Risk evaluation of three apple cultivars affected by Bactrocera dorsalis

Abstract:【Objective】In order to evaluate the risk of Bactrocera dorsalis to apples and to provide a ref-erence for the early warning and control scheme of apple production, the oviposition behavior of B. dor-salis on apples was studied. The correlation between larval development and fruit quality, the suitability of ovipositor and pericarp structure of three main apple varieties,‘Fuji’,‘Starkrimson’and‘Golden Delicious’were observed.【Methods】Four fruits of each variety were placed in the same cage, and marked on their petioles. Among them, two fruit stalks were upward and two downward. The distances among fruits were more than 5 cm. All fruits were weighed in advance. 100 healthy female adults of B. dorsalis were put in the cage at 09:00. And the number of adults on the fruits was recorded per hour. The numbers of spawning holes on each fruit were recorded 24h later. Then, the fruits were put in an ar- tificial climate box for observing the larva hatching and development, under the condition of（25.0±0.5）℃, with 60%-70% RH, and at 16 L: 8 D. The numbers of larvae were recorded at 3 instars, and 12 mature larvae were weighed randomly. Then the microstructure and surface of fruit skin were observed under an electron microscopy, and the ovipositor of adult B. dorsalis was observed under a scanning electron microscopy.【Results】Under indoor conditions, the female adults of B. dorsalis exhibited two visiting peaks on fruits, both at noon and afternoon during the day. For different apple varieties, the vis- iting patterns were different. Further comparing the female selection probability on three apple varieties by B.dorsalis,‘Fuji’was the highest, accounting for 50% , followed by‘Golden Delicious’, and‘Starkrimson’was the lowest, only accounting for 21.8%, far below the theoretical average of 33.3%. Visit volume was significantly positively correlated with the number of larvae in single fruit, flesh firm- ness and water content, indicating that some fruit quality would affect the attacking tendency by B. dor-salis adults. The number of B. dorsalis larvae in three apple varieties was in descending order:‘Golden Delicious’>‘Fuji’>‘Starkrimson’. There were significant differences among the number of spawn-ing holes, the number of spawning per hole and the number of larvae in 100g pulp for three varieties, the number of spawning holes was in descending order:‘Golden Delicious’>‘Fuji’>‘Starkrimson’, the number of spawning per hole was in descending order:‘Starkrimson’> Fuji >‘Golden Delicious’, and the number of larvae in 100 g pulp was in descending order:‘Golden Delicious’>‘Fuji’>‘Starkrimson’. There was a significantly negative correlation between the number of single fruit larvae and the weight of larvae on different apple varieties, which indicated that the nutrition supply of fruit was insufficient when the number of larvae was too large. There was a significantly positive correlation between the number of spawning holes and the content of starch, and a significantly negative correla- tion between the number of spawning per holes and titratable acid content. Larval weight was negative-ly correlated with water content, and the correlation degree was higher. There were significant differenc-es in crack length, width and waxy layer thickness on the skin surface among different apple varieties. The crack lengths of‘Golden Delicious’and‘Fuji’were significantly higher than those of‘Starkrim-son’, and the crack width and wax layer thickness of‘Golden Delicious’were significantly higher than those of two other varieties. In the microscopic state, the surface of‘Starkrimson’skin was the most compact, almost no gaps could be seen, and the skin holes on the surface were scarce. The ovipositor of B. dorsalis was the most likely to enter the flesh through the surface cracks of‘Golden Delicious’, fol-lowed by‘Fuji’. It is difficult to find the position of spawning on‘Starkrimson’because of its few and thin cracks. The number of spawning holes was positively correlated with crack length, width and waxy layer thickness, and negatively correlated with the number of cell layers. It was further demonstrated that the skin structure could affect the oviposition of B. dorsalis.【Conclusion】The ovipositor of B. dor-salis was slender and inserted into the flesh to lay eggs. The oviposition time and position of adult on apple fruit would be affected by many factors, such as fruit color, shape, pericarp structure, maturity, and starch contents, etc., which can give full play to the interaction of vision, smell and touch system. For apples, B. dorsalis preferred the fruit with thin and cracked skin than the fruit with thick and seam-less skin. It is most beneficial for larval growth and development when flesh hardness was low and starch and titratable acid contents were moderate. According to the results, once the B. dorsalis invaded and colonized to the north of the Yangtze River, it would cause a serious threat to apple, especially‘Fu-ji’, which had the largest planting area, and must be strictly controlled. Strengthening quarantine moni-toring, implementing fruit bagging and physicochemical trapping were necessary. In the attacking area by B. dorsalis, appropriate pesticides must be applied before and after bagging of‘Fuji’to prevent B. dorsalis adults from laying eggs on fruits, which would affect fruit yield, quality and marketing.