Mechanism For The Population Increase And Geographical Range Expansion Of The Rice Water Weevil, Lissorhoptrus Oryzophilus Kuschel

The rice water weevil, Lissorhoptrus oryzophilus Kuschel (Coleoptera: Curculionidae), which is native to mid and eastern parts of North America and invaded China in late 1980s, has now become an important insect pest of rice in China. To date, there has been much research on the population biology and management strategy of this pest in this country, but little is known about its invasive and subsequent population expansion mechanisms. This paper studies the implied importance of elytral color morphs and reproductive models (parthenogenesis or bisexual reproduction) of this weevil to its reproductive and spatial expansion capacity, and the effects of major ecological factor including host plants and temperatures on its population development. The aims of this study are to determine the partial mechanisms responsible for its successful invasion and population-increasing worldwide, and thus to provide guidelines for the monitoring and management of this pest.1. Elytral color dimorphism in rice water weevil and it's relationship with female reproductive developmentThe rice water weevil has two elytral color morphs with central pattern of their elytra being black (dark morph) or grey (light morph). Adults were collected from Beaumont, Texas (USA), Yueqing, Zhejiang and Tanghai, Hebei (China), and their color morph frequency was surveyed. At Beaumont TX, both dark and light morph could be easily found with the frequency being 31.6-45.8% and 54.2-68.4%, respectively; in the female, the two morphs have a similar frequency. However, both at Yueqing and Tanghai of China nearly all of females are in dark morph with the light being only 0.5% in frequency. Moreover, at Beaumont, dark males accounted for only 3.5-7.4% of the populations, considerably lower than that of light males (28.8-32.0%). Dark and light female have a similar mating status and ovarian development stage, which suggests that elytral morph in this weevil might be little related to female reproductive development2. Comparisons on the reproductive capacity of females in different elytral color morphsIn this study, females of the two morphs were collected near rice fields in Southeast Texas during April and May 2005. For females supplied with males for 2 d or kept solitary, and then reared on rice seedlings for 48 d, no significant differences were found between the two morphs in oviposition period, number of eggs deposited and survival rate. In both morphs, a proportion of mated females did not oviposit throughout the rearing period, implying that a mating experience might be necessary before reproductive development can be initiated. On the other hand, oviposition occurred in a proportion of females in which no mating experience could be detected and their eggs produced larvae, which suggests the probability of existing parthenogenetic females in southeast Texas.3. The infection of wolbachia in adults of different elytral color morphs and the effect of high temperature on it's detectionThe 16s rDNA sequences in the weevil were detected, cloned, and sequenced with the differential primers 16S rDNA, 76-99F and 1012-994R. The results show that Wolbachia infection extensively occurred in adults of this weevil collected from Texas and California of the USA, and Zhejiang and Hebei of China. The infection in light morph adults was less than that in dark morph ones, implying that females in light morph might have a low potential of becoming Parthenogenetic individuals as compared with those in dark morph. In addition, it was found that higher temperatures and longer disposal time are propitious to wipe off this kind of bacteria in this weevil.4. Diapause in adults of the weevil from southern and northern ChinaThe diapause development of this weevil collected from Yueqing and Tanghai, China was determined. It was found that diapause in adults collected from Yueqing terminated around early September, and at this time 95% of the adults began to onset oviposition after 3 weeks of feeding rice, but for the adults collected from Tanghai, their diapause terminated around early November being much later than the Yueqing population. To determine the effects of temperature on diapause termination, diapausing adults were firstly exposed to constant temperatures (28-8℃), and then transferred them at intervals o a climatically controlled chamber (28±1℃, using rice seedlings as adult hosts) where the dynamics of oviposition onset and the numbers of eggs deposited were observed. The result showed that, at 28℃the diapause in most of adults didn't terminate after 60 d, while the temperatures 20℃and 16℃were much more suitable to terminate the diapause. When exposed to 20℃, both Yueqing and Tanghai populations began to oviposit in 90% individuals at 3 wk after being transferred to 28℃and reared with rice seedlings. The results indicated that the weevil enter summer diapause in both of the two populations examined here, but their diapause terminated prior to winter and maintain in a dormant state during the overwintering.5. Reproductive capacity of 1st-generation adults of the rice