The sorghum midge, Contarinia sorghicola (Coq.), in Nigeria

Abstract

Sorghum midge, Contarinia sorghicola (Coq.), was discovered in Nigeria in 1953, and a survey which indicated the widespread occurrence of the midge in the country was followed by the investigations reported in this paper.Food-plants of the midge in Nigeria are guineacorn (Sorghum vulgare, sensu lato), which annually provides about two million tons of grain for human consumption, and the wild grasses, Andropogon gayanus and Sorghum arundinaceum.At Samaru, Zaria, Northern Nigeria, midges emerge from infested guineacorn heads in the early morning with maximum emergence between 7.45 and 8.15 a.m. After mating, females fly to recently flowered heads where they lay eggs within the spikelets, laying twice as many eggs on the glumes as on the pales. Each female may lay about 50 eggs and both males and females usually die within ten hours of emergence. The egg hatches within four days and, after ten days' feeding, during which the ovary shrivels up, the larvae pupate within the spikelet. The cycle from egg to adult is completed in 19 to 22 days during the growing season but towards the end of the rains larvae spin cocoons and enter diapause.Large numbers of diapause larvae are carried through the dry season in late-flowering heads which, because they are severely attacked by midge, are not harvested and remain on the stems which are kept in stacks and used for building and fencing or as fuel. Smaller numbers of larvae are present in threshing trash. The emergence of adults from the diapause population was observed from 1955 to 1959. In cages, the first adults were observed to emerge half way through the rains, about 5 to 7 weeks after the weekly mean R.H. had exceeded 60 per cent, and emergence continued for 9 to 12 weeks. Field observations confirmed experimental observations and showed that the build-up of the midge population before the main crop comes into flower is largely dependent on the presence of early-flowering varieties. So long as sufficient guineacorn is in flower and the weather is favourable the midge population builds up steadily to a peak in October and early November.A. gayanus appears to be of little importance as an alternative food-plant, and preliminary evidence of the existence of a distinct biological race of C. sorghicola on A. gayanus is presented. Wild sorghum (S. arundinaceum) may be an important alternative food-plant in the south, where it is most abundant, but is of little importance in the main guineacorn-growing areas of the north where it is uncommon. Eupelmus popa Gir., Eupelmus sp., Aprostocetus sp. and two species of Tetrastichus parasitise C. sorghicola, and examples of the three genera are generally present at Samaru in the ratio 3 of Eupelmus: 1 of Aprostocetus: 1 of Tetrastichus. They are only of importance late in the season. Two spiders, a species of Thomisus and a species tentatively referred to as Diaea, prey on ovipositing midges but are apparently of little importance.Experiments show a significant negative linear regression of yield on the proportion of spikelets attacked. There is no evidence of compensation and estimation of the proportion of spikelets attacked by midge gives a direct measure of the loss of yield. In 1957 and in 1958, random samples of guineacorn spikelets were taken from farmers' crops throughout Northern Nigeria. The proportion of spikelets containing midge larvae and pupae was estimated by dissecting 50-spikelet sub-samples taken at random from each sample, and the maturity of the sample was measured in each sub-sample by counting the number of spikelets which had not flowered. Samples should have been taken between 7 and 21 days after 50 per cent, of the crop had flowered, but many of the samples were immature and were discarded. In 52 mature sub-samples obtained in 1957, 10·5 per cent, of all spikelets contained midge and, in 167 obtained in 1958, 4·1 per cent, contained midge. In 1957, the results did not permit estimation of over-all crop losses but in 1958 it was estimated that at least 91,100 tons of grain, valued at £1,822,000 and representing the produce of 218,200 acres, were lost to sorghum midge.The pattern of intensity of midge attack in the survey samples suggests that in the main guineacorn-growing areas, which lie above 9°N. at an altitude exceeding 1,000 ft., midge damage is less severe than in lower latitudes and at lower altitudes where heavier attacks probably result from the longer growing seasons, higher humidities and more abundant wild sorghum.Nigerian farmers recognise the empty heads caused by midge but are unaware of the midge itself and, until they have learned to recognise cause and effect, control measures depending on their co-operation may be unsuccessful. Cultural control by disposal of crop residues and the growing of a uniformly flowering crop would be the most natural and effective method in the main guineacorn-growing areas. There is little possibility of achieving chemical control at present and, though the field resistance of the Nunaba group of varieties (Sorghum membranaceum) from the Gold Coast has been confirmed in Nigeria, laboratory experiments suggest that when the midge is not free to choose between resistant and non-resistant varieties it is able to adapt its behaviour and will then oviposit on resistant varieties.This paper is intended to serve as a basis for further studies of sorghum midge in Nigeria and elsewhere in West Africa.