The Morphological Characteristics, And Variations In Life History Strategy And Adaptive Capacity In The Melanic Form Of Oriental Armyworm, Mythimna Separata(Walker)(Lepidoptera:Noctuidae)

Melanism is one of the common phenomenons in the Lepidoptera. Afterreviewing the causes for the evolution of melanic form, the adaptivesignificances to the changing environment, and variation in behavioral andphysiology in the melanic form, we first1y investigated the melanism of theoriental armyworm, Mythimna separala (Walker), one of the most seriousmigrant insect pests of grains in China and Asia. The variations inmorphological characteristics, and the adaptability to food plants,population density and temperature in the melanic fOrm of M. seParata wereinvestigated in the laboratory. The major progresses obtained from thesestudies are summarized as followstThe morphological variation of the melanic form of oriental armywormpresented only in the moth, black in the body and wings appearance incomparison with the normal form. In the immature stages, the differences inmorphology were generally neglected. However, the larvae at the massrearing condition and pupae closed to emergence were blacker in color thanthose of the normal form under the same conditions. Genetic experimentsshowed that the inheritance of the melanic characteristics fOllowed aMendalian law, with the melanic allele recessive to the normal allele. Singlepair of genes from the same chromosome regulated the melaniccharacteristic. These results indicated that the melanic and normal form ofthe oriental armyworm still belongs to the same species, the melanic formhad already become a new genotype although we were not able to confirmwhat was the cause fOr the melanism of M seParata in this study.Life history strategy of the melanic form seemed to alter from amigrant to a resident in comparison with the normal form of M. separata.Evidences for this statement were that the melanic individual had asignificant shorter developmental period and a higher survival rate fOr theimmature stages. Melanic female had a significant shorter preovipositionperiod and greater fecundity than the normal form under the same normalexperimental conditions. For example, the preoviposition period for themelal1ic form was usually about 4 days after emergence, while that fornormal one was about 8 day, while the average fecundity for melanic andnormal form was about l798 and l0l9, respectiveIy. As a result of that, thepopulation growth index and relative fitness for the melanic form was muchgreater than the normal form. However, evidences from a l2 h tethered-flight test of the unfed newly emerged moths showed that the flightcapacity of the melanic moth was greatly decreased. The average flightdistance and duration for the typical moths were l0.0 km and 2.9 h whilethose for the meIanic one were only 4.6 km and l .2 h, respectively.Furthermore, the proportion of presumed "migrant" fOr the normal aduIt wasmuch greater than that of the melanic adult. These results suggested that thelife history strategy of the melanic form had changed from a migrant to aresident. Thus, the melanic form of M separata would have significantadaptive advantages under environment similar to the experimentaIconditions. On the other hand, the population of melanic form would beeliminated if the habitat were deteriorated, due to the poorer flight capacityof the meIanic moth.Variations in the amount of food intake by larvae and response to thehost plant species demonstrated that the fOod structure and nutritionaIphysiology of the melanic form had aIready changed. The food consumptionand utiIization rate for 1arvae of melanic form was much greater than tl1oseof the norma1 form during the fifth and sixth instar of larvaI stage. Themean relative growth rate fOr larval melanic fOrm was much higher than thatof the typical one suggesting that the 1arval melanic form could eat moreand grow faster. The variation in adaptation to the food plant demonstratedby the fact that the melanic form developed slower and had a lower survivalrate and pupal weight wh...