Flight behavior of the sycamore lace bug, Corythucha ciliata, in relation to temperature, age, and sex

LU Shao-hui , WEl Mei-cai YUAN Guo-jun, CUl Jian-xin, GONG Dong-feng

1 Laboratory of Insect Systematics and Evolutionary Biology, Central South University of Forestry and Technology, Changsha 410000, P.R.China

2 Henan Academy of Forestry, Zhengzhou 450000, P.R.China

3 Henan Institute of Science and Technology, Xinxiang 453000, P.R.China

Abstract The flight capacity of different ages and sexes of the sycamore lace bug, Corythucha ciliata, was studied at different temperatures using a flight mill system. The results of regression analysis showed a significant effect of temperature on flight distance (P=0.0082). Temperature did not influence flight duration (P=0.212) or flight speed (P=0.175). The mean flight distance (1 024 m) and mean flight duration were the greatest at 25.2°C. The age of C. ciliata had a significant influence on flight distance (P=0.0005), flight duration (P=0.0005) and flight speed (P=0.026). The 12-d-old adult had the greatest flight distance (887 m), duration (3 875 s) and speed (0.22 m s-1). Flight distances and flight duration of females were significantly longer than that of males. However, the male had significantly greater flight speed than the female. The insect appears to be capable of long distance flights. The understanding of the optimal age and temperature for the flight of this insect through this study provides a foundation for better management of the insect in China.

Keywords: Corythucha ciliata, flight capacity, tethered flight, temperature, age and sex

1. lntroduction

The sycamore lace bug, Corythucha ciliata (Heteroptera:Tingidae), is a specialist herbivore that mainly feeds on Platanus spp. (Platanaceae) trees. Adults and nymphs feed on the undersides of the leaves, leading to a white stippling.Leaf stippling from C. ciliata feeding can cause chlorotic or bronzed leaf foliage that can inhibit tree growth or cause tree death (Soria et al. 1991; Mazzon and Girolami 2000).

The sycamore lace bug is native to the central and eastern United States and from there it spread to eastern Canada (Halbert and Meeker 1998). The insect has been found in South America (Prado 1990) and Asia (Chung et al. 1996). It has dispersed widely between cities and is now a worldwide pest. The lace bug was first discovered in Changsha, China in 2002 (Streito 2006) and the first population outbreak was recorded in Wuhan in 2006 (Li et al. 2007). The insect has since migrated to many cities in eastern, central, and southwestern China (Ju et al. 2009).It now occurs in most areas except for a few provinces in northeastern, northwestern, and southwestern China.The lace bug can disperse rapidly, for example, 70 km per year in Spain (Soria et al. 1991). In Japan, it was first discovered in Nagoya and within two years, it had spread to many other cities, including Tokyo, Yokohama, Shimizu,Matsuyama, and Kitakyushu (Tokihiro et al. 2003). In China,the northernmost population of C. ciliata in 2007 was in Zhengzhou (Jiang and Ding 2008). However, in 2012 it was discovered in Huairou, Beijing (Yu et al. 2014), a linear distance of approximately 770 km between the two locations.

Since the invention of the first flight mill (Hocking 1953),many studies have used flight mills to study insect flight capacity. Flight mills are used to study insect flight behavior under laboratory conditions, to judge insect capacity for long distance migration, and to measure short-distance movement and dispersal capabilities. Many physiological and environmental factors can influence insect flight behavior (Gatehouse 1989), including age, sex, mating status, ovary development, adult nutrition, diapause status,temperature, humidity, season, and wind speed. The wind tunnel is an important tool to analyze the infochemical effects on insect behavior and therefore, to be used to test the suitable concentrations or component ratios of infochemicals. Wind tunnels can also be used to test infochemicals under controlled laboratory conditions prior to their application in the field, although wind tunnel could only measure insect behavior, not insect flight capacity.

