Effects Of Larval Density On The Growth, Development, Food Utilization, Defense Capacity And Migratory Behavior Of The Beet Webworm, Loxostege Sticticalis (Lepidoptera: Pyralidae)

Larval density has been known as one of the principle environmental factors affecting migratory insect population dynamic. It has a reguratory effect on the insect population growth, larval food utilization and defense capacity and adult migratory behavior. So, understanding the larval density effects on the insect and clarifying mechanisms regularing population dynamic, are always the popular issues in the field of insect physiology and ecology research. The beet webworm, L. sticticalis is one of the major serious pests in the northern of China. In order to clarify the occurrence and damages of beet webworm, improve the forecasting and controlling skills and reduce its disaster loss for this pest. In addition, little is known about the larval density effects on the population dynamic of beet webworm. So this dissertation focuse on the effects of larval density on the insect growth, development and population growth, larval food utilization and digestive enzyme, larval anti-infection ability to B. bassiana and parasitic ability to E. civilis and the induction of adults migratory behavior by the ecological and physiological technique. The major results obtained from these studies are summarized as follows.Larval cuticular color, development period, survival, pupal weight and reproduction of the beet webworm, L. sticticalis were significantly different among five larval density treatments (1,10,20,30 and 40 Iarvae/650ml jar). When the density increased, lavral cuticle melanization score became greater, and the score was significantly larger when the density exceeded 10 larvae per jar. The larval development was also significantly affected by density. The development time of larvae and pupae at the density of 20 larvae jar was shortest, while it significantly prolonged with the larval density increasing. The survival rate of larvae to pupae was highest at the density of 10 larvae per jar, which was significantly higher than that from other density. Pupal weight was significantly different and negtively correlated to larval rearing densities. Larval density had also significant influence on the adult reproduction. The females developed from density of 1 and 10 larvae per jar showed the shortest and longest preoviposition period, respectively, and which was significantly shorten when larval density increased. The total fecundity of per female and the proportion of mating individuals were highest at the density of 1 and 20 larvae per jar, respectively, and they decreased as larval density increased. Adult oviposition period significantly shortened with larval density increased. The longevity of females and males was longest at the density of 10,20 larvae per jar, respectively, and they significantly were shortened when larval density increased. Larval density had also significant influence on the population growth index of beet websorm. The density of 10 larvae per jar was maximum for population growth index, while either higher or lower density was decreased. The results suggested that larval density is one of the major ecological factors for population growth.Both larval food utilization and the activity of digestive enzyme were affected by larval density in L. sticticalis. The change regularity of larval food utilization and digestive enzyme activity was evaluated among three typical densities. The larval food consumption and the amount of voided feces increased with larval density increased. But body weight gain decreased when larval density increased. The efficiency of conversation ingested (ECI) was significantly highest in the moderate density, but the approximate digestibility (AD) was not significantly affected by the larval density. Furthermore, larval density had a significant influence on the activity of protease enzyme. The activity of total protease and leucine aminopeptidase enzyme were significantly higher at the density of 10 than that of 1 and 30 larvae per jar, but there was no obvious difference in the activity of both enzymes between density of 1 and 30 larvae per jar. The activity of amylase enzyme was not affected by larval density.These results provided evidence for clarifying the different damage feature between common and outbreak population and the law of periodic outbreak.Effect of larval density on the larval anti-infection ability to B. bassiana and parasitic ability to E. civilis was reported in this research. Larvae of 5th instar in three typical densites,1,10 and 30 larvae per jar were used to test host resistance to B. tenella and E. civilis, the results indicated that after infected by B. tenella, larval mortality was the lowest at the density of 10 larvae per jar, which was significantly lower than that in other two densities. As larval density increased, the parasitic rate of fly was significantly rose. But the antibiotics capability to E. civilis was strongest at the density of 10 larvae per jar, and significantly higher than that in other two densities. Because both the mortality rate of L. sticticalis and the proportion of fly pupae from the density of 10 larvae per jar were significantly lower than that of 1 and 30 larvae per jar.The larval phenoloxidase (PO) activity, the number of total hemocyte and antibacterial activity of haemolymph were the major cause of the variation of larval denfense capacity. We evaluated the larval phenoloxidase (PO) activity, the number of total hemocyte and antibacterial activity of haemolymph in the 2nd day of 5th instar larvae. Haemolymph PO activity and the number of total hemocyte were significantly higher in the larvae from density of 10 larvae per jar than that in 1,30 larvae per jar. However, the antibacterial activity of haemolymph was higher in the larvae from density of 10,30 larvae per jar than that from density of 1 larva per jar. Furthermore, larval cuticular PO activity significantly rose as larval density increased, but midgut PO activity was not significantly different among three densities. In addition, after larvae parasitized by E. civilis, the pupal duration and reprodution and longevity of adults were not significantly affected in the different densities conditions. The results demonstrated that larval resistance to parasitoids increased within a certain density range, but it would decrease when larval density was too high.The larval density had a significant induced function on the flight potential of the beet webworm. Tethered-flight techniques were used to investigate the flight potential of 1-day-old adult beet webworm, Loxostege sticticalis L. (Lepidoptera:Pyralidae) reared at densities of 1,10,20,30 and 40 larvae per 650-ml jar. Larval density had a significant effect on the flight potential, including total flight duration, distance, average velocity and longest flight duration. Adults reared at a density of 10 larvae per jar and those reared in isolation displayed the greatest and poorest flight capacity, respectively, relative to the other density treatments. Females with greater flight potential usually showed longer preoviposition period, and flight potential was positively correlated with preoviposition period. Body weight, water content, and triglyceride content of the moths were significantly affected by larval density, whereas wing loading and glycogen content was not. Triglyceride content of adults reared at the different larval densities corresponded to the observed differences in flight potential, while water content and body weight were not related to their flight potential. These results suggest that larval density exerts a significant influence on preoviposition period and triglyceride content, which in turn influence flight potential of adult L. sticticalis.