| The red imported fire ant Solenopsis invicta Buren (Hymenoptera: Formicidae) is an invasive pest that was found firstly in 2004 and affirmed to establish permanent colonies in Mainland China in 2005. At present, it distributes sporadically in parts of the Guangdong, Guangxi, Hunan, Fujian and Jiangxi, Taiwan, Hong Kong and Macao, with the largest areas spreading in Guangdong province. The S. invicta, as one of the most detrimental pests that has tremendous capacity to injury and threaten to the safety of human being, living storks, wild animals, crop production and public utilities has been treated as a quarantine and eradicating species since it was found in China. However, the monitoring, quarantine and control efficacy of S. invicta in China is limited for it is a new pest to the country. The variations in social forms in different geographic locations, composition and supercooling points of various castes and developmental stages in the colonies, out-nest activity of worker ants and nuptial flight of alates, potential expansion range and the integrated risk assessment for S. invicta in China were studied to improve its monitoring, quarantine and management techniques, which are necessary to reduce the population and damages of fire ant in mainland China. The main results obtained were summarized as follows:Results on the social forms, variations in proportions of the worker ants, larvae and pupae in the colonies, relation between nest volume and the number of individuals were demonstrated through a year field investigation by applying ecological and behavioral principles and methods. Results obtained through combined investigations of aggressive behaviors of worker ants and nest dissection for queen numbers showed that 92.4% (73/79) of the colonies identified in four locations of Guangdong and Guangxi were belonged to polygene, while a small portion (6/30) investigated in Guangzhou was monogyne. These results were consistent to those identified by multiplex PCR of Gp-9 alleles and hence, provided a reliable and simplified method for the social forms identification methods that may used in the field. The results obtained from bimonthly field investigations from September to July showed that volume of the active mounds continued to increase from July and decrease after the mounds reached to their maximum volumes in January. The mounds kept to decrease afterwards until they reached to the minimum in March. Variations in mound volume made their annual growth curve look like a V shape and differed significantly. Total number of individuals and percentage of the worker ants in the mound corresponded to the size of mound and differed as the season varied. That is, total number of individuals in the mound increased and reached the maximum from September to November, then decreased until it reached the minimum in March, and increased again until July. Percentages of the larvae, pupae and alates in the mounds however, showed an opposite trend to that of the worker ants. There was a close relationship between the size of the mound and number of worker ants (r =0.78, P<0.05), which could be described by the formula Y = 1.335x2.894 (r2 = 0.765, P<0.05). All these results not only demonstrated the ecological and behavioral aspects of S. invicta in mainland China but also provided a substantiate basis to improve its monitoring and managing techniques. Daily and seasonal out-net activity of the workers and nuptial flights of the reproductive forms (alates) investigated in Guangzhou area (113°45E′, 22°43′N) of Guangdong province demonstrated that these activities of S. invicta differed significantly as season or month varied. Annually, the out-net activity of worker ants peaked in June and October, while that occurred in January and February was the lowest although they could be found in sunny and warm days during these period. The out-net activity of worker ants in a day also differed in different months. From January to February, worker ants hardly left their nest in general except the time when it was at noon on a warm and sunny day. From March to April, workers'activity peaked at noon, but from July to October, workers were more active in the morning and afternoon. From May to June, and November to December, the out-net activity of worker ants declined at noon in comparison with that in the morning and evening. All the out-nest activity of worker ants is dominated by temperatures. On the other hand, the out-nest activities of worker ants were enhanced before and after reining but was held back by depositions. Nuptial flights of alates took place all the year around, but occurred mainly in the afternoon from March to May. The nuptial flights of alates were also enhanced before or after raining.Results obtained showed that worker ants responded differently to various degrees of nest intrusion and size of the colonies. In general, the response of worker ants to the nest intrusion were speeded up as the intention of disturbance