The spread of C. ciliata can be rapid. In Turkey, in the two years after the first discovery of C. ciliata in 2007, it had spread into the 120 km2northern region (Mutun 2009).Spread of C. ciliata in cities can be equally rapid. All the streets within the city of Turin were infested within one year and the occupied area exceeded 100 km2(Arzone 1975).The rapid spread of C. ciliata may be due to the strong flight capacity or the short distances by autonomous flight or even long distances by air flow.

The effects of temperature, age, and sex on insect flight capacity have been well documented (Horn et al. 1983;Battisti et al. 1985; Santini et al. 1986; Kim and Jeong 1999; Kukedi 2000; Liu et al. 2011). C. ciliata ontogeny is significantly influenced by temperature (Santini et al. 1986;Kim and Jeong 1999). In such areas where the insect has three generations a year, the peak period of adults within each generation was early July, early August and the middle of September (Horn et al. 1983; Battisti et al. 1985). We have observed that the young adults of C. ciliata are less active than older adults. There are sex-related differences in the flight abilities of many insects. In the present study,the effects of the environmental temperature, age and sex on the dispersion of C. ciliata were studied.

2. Materials and methods

2.1. lnsects

Nymphs of the insect were collected from host trees in Huiji District of Zhengzhou, China. They were fed with fresh sycamore leaves. Moist, absorbent cotton was placed around the ends of the petioles to reduce leaf desiccation.The nymphs were fed separately in a screened cage. They were placed in a climate chamber with (25±1)°C, 70%relative humidity and a 12 h L:12 h D photoperiod until adult eclosion. Longevity of adult C. ciliata is not much different between the insect from long-term indoor feeding and the ones in the wild.

2.2. Test device

Flight mill system was provided by Henan Institute of Science and Technology. The system consisted of a sensor and data acquisition board, mill, and computer software(Fig. 1). A total of 26 individual flight mills were linked to a recorder, which was connected to a computer and placed on glass shelves in a room with adjustable temperature, relative humidity (RH), and light intensity. A copper cantilever was placed between two miniature magnets on the flight mill,thereafter ensuring the insect was positioned horizontally for the sake of smooth flight (Feng et al. 2004). Data recorded by the software include the time of flight initiation and each revolution, and the number of mill revolutions that occurred in consecutive 5-s intervals. Flights interrupted by a 5-min interval of inactivity were considered as separate flights.This is a simple, cheap and easy method to indirectly measure the flight capability of this insect.

2.3. Tethered flight test

Before testing, individual C. ciliata was lightly anaesthetized with diethyl ether and hairs on the thorax and abdomen were removed using a brush pen. The insect was then attached to a thin copper wire hanging from the arm of the flight mill.The flight distance, speed and time were recorded during each 24-h test with no supplementary food.

Fig. 1 Schematic diagram of a flight mill apparatus. a, miniature magnets; b, reflector (that was made of a black oval slice of plastic); c, mill arm (made of two copper threads); d, electrical cable.

2.4. Effect of temperature on C. ciliata flight capacity

The tethered flight tests were performed using 10-d-old adults at 19, 22, 25, 29 and 31°C, respectively. The ratio of males to females was 109:102. For each temperature,25 adults were studied. The tests were conducted at 70%RH and 24 h light, 900 lx, under totally enclosed conditions.The experiments were performed in July 2016.

2.5. Effect of age and sex on C. ciliata flight capacity

The experiments were conducted in July 2016. C. ciliata adults of 1- to 12-d-old were tested. For each age (day),20 C. ciliata adults were tested. The tests were conducted at (25±1)°C with 70% RH and 24 h, 900 lx, under totally enclosed conditions. Tests of same age insects were carried out on different days. After the experiment, the sex and the number of all the bugs for the test were recorded and statistical analyses were performed.

2.6. Statistical analyses

Statistical analyses were performed using SPSS (version 19.0). Flight distance, flight speed, and flight duration under different temperatures and ages of the insect were compared by ANOVA F-test followed by the multiple range test of least significant difference (LSD), and these relationships were also analyzed using regression. The relationships between tested temperatures and the three parameters of flight distances, flight speed and flight duration were fitted with polynomial regressions,respectively. The relationships between tested age and the three parameters mentioned above were fitted with linear regression, respectively. Data from both sexes were pooled for the regression analyses. Parameters of the C. ciliata flight test with different sexes were compared using Student’s t-tests. When the cumulative flight distance of a single insect was less than 5 m, it was considered as unreliable data and omitted.