water weevil with the effect on different plants and emerge timeFirst-generation adults of the rice water weevil were collected from Yueqing during late June and mid-July, and reared on rice plants (cultivar Shanyou 63) in the laboratory (28±1℃, 16L:8D) to examine their reproductive capacity. On plants aged 16-22 days, field adults collected in mid-July could reach oviposition onset more rapidly and deposit more eggs than those collected earlier. Adults that emerged in late June and early July displayed more tendency of becoming reproductive ones, as compared with those emerged in mid-July. But each of these groups of adults, either field collected or newly emerged, deposited only 3-14 eggs within 60 days of feeding. Reproductive capacity was greatly related with plant age, with plants aged below 20 days being most favorable; on plants aged over 20 days, reproductive development could take place more or less, but little oviposition was observed. These results indicate that reproductive capacity of 1 st-generation adults in Zhejiang depends upon the time of adult emergence and post-emergence, as well as the age of rice plants.6. Effect of the weed Leersia hexandra Swartz on formation of the 2nd-generation adults of the rice water weevil in Zhejiang, ChinaThe grass Leersia hexandra Swartz is one of important hosts for rice water weevil in Zhejiang. Adult feeding and fecundity on L. hexandra with different quality were surveyed in the laboratory. Adults were collected from rice fields in late 26, mid-July an late July, and supplied with three kinds of L. hexandra plants respectively bearing small, mid-, and large leaves No oviposition occurred in all groups of adults after 30 days of feeding on L. hexandra, but after transferred subsequently to rice feeding 80-95.8% of individuals had oviposition onset in all groups within 15 d, and this ovipositing percentage was significantly higher than the adults that were reared consistently on rice. Moreover, surveys on the first-generation adults feeding on L. hexandra plants on field ridge revealed that, in early July all of such adults had very little ovarian development; however, in middle mature eggs occurred in about 10% of individuals, which might become the source of second population. These results indicate that feeding on L. hexandra can increase the potential of first-generation adults development into reproductive individuals.7. Reproductive capacity and survival of adults on different temperature with starvation stressReproductive capacity and survival of newly emerged adults and filed collected ones (collected from Yueqing, Zhejiang, China) under different temperatures with starvation stress were studied. Newly emerged adults and filed collected ones had a similar survival-percentage curve at different temperatures with lower survival occurring at higher temperatures. For the newly emerged adults, 28% of individuals had begun to oviposit after 36 days of feeding at 28℃, and at other temperatures no oviposition was observed after the same period of feeding. In comparison, for adults collected from rice fields, the cumulative of oviposition onset percentage reached 50-80% after the 36-d feeding, and the percentage was higher at higher temperatures (31 and 34℃) than at lower ones (28℃). The onset of oviposition was greatly promoted as the previous starvation was prolonged. After the starvation at each temperature for 1, 2 and 3 weeks, cumulative percentage of oviposition onset peaked at 29, 15 and 9 d after transferring to rice, respectively.8. Reproductive behavior of rice water weevil in its native areasAdults of the rice water weevil were collected from rice plots, a light trap and ratoon rice in Beaumont, Texas, USA, and their reproductive status and mating success were surveyed from April to September. Most of the adults reproduce sexually in this area; parthenogenetic females also occur in this area, but its percentage at the location is below 10%. The laboratory experiments showed that adults collected from late July can lay eggs when not supplied male ones, but the viability of offspring eggs was very low, indicating that in this area the contribution of Parthenogenetic females to population development is low. High percentage of oviposition onset occur in field from early July to middle September in rice fields, and such percentage reached 50.4-77.4% in light trap, which substantially differ the situation in Zhejiang, China where percentage of reproductive females at this season is very low.9. Reproductive superiority of parthenogenetic femalesParthenogenetic females were collected from the Sacramento Valley (Butte Co.), California in the spring of 2005, and their mating success, reproductive capacity and progeny viability were observed under laboratory conditions. Females mated successfully when supplied with males collected from Texas, but they appear to be less preferred by males compared to gamogenetic females. Fecundity and progeny viability were significantly reduced in mated females, suggesting a considerable cost of mating to reproductive potential. This results implies that the parthenogenetic females might have a reproductive superiority over bisexual reproductive females.