Bulletin of Entomological Research (1999) 89, 589–597 589 The host range, annual cycle and parasitoids of the African rice gall midge Orseolia oryzivora (Diptera: Cecidomyiidae) in central and southeast Nigeria C.T. Williams1*, O. Okhidievbie1, K.M. Harris1 and M.N. Ukwungwu2 1West Africa Rice Development Association/CAB International African Rice Gall Midge Project, c/o CABI Bioscience, Silwood Park, Buckhurst Road, Ascot, Berks, SL5 7TA, UK: 2National Cereals Research Institute (NCRI), Badeggi, PMB 8, Bida, Nigeria Abstract Host range experiments and field sampling in Nigeria produced no evidence that African rice gall midge, Orseolia oryzivora Harris & Gagné, can develop on plants other than Oryza species. Sampling in three outbreak areas during 1994 showed that the insect’s annual cycle varied according to the agroecological zone and rice cropping pattern. In the humid forest zone, Orseolia oryzivora persisted through the short dry season on ratoons of cultivated rice Oryza sativa at a rainfed site and on dry season rice crops at an irrigated one. In contrast, at rainfed sites in the moist savannah zone the pest survived the longer dry season on the perennial wild rice O. longistaminata, while ratoons and volunteers of O. sativa provided ‘bridges’ between the wild host and wet season rice crops. Early in the wet season at rainfed sites, galls of Orseolia oryzivora were not found at high density on wild rice, ratoons or volunteers. The heavy infestations which developed by October resulted primarily from rapid multiplication on rice crops themselves during the wet season. At all 13 sites sampled, the large majority of galls were found on fallow or cropped rice fields, rather than in ditches, bunds or uncultivated wetland, irrespective of the time of year or the hosts involved. From gall dissections, the parasitoids Aprostocetus procerae (Risbec) and Platygaster diplosisae Risbec caused over 30% mortality at some sites by October but generally increased too late to prevent crop damage. Implications of the results for the management of Orseolia oryzivora are discussed. Introduction which does not occur in Africa. Since the late 1970s outbreaks of O. oryzivora on wet season rice crops have The African rice gall midge, Orseolia oryzivora Harris and increased in Burkina Faso (Bonzi, 1980; Dakouo et al., 1988), Gagné (Diptera: Cecidomyiidae) is an indigenous pest of Nigeria (Ukwungwu et al., 1989; Ukwungwu & Joshi, 1992) rice in sub-Saharan Africa (Commonwealth Institute of and several other African countries (e.g. Alam et al., 1985; Entomology, 1984). It was first identified as a distinct species Taylor et al., 1995). Those in Nigeria have damaged over by Harris & Gagné (1982), having previously been confused 50,000 ha of rice, causing total yield loss on the worst with the Asian rice gall midge Orseolia oryzae (Wood-Mason), affected fields (Ukwungwu et al., 1989). In rainfed lowland conditions even moderate infestation levels can cause substantial losses (Williams et al., 1999). *E-mail: [email protected] Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 10 Jul 2017 at 16:17:01, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0007485399000747 590 C.T. Williams et al. Table 1. Locations, rainfall characteristics and agroecological zones of the Orseolia oryzivora outbreak areas studied, and the number of sites sampled in each area. Location Latitude and Annual Rainy Agroecological Number (nearest large town) longitude of town rainfall (mm)2 season3 zone of sites sampled Uyo, 05° 01’ N 2444 March to November Humid forest 3 Akwa-Ibom State 07° 56’ E Abakaliki, 06° 17’ N 1833 April to October Savannah/forest 6 Ebonyi State1 08° 04’ E transition Bida, 09° 06’ N 1183 May to September Moist savannah 4 Niger State 05° 59’ E 1 Until Ebonyi State was created in 1996 this outbreak area fell within Enugu State. 2 Average annual rainfall for 1985–1996. Data for the Uyo area were recorded at Onne, 90 km WSW of Uyo. Data sources: IITA Agroecological Studies Unit (Onne and Bida), Ikwo Agricultural College and Sudan United Mission, Ebonyi State (Abakaliki). 