3. Results

3.1. Effects of different temperatures on the flying ability of C. ciliata

The actual flight distance, flight speed and flight time under different temperatures were measured. The results demonstrated that the mean flight distance was 637 m for 24 h of tethered flight (range: 5-6 060 m) and the mean flight speed was 0.18 m s-1(range: 0.02-0.81 m s-1). The mean flight duration was 4 016 s (range: 19-54 706 s). At 25°C,the average flight distance reached the maximum of 1 160 m.At 19 and 31°C, the average flight distance decreased significantly (P=0.0082), to 242 and 351 m, respectively.

The insect exhibited significantly different flight distances under different temperatures (F=2.67, P=0.036). The flight distance under 25.2°C was significantly farther than those under 18.89°C (P=0.004) and 31.51°C (P=0.012). The relationship between temperature and flight distance was fitted by a polynomial regression equation (y=-20.788x2-1 047x-12 159, R2=0.9293). Maximum predicted flight distance occurred at 25.2°C where predicted flight distance was 1 024 m. From 18.89 to 25.2°C, with the increase of temperature, the flight distance increases. From 25.2 to 31.51°C, with the increase of temperature, the flight distance gradually decreases (Fig. 2). For C. ciliata in tethered flight for 24 h, temperatures lower than 18.89°C or higher than 31.51°C reduced the flight distances to less than 200 m.Summaries of regression models presented in Figs. 1-6 are shown in Table 1.

No significant difference was found in the flight duration under different temperatures (F=0.768, P=0.470). The relationship between temperature and flight duration was fitted to the polynomial regression equation (y=-124.13x2-6 300.4x-74 135, R2=0.6458. Flight duration was not affected by temperature (P=0.212). At 25°C, the maximum flight duration reached 54 706 s (about 15 h), indicating the strong flight ability of C. ciliata (Fig. 3).

No significant difference was found in the flight speed under different temperatures (F=1.52, P=0.201). The relationship between temperature and flight speed was fitted to a polynomial regression equation (y=-0.0003x3-0.021x2-0.4743x-3.6502, R2=0.768). Flight speed was not affected by temperature (P=0.175). The average flight speed was the slowest (0.15 m s-1) at 19°C. The average speed was the fastest (0.25 m s-1) when the temperature was 28°C. The flight speed decreased to 0.16 m s-1when the temperature rose to 31°C (Fig. 4).

3.2. Effect of age on C. ciliata flight capacity

Fig. 2 Corythucha ciliata flight distance at different temperatures.

Table 1 Summary of regression models of temperatures and age on flight performance of Corythucha ciliata during 24-h flight

Fig. 3 Corythucha ciliata flight duration at different temperatures.

The flight distance, flight speed and flight duration of all ages were calculated and the results revealed that mean flight distance was 474 m in 24 h of tethered flight (range:9-6 060 m) and mean flight speed was 0.19 m s-1(range:0.04-0.58 m s-1). Mean flight duration was 2 287 s (range:41-18 589 s). The maximum flight distance and the longest flight time of 10-d-old insects were 6 060 m and 18 589 s.The maximum average flight speed of 12-d-old adults was 0.23 m s-1. However, the results of ANOVA test indicated that there was not significant difference in the flight distances(F=1.72, P=0.111), the flight speeds (F=0.61, P=0.747), or the flight durations (F=1.30, P=0.256) among different ages of the insect.

Fig. 4 Corythucha ciliata flight speed at different temperatures.

The relationship between age and the flight distance was fitted to a linear regression equation (y=72.774x-14.118,R²=0.5231). Based on the results of statistical analyses,age had a significant effect on flight distance (P=0.0005),the estimated maximum flight distance (887 m) would occur in 12-d-old adult (Fig. 5). However, the actual experimental results showed that it reached the maximum (1 361 m) in 10-d-old insects, and then decreased in 11- and 12-d-old insects.

