snout beetles. At times the damage can be extensive, but on the whole it is probably more unsightly than economically important on field crops. Garden roses can be damaged particularly badly, often with almost total foliage loss and a great deal of damage to the flower buds. TOP

Text Box: The beetles emerge from the soil with the first soaking rains of the season, in about November. It is at this time that the adult Eulepida mashona can be seen (and heard!) swarming around Msasa trees towards dusk. Copulation takes place soon after emergence and the females return to the soil to lay eggs at depths of 10 to 22 cm. Freshly ploughed land is thought to be an attractive oviposition site, due to the ease with which the beetles can burrow into it. Sandy soils tend to be more readily affected than heavier soils. In pastures, it may be dung that is attractive to the female beetles. The female lives for two to three weeks, during which time 30 to 60 eggs are laid. Beetles may be seen clustering among the tree foliage during the day, and only become active towards evening. The incubation period of the eggs is 14 to 18 days, after which a tiny white grub emerges. These larvae feed on roots and moult four to five times before attaining their full size in about May (about 5 cm long, although curled into a C-shape). It is usually from about March onwards that the grubs are first noticed because the damage to plants becomes obvious. With the advance of the dry season, the mature grubs usually penetrate deeper into the soil and in late September/early October they pupate in earthen cells. After some weeks, adults are fully developed, and await a good soaking rain before they emerge. Thus each generation lasts a full year. The life cycle of the chafer beetles mentioned takes a very similar form to that described, but it is possible that the smaller species may actually have two generations in a year, rather than one. TOP

Text Box: Control of soil-dwelling pests has always been a problem, and this was only really solved when the organochlorine insecticides came onto the market. They worked well for the very reason that resulted in their banning – they lasted several months, if not years, in the soil, obviating the need for repeated applications. Other newer chemicals, which do not last nearly as long, have shown some efficacy, but they must obviously be timeously applied, since they need to be in the soil when the grubs begin their cycle. In addition, since the grubs are subterranean, surface application of the chemical is often ineffective, and the properties of individual chemicals have to be carefully taken into account (soil mobility, solubility, adsorption to clay particles, etc). And lastly, since white grub infestations are sporadic and patchy, preventative treatment of whole fields would be wasteful, not to mention very costly. So, what can be done? Firstly, white grubs prefer sandier soils, and certain areas tend to have a history of infestation. If a field comes into this category, and plants show signs of possible root damage, check whether an infestation is present by digging for the white grubs. The best time to do this would be from about February, when the grubs are medium-sized and have not moved down too deeply. This would at least answer the question of presence, if not control. If any new plants are being planted at this stage, at least there is a reasonable chance of incorporation of insecticide into the planting holes. For those who are already aware of the problem, ploughing during the winter would expose many grubs to the sun and to predators, since the grubs are in the middle of their cycle at this time. It would, however, only deal with the current year’s infestation and would not stop re-infestation in the summer. New plantings going into the field at the beginning of summer could receive an application of insecticide that is well incorporated into the planting hole. This should give protection from the new generation of white grubs. Among the chemicals that have shown some control of white grubs are the following: Aldicarb granules – a systemic insecticide to be incorporated; Carbofuran granules – a systemic insecticide to be incorporated; Chlorpyriphos – a contact insecticide to be incorporated; Ethoprophos granules – a contact insecticide to be incorporated; Fipronil – a contact insecticide to be incorporated; Imidachloprid. – a systemic insecticide with reasonable soil mobility; Isofenphos– a systemic insecticide with reasonable soil mobility. It should be noted that most of these are EXTREMELY TOXIC, making their use undesirable. Most of them have very limited registrations for use and would make a costly exercise if prevention were the aim since one can not know in advance where the infestation limits will be. Another tactic that has been considered is to watch for the emergence of the beetles after the first main rains, and to apply contact insecticides to the foliage of the trees where they swarm. Since indigenous trees are the hosts, it would be a fairly impossible task to cover all the possibilities, not to mention being an extremely undesirable exercise in terms of environmental care. Newer approaches to control include research into controlling some pests with naturally occurring fungal diseases, known as myco-insecticides, which are far more environmentally friendly than the conventional insecticides. A good deal of success has resulted along these lines with control of locusts, and this may well be an avenue of promise for some of the soil pests. In addition, further approaches for research could include the application of mulching sprayed with repellents, on the grounds that the female beetles have to penetrate the soil to lay the eggs that start a new generation. Little research is currently being carried out on white grubs, however, mainly because of the sporadic nature of infestation, which makes chemical control a chancy and costly business. TOP

Whitegrub

Chafer beetle life cycle. Only the winged adult is found above ground.