Snout Beetles

Pests of Field Crops in Southern Africa

SNOUT BEETLES

(Coleoptera: Curculionidae)

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Text Box: The small, dark snout beetles that attack maize and various other crops are well known to farmers, despite the fact that they are mostly active at night. All snout beetles can be recognised by the fact that the front of the head is drawn into a snout, and the antennae originate some way along this snout, rather than next to the eyes. They usually drop to the soil and remain motionless when disturbed, making them difficult to spot. Although there are several species, the three main ones in Zimbabwe are probably Systates exaptus, Tanymecus destructor and Mesoleurus dentipes, while in South Africa, Protostrophus spp. are common. S. exaptus is about 8 mm long, brown in colour, with a darker V-shaped mark bordered to the posterior by a whitish band on its back. It is quite broadly oval in shape compared to the next two species. T. destructor is about the same size, but is darker brown or black and much more slender, with no identifying markings. M. dentipes is about 6.5 mm long, shiny black in colour and has a light streak down each side of its body. Other species of Systates have been found on occasion in abundance in particular fields (notably S. smeei) and sporadic damage is also caused by a smaller, more stubby-looking species called Analeurops cuthbertsonii, particularly to young maize. Protostrophus spp. are from 4 to 8 mm long, quite stubby with a broad snout and are greyish-brown in colour. None of these beetles can fly. The larvae are white, C-shaped grubs found in the soil, but unlike "whitegrubs" of the scarabaeid family, they are legless, have an inconspicuous head capsule, and are less than 10 mm long when fully grown. TOP

Text Box: Most of the species mentioned have a very wide host range, although they may be of little real importance to many of their hosts. The beetles feed on foliage of soyabeans, groundnuts, lucerne, maize, sorghum, sunflower, coffee, deciduous fruits (particularly plums and apples), cotton, potato, sweet potato, tobacco, citrus, grape vines, strawberry, young Eucalyptus trees, as well as many garden ornamentals such as roses, dahlias, begonias and Plectranthus, and various flowering annuals. The grubs are probably indiscriminate feeders, and have been recorded as damaging maize, cotton, soyabeans and other seedlings, potato tubers and groundnut pods. TOP

Text Box: Damage by the beetles typically takes the form of "ragging" around the edges of leaves, but while this may be annoying and unsightly in the garden, it is normally of minor importance to crops. In Zimbabwe, unlike the other two species, T. destructor beetles can cause severe damage to maize by their feeding, since adult emergence tends to coincide with germination (see life cycles). The grubs of the other two species attack germinating maize seedlings by feeding on the underground stem, thereby causing stunting and occasionally death. Protostrophus spp. tend to emerge towards the end of the year (early in the season) and can cause extensive foliage damage to young crops in sporadic outbreak years. Once an infestation has become established in a field, it can be a recurring problem if successive crops are favourable for the grubs and beetles. Maize and soyabeans are particularly susceptible hosts. Snout beetles can also cause considerable damage to apples, since the feeding by the beetles causes unsightly scarring on the fruits as they expand . TOP

Text Box: Female S. exaptus lay about 20 eggs in a batch on the edge of a leaf where they have been feeding, folding a flap over to protect the eggs. This occurs in February or March. The eggs hatch in seven to nine days, and the tiny grubs fall to the soil where they feed on plant roots. Feeding continues throughout the season, but as the dry season approaches, the grubs cease to feed, and hibernate in cells in the soil. When the soil is soaked with the first rains of the next season, the grubs resume development and continue to feed, severe damage being caused to young maize or other seedlings at this time. They pupate two to four weeks after the rains start (December/January), and the pupal period lasts from nine to 12 days. Adults, then, usually emerge from the soil some time after the crop has germinated. The beetles are not immediately sexually mature, being unable to produce eggs until a week or two after emergence. They shelter in the funnels of the young maize or growing points of other crops during the day. The life cycle of M. dentipes is very similar to the one described, except that adults tend to shelter at the bases of plants under clods of soil. T. destructor differs to some extent in that the beetles emerge much earlier, shortly after the first rains of the season (November), making them the first snout beetles to appear. They too, shelter at the bases of the plants but are believed to lay their eggs in the soil rather than on the leaves. Protostrophus beetles lay their eggs during the summer rainy season shallowly in the soil, and these hatch in a week or two. As the soils dry out towards the end of summer, the grubs penetrate deeper and then pupate in October to November. Adults usually emerge in December and January, although earlier emergence can occur following irrigation in warmer years . TOP

Text Box: So far, it has proved almost impossible to protect crops from grub damage by direct chemical means. One has to kill adults soon after they emerge, and before they become sexually mature and lay eggs, with the result that few grubs will be present the following season. Carbaryl is generally used for this purpose, but as it is not very effective on all three species, a stronger chemical such as monocrotophos is sometimes recommended. Synthetic pyrethroids have also been used with success, but care should be taken to adhere to the restricted usage periods if these are recommended. On deciduous and other fruit trees, a physical barrier preparation such as is used for prevention of access by ants, can also effectively exclude snout beetles from the trees. This is a safe and environmentally friendly method, using the fact that the beetles are unable to fly, and usually consists of either a wrapping around the trunk with a thick, viscous grease band applied to it, or a baffle which is sprayed on its under side. There must be no tall weeds or low-hanging branches to allow the beetles alternate access routes into the tree canopy . TOP

Snout beetle and damage.

Typical snout beetle damage to leaf edges.