The relationship between age and flight duration was fitted to a linear regression equation (y=280.65x-507.74,R2=0.6031). Based on the results of statistical analyses, age had a highly significant effect on flight duration (P=0.0005).The estimated flight duration possibly increased with increased insect age, and the maximum of 12-d-old insects would be 3 875 s (Fig. 6). However, the actual experimental results showed that flight duration of 10-d-old insects reached the maximum of 4 580 s while flight times of 10- to 12-d-old insects decreased.

Fig. 5 Corythucha ciliata flight distance at different ages.

Fig. 6 Corythucha ciliata flight duration at different ages.

Fig. 7 Corythucha ciliata flight speed at different ages.

The relationship between age and flight speed was fitted to a linear regression equation (y=0.0059x-0.1478,R2=0.4772). It can be seen that the linear equation does not fit the data well. However, age does have a significant effect on the flight speed (P=0.026). Generally, with increased age, the flight speed increases, until it reaches the maximum of 0.23 m s-1for the 12-d-old adult (Fig. 7).

3.3. Effect of sex on C. ciliata flight capacity

C. ciliata female had significantly longer flight distance(t=2.191, P=0.030) and flight times than the male (t=2.919,P=0.004). Both flight distance and flight time for the female were almost twice as much as those of the male. However,the flight speed of the male was significantly greater than that of the female (t=-1.274, P=0.020) (Table 2).

4. Discussion

Many insects with similar size of C. ciliata have been measured for their flight capacity. The orange wheat blossom midge, Sitodiplosis mosellana had the longest flight distance of 2 181.52 m and mean flight distance of 735.10 m(Miao et al. 2013). The insect could be collected at a distance of 75 m above ground. S. mosellana can disperse,with the aid of wind, for longer distances (Hao et al. 2013;Miao et al. 2013). Liriomyza sativae has an average flight distance of 9 500 m in life. It is less likely to move long distances based on flight capability, but may travel long distances in air flow (Lei et al. 2002). C. ciliata is also able to fly into low altitude laminar airflow (60-350 m) (Zhang and Cao 1997) and migrates using air currents. Current reports on C. ciliata dispersal are limited and most dispersal hypotheses lack supporting data. Seedling transportation could be one of the methods for the long distance spread of C. ciliata. In addition, with the help of wind, C. ciliata can possibly migrate thousands of meters in a single episode,which could be the major means for the rapid spread of the bug in a short time after initial introduction into a new area (Ju et al. 2009). Both the nymphs and adults of the insect have some active dispersal capacity (Wu and Liu 2016). These results suggested that C. ciliata has strong flight capacity and may move and spread short ranges by its own flight and readily disperse over significant distances with the aid of air current.

Other studies have shown that temperature can affect insect flight behavior (Duan et al. 1998). The flight distance of S. mosellana peaked at 16°C, which is close to the normal field conditions when adults emerge and infest wheat plants(Doane and Olfert 2008). The maximum predicted flight distances occurred at 27.9 and 26.5°C for the emerald ash borer and its parasite Tetrrasticus planipennisi. Spodoptera exigua (Lepidoptera: Noctuidae) showed the strongest flight capacity at 24°C (Jiang et al. 2002). The highest value of net reproductive rate and fecundity of Mythimna roseilinea(Walker) were observed at 21 and 24°C (Qin et al. 2018), the developmental duration of each stage of Mythimna separata(Walker) was negatively correlated with temperature (Li et al. 2018). Temperature affects the flight capacity of C. ciliata adults. At 25°C, adults had the strongest flight capacity. The tolerances of C. ciliate adults to high temperatures and low temperatures are relatively great. In North America, it tolerated a -23.3°C low temperature and was able to survive at high (＜41°C) temperatures (Halbert and Meeker 1998; Ju et al. 2011).

