Biological Habits Of Curculio Megadens And Its Adaptive Evolution Against Host Plant Defense

Plants and herbivorous insects belong to the most diverse biological groups on Earth.During long interaction,plants and herbivores have developed a series of defense and anti-defense strategies.Study on interaction between two strategies is a focus in evolution ecology.Taking a subject of a key pest Curculio megadens（Coleoptera:Curculionidae） damaging Fagaceae plants in the middle of Yunnan province（China）and can lay eggs directly in living host nuts with a referring species of Mechoris ursulu（Coleoptera:Attelabidae）,the author comparatively analyzed the behaviors,oviposition periods,egg characters,rostrum shapes of two species of insects through experiments in both field and laboratory together with field investigation.The mutual relationship between female’s oviposition of C.megadens and its host plant defense was meanwhile researched.The paper’s aim is to compare the anti-defense strategies of two weevils against the same host defense system and explore the evolutionary adaptation of C.megadens to avoid triggering the host defense system.The results are the follow:1.Sizes of adult,egg and egg chamber of C.megadens are lower than ones of M.ursulu,but their rostrum sizes are inverse;the adult of C.megadens emerged out of soil later than one of M.ursulu,the emerging peak of C.megadens is in August and one of M.ursulu is in the middle May;C.megadens and M.ursulu are of no difference in effect of temperature and host fruit water content on their egg development with egg period of seven days under constant temperature of 25℃;the soil depth of their mature larvae’pupariation are mainly 4 to10 cm underground.The results show that the insect C.megadens possibly resist against the callus’s squeezing egg in host fruits just by reducing sizes of its body and egg chamber,not by shortening egg period.It is early or late for two species adult to emerge out of soil is consistent along with that it is early or late for them to lay egg in their common host nuts.There was no relationship between the depth of the mature larvae in the soil and the emergence time of the adults in the next year.2.The ovipositing habits of female C.megadens observing in field and lab.discovers that preoviposition period of the female is about 30 days and its oviposition period in lab.is more than 60 days,and therefore the suitable period of every host for female’s laying-egg of C.megadens is shorter than the female oviposition period.So the host transfer of C.megadens was in field observed,and C.megadens earlier laid egg in nut of Q.serrata with early mature period than in nuts of Q.franchetii with late mature period.The earlier laid-egg fruits,which were chosen by females of the C.megadens,would drop down earlier.The four instars of C.megadens have been determined by the indexes composed of length and width of larvahead and its palate width measured under light microscope.These indexes are basic for identifying larval instar.3.The measuring and dissecting results of laid-egg nuts by C.megadens shows that it is random and is not related to large nut or small one for the female to lay egg in nuts.The female lays one egg to five eggs in one nut of Q.serrata,and however,one egg per a nut is common with 40 percent of all accounting laid-egg nuts and five eggs is less with only 1.4percent.There is neither linear relationship between laid-egg number per a nut and nut’s size of Q.serrata,nor linear relationship between mature larval weight of C.megadens and it.The experimental results also shows that it is not because the nut size of host is large for that the female of C.megadens lays egg into being-mature nut suitable for larval growth and development,and the causes are possibly that the female is to avoid the host defense response and the nuts in late development more easily fall off and do not produce defense materials.It needs the further study whether the such behavior of oviposition is related to avoiding host’s defense materials.4.Although female of C.megadens does not cut off laid-egg fruit branch,it damages part of host nut cotyledon.And however,nutrient substances in the damaged cotyledon does not obviously reduce compared to ones of the cotyledon without any damage.And only,Ca²⁺content as the second messenger obviously increases,which is not different from feeding feature of weevils with chewing mouthparts.With comparison of laid-eggs nuts by female of C.megadens from Q.serrata and Q.franchetii to the nuts without any pests,contents of defense messenger materials and defense ones do not obviously change.The analysis of defense materials with larva of M.ursulu,which shares the common host with C.megadens and is sensitive to host defense material show that the laid-egg nut by female of C.megadens does not induce the defense materials.The experimental results show that the behavior of female’s damaging nut cotyledon is not related to feeding during its oviposition,but related to the nut’s falling down earlier,during which a long and bending rostrum is evolved and plays an important role.Oviposition of C.megadens neither induce defence response of host nut,nor produce any defense materials affecting larval growth and development.