The Regulation And Interaction Of Migration And Reproduction In The Beet Webworm, Loxostege Sticticalis (Lepidoptera: Pyralidae)

The beet webworm, Loxostege sticticalis L.(Lepidoptera: Pyralidae) is a very destructive insectpest of crops and fodder plants in the Northern, Northeastern and Northwestern China. It is also along-range migratory pest with larval populations often exploding in regions receiving immigrants.Great yield and economical losses have been caused by the upsurge population of L. sticticalis duringthe26-years since1949. It is known that migration is a major life history strategy evolved fromadaption of the seasonal variations and a major cause for the frequent outbreaks of L. sticticalis.However, the cues that may induce the migratory flight behavior and roles of migration may also playan important role in the formation of outbreak population of L. sticticalis have been less understood inboth home and abroad. In order to demonstrate the environmental cues induced the migratory flightbehavior, and roles of migration played in the formation of outbreak population of L. sticticalis andprovide evidences that may raise the level of forecasting technology. The effect of temperaturesexperienced by the immature stages, and the supplemental nutrition on the migratory and reproductivebehavior, and the interaction between reproduction and migration were investigated by applying theprinciple and methodology of eco-physiology. The major results obtained are summarized as follows.The temperature experienced by the immature stages (from egg to pupae) is an important cueaffecting the migratory behavior of the adult L. sticticalis. The flight capacity, dry weight of flightmuscle, ovarian development and the preoviposition period (POP) of the adults developed from thetemperature of18,22,26and30℃were reared at22℃to determine the impacts of temperatureexperienced by the immature stages on the migratory flight behavior. The results showed that the flightcapacity (distance) of the adults developed from the4temperature conditions were not significantlydifferent during a12-h tethered flight test, even though the adults developed from the temperature of26℃possessed significant greater flight muscle than those derived from other temperatures. Besides, theovarian development or POP, oviposition period, and longevity of the adults developed from thetemperature of22and18℃were significantly slower or longer than that of adults developed from22and18℃. Thirdly, the oviposition period of adults derived from26℃was less than that of adults from22and18℃but was significantly greater than that of adults from30℃. The lifetime fecundity, matingfrequency and egg hatching rate of the adults developed from26℃were the greatest amongst the4treatments. Finally, the POP and longevity of the adults developed from the temperature of30℃aresignificantly shorter than those of the adults from other temperatures. All these results suggest that theadults developed from18and22℃intended to migrate while those developed from26and30℃intended to reproduce in the natal habitat.The roles of supplemental nutrition in the reproduction and migration of L. sticticalis wereconfirmed and the first2days of adult life were the final sensitive period for the adults to determine if they will migrate or not. Results from the present and absent of supplemental nutrition, and starvationdegree and in different stages of the adult life showed that the supplemental nutrition has a significantbut different effect on the migration and reproduction of L. sticticalis.(i) The POP, preovipositionperiod, lifetime fecundity, longevity, flight capacity, weight of flight muscle and content of triglycerideof the adults fed with only water were significantly shorter or less than that of the adults fed with10%glucose solution, indicating that the supplemental nutrition is crucial to both reproduction and migrationof L. sticticalis.(ii) POP of the moths starved at the first3days of the adult life was curtailed, incomparison with that of the control. However, POP of the moths was significantly prolonged when theirfood was deprived at the3to4day of the adult life.(iii) It was also showed the POP of the moths wassignificantly decreased when the moth was starved for24h at the1st or2nd day of the adult life while itkept unchanged when the moths were starved for24h at the3rd day of the adult life.(iv) It was showedthat the development of both flight and reproductive system is significantly affected when the mothswere starved for24h at the1st day of the adult life. The flight capacity, flight muscle weight, ovariangrade and basic ovariole width at the1st and2nd day post starvation did not significantly differ fromthat of the control moths at the same age. The flight capacity at day3and flight muscle weight at day3and4of the treated moths was greater than that of the controls, while the basic ovariole width at day4of the treated moths was significantly greater than that of the control moths with same age. The resultsobtained showed that the development of reproductive system is faster than the flight system althoughdevelopment of both reproductive and flight system are enhanced.These results implied that themigratory flight behavior of L. sticticalis could be changed at the first2day of the adult life, whichcould be considered as the sensitive period for the shifting of migrants into residents. That is, the mothswould