3 Months for which average rainfall exceeds 100 mm. Orseolia oryzivora develops from egg to adult in three to on O. oryzivora and O. oryzae and the abundance of different five weeks and a female can lay 100–400 eggs within two to grasses in the outbreak areas studied were used to decide three days of emergence (Descamps, 1956; Anon., 1987; which species to screen. Four species of Oryza and six other Bouchard et al., 1992; Umeh & Joshi, 1993). Eggs are laid on species of grasses (Poaceae) were tested (see table 3). The the plant surface and the neonate larvae migrate between three annual species of Oryza were grown from seed the leaf sheaths to attack the growing points of vegetative obtained from the IITA Genetic Resources Unit. The tillers. An infested tiller cannot initiate more leaves or a remaining species were grown from seeds, seedlings or panicle and instead produces a hollow gall, within which the rhizomes collected from the field. Plant identifications were larva develops. When the insect reaches the pupal stage the confirmed by the Weed Science Unit at IITA. gall elongates to form a tube, known as an ‘onion leaf’ gall, For each experiment, a cage containing potted plants of and within a few days the pupa matures, moves up the gall the test species at the seedling or early tillering stage was and cuts a hole near the tip, through which the adult midge heavily infested with adult Orseolia oryzivora which had emerges. The exposed eggs and neonate larvae require emerged within the previous 24 h in an insectary culture. humid conditions, so outbreaks are associated with wet The same number of midges from the same culture cages weather, and lowland (swamp) rice is more seriously were also introduced into a ‘control’ cage containing pots of affected than upland rice. tillering ITA 306, a susceptible variety of Oryza sativa. For The literature contains conflicting information on the five to seven days after infestation, both cages were shaded host range of O. oryzivora. Some authors give only wild and and the plants sprayed with water at two-hourly intervals cultivated rice (Oryza spp.) as hosts (Descamps, 1956; Anon., during the day to maximize survival of midge eggs and 1987) but Bonzi (1980) and Brenière (1983) include other neonate larvae. Tillers and galls were counted on the test grasses. And although some data are available on the pest’s species and the control plants 30 to 35 days after infestation. population dynamics on cultivated rice during the wet All apparently healthy tillers were then dissected to check season (Dakouo et al., 1988; Umeh & Joshi, 1993) little is for small galls, usually after a further two to three weeks. known of its ecology when rice crops are unavailable. The culture used was derived from galls collected from rice Descamps (1956) provides the only detailed observations on crops at IITA, Ibadan, and was maintained on ITA 306. the survival of Orseolia oryzivora from one rice-cropping season to the next, based on an early study in northern Field sampling Cameroon. Chemical control of O. oryzivora is difficult, costly and Three outbreak areas in different agroecological zones involves risks to human health, biological control agents and and with different seasonal rainfall patterns were selected other non-target species, so improvements to cultural, for study (table 1). All have had regular outbreaks of Orseolia biological and varietal control methods are urgently needed. oryzivora since the late 1980s. Three to six sites were sampled These require more data on the pest’s host plants, annual in each area, the number varying according to site size and cycle, natural enemies and ecology off the crop. To provide accessibility. The smallest sites were valleys less than 200 m some of this information, host range tests were carried out across. The largest were floodplains over 1 km wide. Twelve and year-round sampling was undertaken during 1994 in of the 13 sites were rainfed, so farmers only grew lowland three major outbreak areas in Nigeria. rice in the wet season, but one site near Uyo had year-round irrigation, so both wet season and dry season rice crops were grown. All the rice was transplanted except at two Bida sites, Materials and methods where some fields were direct-seeded. Around Bida and at one Uyo site both improved and traditional rice varieties Host range experiments were grown but at the others farmers only grew improved Simple ‘no choice’ tests of possible host plant species varieties. Most farmers applied mineral fertilizer but none were carried out in mesh-covered 80 3 80 3 80 cm insect used insecticide on their rice in 1994 (the year of the study). cages in a screenhouse at the International Institute of Planting of wet season rice was staggered over several Tropical Agriculture (IITA), Ibadan, Nigeria. The literature months at all sites. In 1994 the first fields were planted in Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 10 Jul 2017 at 16:17:01, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0007485399000747 Host range and annual cycle of Orseolia oryzivora 591 mid-June at most Uyo and Abakaliki sites and in late July at suspected hosts were genera mentioned in the literature as most Bida sites. The last were planted in August or hosts of O. oryzivora or O. oryzae, and grasses which were September. The harvest was also staggered, starting in particularly abundant.