Study on the oviposition preference of the litchi fruit borer, Conopomorpha sinensis Bradley (Lepidoptera: Gracillariidae)

Astract:【Objective】The litchi fruit borer (LFB) , Conopomorpha sinensis Bradley, is the most distructivepest of litchi and longan in China and causes severe damage and yield reduction during the litchi's fruitstage due to boring by larvae in the fruit. Control of LFB by spraying pesticides is hampered because thelarvae live inside the plants. In addition, most pesticides kill a large number of beneficial natural enemiesand result in pesticide residues. LFB is less harmful at the litchi shoot stage with its population rapidly de-creases. The adults prefer litchi fruits over shoots for oviposition and lay fewer eggs on leaves than fruitsunder natural conditions. The purpose of the present study is to investigate the oviposition preferences ofLFB.【Methods】In this study, factors such as litchi fruits, surface texture, color, shape and size, litchiyoung and old leaves, were conducted to test for their attractancy to ovipositon of LFB adults. The oviposi-tion preference test design in the laboratory was used to examine the preference by observing the amountof eggs that were laid by the LFB on the different oviposition media. The cultivar of the litchi used to con-duct the test was‘Feizixiao'and it was collected from the Agricultural Scientific Experiment and Demon-stration Farm, Guangdong Academy of Agricultural Sciences. With references to the experimental meth-ods from former studies, the oviposition media made from litchi fresh and dried fruit, litchi young leavesand old leaves were used as the treatments in the test. To test the oviposition preference of the LFB to li-tchi fresh and dried fruits, two plastic cylinder containers were filled with a litchi fresh fruit and a driedfruit respectively and sealed with tissue paper to be used as the oviposition media. They were then put in-to the cage together to observe the amount of eggs laid by the LFB adults on their surface with a litchifresh fruit and a dried fruit used as the control. To test the oviposition preference of the LFB to surface tex-ture, seven litchi fresh fruits wrapped in 80, 100, 200, 300, 400 mesh screens, copy paper and tissue pa-per respectively were used as the oviposition media and were put into the cage together to observe theamount of eggs laid by the LFB adults on their surface with a litchi fresh fruit used as a control which waswrapped in tissue paper. To test the oviposition preference of the LFB to litchi young and old leaves, threeplastic cylinder containers filled with a litchi fresh fruit, young leaves and old leaves respectively and allsealed with tissue paper were used as the oviposition media and put into the cage to observe the amount ofeggs laid by the LFB adults on their surface.【Results】The fresh litchi fruit induced egg laying by theLFB adults, even when it was placed in a plastic cylinder container and there was no contact, adults stilllaid their eggs on the surface of the container, where the amount of eggs showed no significant differencefrom those that were laid directly on the fresh litchi fruit. But LFB adults did not lay any eggs once thefresh fruit was sealed with parafilm or replaced by the dried litchi fruit. It suggested that the ovipositionstimuli were primarily from the volatiles of the fresh fruit. Surface texture affected ovipositon preference ofthe LFB adults as well. The adults laid many more eggs on the surface of 200, 300 and 400 mesh screenthan the surface of 80 and 100 mesh screen and copy paper. The pericarp of the litchi fruit presents in-tense protuberances between which small crack textures appear in the surface. Under natural conditions, LFB adults usually lay eggs in the crack texture on the surface of the litchi fruit. It suggest that LFB adultsmay prefer to lay eggs on a rough surface with a small crack texture and that egg laying was reduced whenthe surface became smooth or the crack texture was too large. However, surface texture did not directly in-duce egg laying. Without the stimuli from the volatiles of the host plant, LFB adults did not lay any eggson the surface even when its texture was appropriate for egg laying. In addition, the volatiles of litchiyoung and old leaves induced egg laying as well as those of the fresh fruit. LFB adults laid eggs on the sur-face of the container used as the oviposition medium which had the same weight no matter if litchi youngleaves, old leaves or a fresh fruit was placed in it. The amount of eggs laid on the above three containershad no significant differences. However, the LFB adults had no oviposition preference to visual factors, such as color, shape and size.【Conclusion】The odors of litchi fruit, young and old leaves can induce egglaying by LFB adults and the oviposition preference of the adults was influenced by the surface texture.That the surface of the leaves was too smooth was one of the reasons why the LFB adults laid significantlyless eggs on leaves than fruits under their natural conditions.