increased, which was exemplified by the evidences that the worker ants needed 0.80±0.03 s, 0.4±0.04 s and 0.51±0.02 s to rush out from the mounds that were hit by the ball fallen from the heights of 0.5 m, 1.0 m and 1.5 m, respectively. There were no significant difference between the percentages of large workers by the intrusions of balls fallen from the height of 1.5m and 0.5m. Besides, the response and number of worker ants from the larger mounds were much quicker and greater than those from the smaller mounds at the same degree of intrusion. As a consequence of that, the greater mounds would have more worker ants rush out when they subjected heavier intrusion. The processes of worker ants in different size of the mounds responded to different intrusion could be described by the Fang's formulas. One thing worthy to be mentioned was that the number of worker ants was decreasing as the distance from center of the disturbing mound increasing but were insignificantly different at different orientation. These newly obtained data provided not only a basis to understand the aggressive behavior of S. invicta but also some helps to human beings to protect themselves from its attacks.To elucidate the cold hardness and provide basis for its risk assessment in mainland China, the supercooling points (SCPs) of various castes of adults, pupae and larvae, workers collected in different geographic locations and seasons and treated by durative starvations were examined and reported here. Our results indicated that the SCPs of fire ants differed significantly among castes and developmental stages. The worker ants had a significant lower SCP (-11.8℃) than those of the reproductive adults. Adults had a higher mean SCP than immature individuals. Queens had the highest SCPs (-7.9℃) while the pupae of workers had the lowest (-13.6℃). The SCPs was not significantly different in workers collected from different geographic locations of Luchuan, Guangzhou, Zhuhai and Shenzhen in spring, autumn and winter except those colleted in summer. SCPs of worker ants decreased as the season progressed, with the highest for spring (-4.3℃), the second for summer (-6.6℃), and third for autumn (-9.4℃) and lowest (-12.0℃) for winter colonies. The SCPs of worker ants increased greatly by a durative starvation of 15 days. The distribution of their SCPs shifted from a range of -14.7℃and -8.6℃before starvation to -10.4℃and -2.0℃at end of 15-days test. The SCP of major workers and minor workers increased 7.0℃and 4.9℃, respectively during the test period, indicating that the major workers are less resistant to starvations than the minor workers.The potential distribution range of S. invicta in China was predicted by using Kriging spatial interpolation function in the geographic information system (GIS) software, the number of degree-days as main determinant, and annual precipitation of 500 mm and the average temperature of -2℃in January as limiting factors. Results obtained demonstrated that the potential distribution range of S. invicta in China could be divided into four regions. They were named as the most suitable region (MSR), suitable region (STR), less suitable region (LSR) and unsuitable region (USR). The MSR included Hainan, Hong Kong, Macao, the great part of Guangdong, Jiangxi and Hubei, central and southern Guangxi, and part of southern Yunnan and Taiwan. The STR included northern and western Guangdong and Guangxi, the great part of Fujian, Hunan and Chongqing, northern Taiwan, southern Guizhou, the area of Sichuan near Chongqing, the southwestern area near the border line in Yunan, the southern and the northeastern of Zhejiang near to Shanghai, a small area of southern Jiangsu and the southwestern Anhui. The LSR included the central and eastern Guizhou, the area near the common boundary of Sichuan, Gansu and Shanxi, the area near the common boundary of Henan and Shandong. The other regions in the country that were not previously mentioned were the unsuitable area because of the limitations of drought and low temperature. The MSR, STR, LSR possesses 7.4%,8.4% and 6.0% of China's territory, respectively. Briefly, the total ranges suitable for the expansion of S. invicta in China including large or small parts of 19 provinces, which equals to 21.8% of the territory in the country.The rank of quarantine importance of S. invicta was evaluated by an integrated risk assessment method combining analysis of the data on its biology, destructive capacity, potential geographic rage, economic impact and cost of eradication. The result showed that the integrated risk score for this pest is 2.23, suggesting that China is being at risk for S. invicta infestation and detrimental impacts. These results implied that quarantine officials should be vigilant for any accidental introductions of S. invicta in suitable regions of China and the quarantine regulations against this pest should be strictly followed in the country.
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