These flight parameters reached their highest levels in 10-d-old adults. However, age did not significantly affect flight speed of C. ciliate in our study. In Carpomya vesuviana(Diptera), age does affect flight speed. Flight speed of C. vesuviana adults increases as they grow from 1- to 12-d-old adults (Ding et al. 2014). Apolygus lucorum(Hemiptera) adults had stronger flight capacity when they grew older, i.e., the lowest flight capacity at 1-d-old and thestrongest flight capacity at 10-d old (Lu et al. 2009). The flight capacity of C. ciliata adult increases as it gets old.Adults of 1-10-d-old may have inadequate energy reserves to maintain flight and their flight muscles may not have been fully developed. As age increases, the nutrition from feeding continues to enable flight muscle development and flight ability may gradually increase.

Table 2 Flight capacity of Corythucha ciliata as a function of sex

Flight capacity shows sex-dependent. Study has confirmed that the flight ability of male houseflies is less than that of females (Liu et al. 2011). Females had longer flight distances and flight times than males, but males have faster flight speeds than females. Differences between male and female flight capacity in insects appear to be common.In A. lucorum, females have stronger flight capacity than males (Lu et al. 2009). Agrilus planipennis (Coleoptera)females had longer flight distance and higher flight speeds than males, but there was no significant difference between males and females in flight times (Wang et al. 2015). The flight capacity of insects often depends on body size and in many insect species, and females have a larger body size than males. Thus, the females have stronger flight capacity.Insect flight capacity also depends on flight muscle status.In some species, flight muscles are degenerated and the proteins are reused in the reproductive system for ovary development. Flying for a certain threshold distance can be beneficial for sexual maturity (Nair and Prabhu 1985). In C. ciliata, the relatively stronger flight capacity of females might due to their larger body size and reproductive physiology. The reason for C. ciliata males having greater flight speeds is unclear. There are also effects of insect mating status and female ovarian development on flight capacity. The flying frequency of mated Riptortus clavatus(Thunberg) is higher than that of unmated individuals(Maharjan and Chuleui 2009). Zhang and Wang (1991)studied peach aphid dispersal departure time. The take-off angle and in vivo ovarian tubule number negatively correlate with the number of ovarioles and migratory potential.However, our study tested insects before being attached to a flight mill and did not report matings. Effects of C. ciliata mating status and ovarian development stages on flight capacity will require further investigation.

Environmental conditions can have a significant influence on insect activity (Kroder et al. 2006). Low RH (30%), which could be to the result of water loss during tethered flight assays (Lu et al. 2009). The distance, and flight duration differed significantly among the various light intensities. Our study was conducted under fixed 70% RH and 900 lux. The effects of different relative humidity and light intensity on the flying ability of C. ciliata require further study.

More attention should be paid to interpret of the results in many flight mill studies. The results of flight mill bioassays may not necessarily reflect flight activity under natural conditions (Hao et al. 2013). Handling can reduce(Kennedy and Booth 1951, 1954) or increase (Cockbain 1961) tendency for flight. The smaller the insect, the greater the impact of handling. C. ciliata is a very small insect. Therefore, size should be considered, in combination with the mechanics of the flight mill, on flight capacity measurements. Under natural conditions, C. ciliata adults are free to feed. Our tests were carried out over 24 h without supplemental feeding. Greater flight activity and flight distances may occur when insects are allowed to rest and consume water and/or feed in natural settings (Taylor et al.2010). In addition, the distance of the insect dispersal can be increased over many generations. T. planipennisi may spread up to 5 km per year, though this distance may be achieved by incremental dispersal of successive generations within a given season (Duan et al. 2013). Further field experiments are needed to collect more data to confirm the fit of the hypothetical model, for example, using the markerrelease-recovery method.

5. Conclusion

The results show a significant effect of temperature on flight distance. However, temperature did not influence flight duration or flight speed. The age of C. ciliata had a significant influence on flight distance, flight duration and flight speed. Flight distances and flight duration were significantly longer for females than for males. However, the male had significantly greater flight speed than the female.

Acknowledgements

The study was funded by the Fundamental Research Funds of Henan Academy of Forestry, China (162102410033).