Note: Armoured ground crickets are at present regarded as the most serious pest of pearl millet crops in north central Namibia. COMMON NAMESarmoured ground cricket, armoured bush cricket, Gobabis prawn (English); /khînibeb (Damara); endindi (Herero); endindi (Dhimba); malindilindi (Oshiwambo); omahelindi (Oshikwanyama); koringkriek (Afrikaans)PEST STATUSAcanthoplus discoidalis may cause severe pre-harvest losses of pearl millet (Pennisetum glaucum) by feeding on the cereal heads. Damage has been reported from most of the northern regions of Namibia, where pearl millet is the staple cereal. The development of Acanthoplus discoidalis is in phase with that of pearl millet, which, combined with the size of the insect, the number of the crickets, and its preference for feeding on millet, causes severe problems. It was estimated that Acanthoplus discoidalis may cause a pre-harvest loss of up to 30% of the expected yield in northern Namibia.DESCRIPTIONThe eggs are laid in foam egg cases (egg pods) during April to May, with up to 15 eggs per pod. The eggs are dormant (stay in diapause) until early spring, after which embryos will develop in about 50% of the eggs between September and November. During dry years, only the latter eggs will hatch after rain, and the rest of the eggs will remain in diapause until the next wet season. The remaining eggs will hatch after good rain (>25 mm), with most eggs hatching within 20-28 days after heavy rain. Crickets usually hatch from December to January.Acanthoplus may have up to 8 development (growth) stages, with the immature (nymph) stages lasting about 70 days. The young crickets eat significantly more from the 4th instar (stage 4) onwards. At that time the mouthparts are well developed, which allows for feeding on more substantive vegetative growth and crickets may start immigrating into fields, where the cereals are then reaching the heading and flowering stage. Population build-up within fields may be rapid, and farmers may experience severe losses during the following weeks. If the Acanthoplus population is predominantly in development stage 5 or higher, they may cause particularly severe damage to millet during the milk-ripe stage. At this time the developing crickets require more protein-rich food and are feeding more selectively on millet heads. CONTROLArmoured ground crickets were traditionally collected and destroyed by hand. Manual control is labour intensive and only partially successful, and, with compulsory schooling, the main labour force (children) is no longer available. Due to the small size of most fields and cricket migration, pesticide application is not recommended.The best long-term solution is through early sowing and the use of short-term or bristled millet varieties. The crop may then be harvested before the crickets cause damage. An alternative method is induced egg-laying and egg-destruction: Millet heads should be harvested fairly early and then be allowed to dry on a platform in the field. Many female crickets will lay their egg packets below the platform, and the eggs may then be destroyed by plowing and tilling. That will reduce the number of crickets during the next growing season. This method has to be repeated every year in order to be effective. Tests for barrier spraying (to reduce immigration) and the spraying of crickets in the fields have not been successful. The best control is through bait application: A bait consisting of millet flour, fishmeal, and a pyrethroid pesticide, e.g. Bulldock, should be mixed. The bait should be liberally sprinkled around the field (against migrating crickets) as well as in the leaf sheathes of millet. The crickets eat the bait and are killed by the poison. If chemical control is found necessary, please contact the FSRE pest officers for precise advice on chemical control. The contact numbers and addresses are listed on the cover. OTHER NOTESIn most parts of Namibia, with the exception of the north-eastern Caprivi and Okavango regions, Acanthoplus are very common during March and April. Adults of all species are large (40-50 mm; 5 - 6,5 g) and reach very high population densities, although the southern and western species, Acanthoplus longipes, is less of an agricultural problem than the central and northern Acanthoplus discoidalis.The Ministry of Agriculture, Water and Rural Development is presently testing a low-cost monitoring and early warning system in north central Namibia. Acanthoplus egg pods are buried in open ground cages, and the number of crickets hatching is counted each day. The system may provide farmers with early warning about outbreaks.
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