Zoology and Entomology
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Item Open Access The bio-ecology of the grass leaf miner, Agromyza ocularis (Diptera: Agromyzidae), on wheat and barley in the Northern Cape Province, South Africa(University of the Free State, 2010-06) Adendorff, Joan; Prinsloo, G. J.; Louw, S. vdM.English: The grass leaf miner fly, Agromyza ocularis (Spencer) (Diptera: Agromyzidae) was first described from specimens collected at Ceres (1959), Giant's Castle (1961) and the Maseru (1963) district. These specimens were collected with a sweep net from grass and no information was recorded on the host range and biology of the species. During 2000, A. ocularis was recorded on irrigated wheat in the Prieska district, Northern Cape and since then the species has spread to many wheat and barley fields in the Prieska and Douglas area. Two distinct types of injury are caused by A. ocularis, namely feeding punctures and leaf mining. The adult femail cause's circular punctures on the upper leaf surface of the host plant with her ovipositor, the ovipositor puncture can provide a feeding or egg-laying site. Mines are caused by the larvae feeding on tissue inside the leaf. The mine commences at the egg laying site where the larvae hatch. The mines are usually linear in the direction of the stem attachment or apex of the leaf. The area mined is dead, necrotic tissue and could not be revived through plant growth compensation. Photosynthesis rate by the leaves is therefore diminished. A mature larva cuts a slit in the leaf epidermis, escapes from the mine, drops to the ground and burrows into the damp soil adjacent to the plant and pupates. The pupal stage can last 23 days at 25°C under laboratory conditions. A pupal diapause stage which lasts for ±10 months in the laboratory can occur. The function of this stage is not clear, but it could be a mechanism to survive through periods when food resources are not readily available and extreme high temperatures occur. The lifespan of the adult fly is 30 days at 25°C, also under laboratory conditions. This can lead to overlapping generations of flies in a field and increase of damage to the crop. The flies were found to be active in the field when mean daily temperatures ranged between 10°C and 30°C and high relative humidity prevailed due to irrigation. This could be the optimal conditions the flies need to survive and thrive. A number of alternative host plants for A. ocularis were identified, i.e. Phalaris minor (small canary grass), Bromus catharticus (rescue grass), Lolium perenne (perennial rye grass) and Avena fatua (common wild oats). These grasses were recorded on the edges of the wheat fields during the wheat growing season, but not during the peak of the summer or in the natural vegetation surrounding the fields. The appearance of the flies in the cultivated fields differed by 14 days between 2008 and 2009 growing seasons. In spite of this difference, the crops in both years were older than plant growth stage (GS) 5 (Joubert scale) and the plants had already entered the stem elongation phase when the leaf miner outbreak occurred. It thus seems as if the appearance of flies is not linked to the specific plant growth stage, but rather to climatic conditions. At plant GS 5 the number of tillers per plant is already set and the plants' energy is utilized to initiate the number of heads per plant and the number of florets per head. If the plant is heavily damaged by this time, the number of heads and florets per plant will be influenced. The leaf miners commence oviposition on the lower older leaves of the plant, which are then obviously mined first. The rest of the leaves are attacked as they develop. In 2008, 100% of both barley and wheat plants sampled in the Douglas area were damaged by leaf miners when evaluated at GS 5. This damage intensity continued in all subsequent investigations. The number of damaged tillers per wheat plant increased from 48% - 63% between GS 5 and GS 17, the latter being the stage immediately prior to flowering. The number of damaged wheat leaves per tiller increased from 33% - 58% between GS 5 and 17, whilst indices ranging from 10% to 100% of leaf area damage were recorded. This amply demonstrates the severity of damage that is caused by the leaf miner. The damage varies, however, between fields and between years and could be due to specific spatial and temporal parameters. Damage to the crop and its yield is thus expected. However, insecticide trials conducted under similar conditions during 2008, demonstrated no difference in yield, albeit that a number of larvae were still present per tiller. The plant therefore seems to be able to compensate for the damage inflicted by the leaf miner. If the optimal conditions under which the crop is produced, i.e. sufficient water and fertiliser, is considered, this could be possible. The testing of different insecticides in field trials between 2007 and 2008 provided variable results, with a double dosage of Unimectin® resulting in 80% reduction in larval numbers on barley, which is the only reduction figure which is according to the pesticide registration application (Act 36 of 1947). In 2008 only Abamectin® double dosage met these standards and was the most successful in larval reduction, resulting in figures of 53% - 85% on barley. In spite of all these variables no significant decrease or increase in yield could be measured on any of the treatments in any of the years, suggesting that the plants could absorb the damage levels through compensatory growth.