be shifted from a migrant to resident when they do not have food during the first2days afteremergence.It is generally believed migration in most insects will stop once reproduction begins. However, theunderlying mechanisms have rarely been investigated and understood. In this study, the variations inflight capacity, flight muscle weight, and triglyceride content of adult L. sticticalis at1to5days aftercopulation or oviposition were investigated to understand the impact of reproduction on the flightcapacity. Flight capacity of the oviposited females was decreased as the number of egg lay increased orthe oviposition period extended (y=31.39-0.08x). Furthermore, flight capacity of oviposited femalewas significantly weaker than that of the virgins with same age. Flight capacity of the mated males, ascompared with that of the virgin with same age was also significantly declined although it was notsignificantly different within the observed period. Flight muscle weight in the oviposited females wasobviously declined as the number of eggs they laid or as their oviposition period increased. Besides, itwas significantly lighter in the ovipositing females than in the virgin females with the same age. Thedifference in flight muscle weight between the mated and virgin males was mostly significant althoughit was not significantly different between the days observed. Triglyceride content of the oviposited female was dramatically decreased and was only around10%of that virgin female at any day observedalthough it was not significantly decreased as their oviposition period extended or as the number of eggsthey laid increased. Triglyceride content of the mated males was also significantly less than that of thevirgin male although it was not significantly different between any days within the observed period.These results suggest that the migration will stop once the moths start to reproduce, which can provide areliable basis for judging the sources of adult population in the field.Identifying the reproductive consequences of insect migration is critical to understand itsecological and evolutionary significance. However, many empirical studies are seemingly contradictory,making recognition of unifying themes elusive and controversial. In this study,(i) reproductive costs ofmigratory flight, and (ii) reproductive traits contributing to larval outbreaks of immigrant populationswere examined. It is suggested that the beet webworm does not initiate migratory flight until the3rdnight after emergence. POP, lifetime fecundity, mating capacity, and egg hatch rate for adults thatexperienced prolonged flight after the2nd night did not differ significantly from unflown moths,suggesting these traits are irrelevant to the severity of beet webworm outbreaks after migration.However, the period of first oviposition, a novel parameter measuring synchrony of first egg-laying bycohorts of post-migratory females, for moths flown on days3and5of adulthood was shorter than thatof unflown moths, indicating a tightened time-window for onset of oviposition after migration. Theresulting synchrony of egg-laying will serve to increase egg and subsequent larval densities. A densepopulation offers potential selective advantages to the individual larvae comprising it, whereas the effectfrom the human standpoint is intensification of damage by an outbreak population. The strategy ofsynchronized oviposition may be common in other migratory insect pests, such as locust and armywormspecies, and warrants further study.The major discovery points of this dissertation are summarized as follows:1. It is demonstrated that the temperature experienced at the immature stages is an important cueaffecting the migratory flight behavior in L. sticticalis. The migration propensity is preferred by adultsdeveloped from18-22℃. On the contrast, adults developed from the temperature of>26℃prefer toreproduce in the natal habitat for their shorter POP.2. It is confirmed that supplemental nutrition is critical to the reproduction and migration of L.sticticalis. Both reproductive and migratory flight capacities are significantly decreased when the mothsdo not have food. POP is significantly decreased when the moths are starved at the first2day of adultlife. Neither reproduction nor migration is significantly affected when the moths are starved after twodays of emergence. Development of reproductive and flight system is speeded up when the moths arestarved at day1of adulthood, but the former is favored.3. It is found that the adults of L. sticticalis will not migrate when they are mated or oviposited. This is because the flight capacity, triglyceride content and flight muscle weight in both male andfemale are all remarkably decreased as the number of eggs they laid increased or their ovipositionperiod extended. Besides, any of these3parameters of the reproduced adults is decreased in comparisonwith the virgin male or females at the same age, regardless of the significance.4. It is demonstrated that there is no reproductive cost for the migration of L. sticticalis, andmigration of the moths initiates at the3rd day after emergence based on the evidences from the flightcapacity, and variations in major reproductive parameters in the moths experienced various flight test.Besides, migration can increase the larval population density and thus enhance their damages thoughdecreasing their period of first oviposition.