a comparison of the pre-calling period between japanese and southeastern asian populations of...

Insect Science 13, 211-216

A comparison of PCP of Spodoptera litura 211

Introduction

Some migratory lepidopteran insects are known to displaygeographical variation in reproductive characteristics. Forexample, the pre-calling period (PCP) from adult emer-gence to the first release of sex pheromones varies amongdifferent populations in Helicoverpa armigera (Colvin &Gatehouse, 1993; Casimero et al., 1999) and Pseudaletiaunipuncta (McNeil et al., 1996). Many insect speciesundergo migration as sexually immature adults (Johnson,1963). The flight activity is high when the adult is stillimmature (Colvin & Gatehouse, 1993). In the insectsdistributed in a wide range, it is predicted that there isvariation in reproductive traits and behavior because ofclimate differences. Therefore, the PCP may be an impor-

Correspondence: Mika Murata, Food Entomology Laboratory,National Food Research Institute, Independent AdministrativeInstitution, 2-1-12 Kannondai, Tsukuba, Ibaraki, 305-8642; Japan.Tel: +81 29 8388081; fax: +81 29 8388081; e-mail:[email protected]

A comparison of the pre-calling period between Japaneseand southeastern Asian populations of Spodoptera litura

Abstract The pre-calling behavior of female adults of the common cutworm, Spodopteralitura (Lepidoptera: Noctuidae) collected in southern Japan and southeastern Asian coun-tries was investigated. Most females started calling on days 1-4 after adult emergence. Inthree populations obtained in Saga prefecture in Kyushu mainland, Japan, two differentpatterns of pre-calling period (PCP) were observed: one was the above-mentioned patternshown by most individuals, and the other was a pattern in which females sexually maturedwithin several hours after emergence. Diel periodicity was shown in the time of callingactivity, and its pattern varied among the populations. Pre-calling period was stable oversuccessive generations in the laboratory.

Key words geographical variation, long distance migration, pre-calling period, SpodopteralituraDOI 10.1111/j.1744-7917.2006.00084.x

www.blackwellpublishing.com/ins 211

Insect Science (2006) 13, 211-216

M I K A M U R ATA 1 , 2 , H I R O Y U K I M AT S U K I 3 , R Y O H E I YA M A O K A 4 a n dS U M I O T O J O 3

1Food Entomology Laboratory, National Food Research Institute, Independent Administrative Institution, Tsukuba, Ibaraki;2Venture Laboratory and 4Faculty of Textile Science, Kyoto Institute of Technology, Kyoto, and 3Faculty of Agriculture, SagaUniversity, Saga, Japan

tant characteristic in understanding the variability of thelong-distance migration and the survival strategy to avoiddeteriorating conditions.

The common cutworm, Spodoptera litura, widely dis-tributed throughout southeastern Asia, is a serious pest ofvegetables, and more than 70 host plant species are known(Okamoto & Okada, 1968). It does not enter diapause atany developmental stage and overwinters only in warmareas (Matsuura & Naito, 1997). One of the most outstand-ing characteristics of this pest insect is its high migratoryability. Adults of S. litura were observed in the East ChinaSea (Asahina & Tsuruoka, 1968). This insect has a physi-ological capability to fly longer than 18 hours (Murata &Tojo, 2004). It has been suggested that individuals in Japanmight be overseas immigrants carried by typhoon windfrom southern areas (Murata et al., 1998).

Although much information is available about the mi-gratory activity of S. litura, the details of its reproductivetraits are not well understood. Variation in reproductivetraits is common in other migratory species as above. In thepresent study, we compared the PCP and the calling timeas indices to discriminate immigrant populations from


212 M. Murata et al.

resident ones in S. litura in those derived from Japan andsoutheastern Asia. We also investigated the stability of thevariation in PCP between different generations in somepopulations. The purposes of the present study are todescribe these results and to discuss how Japanese popula-tions of this species pare built up in relation to the migration.

Materials and methods

Insects

To establish laboratory colonies, S. litura eggs andyoung larvae were collected at various sites, as listed inTable 1. In Japan, they were found in eddo fields atYamato, in July, 1999, and in soybean fields at Saga cityand Higashiyoka, Saga prefecture in September, 1999.Eggs and young larvae were collected in taro fields at Kinand Ginowan, Okinawa Main Island, Okinawa prefecture,from September to October, 1999, and on coral trees atHirara, Miyako Island, Okinawa prefecture in May, 2000.Eggs and larvae were collected in soybean fields at Tainan,Taiwan region in October, 1998, and in eddo fields atRosales, Pangasinan prefecture, the Philippines in January,1998. Populations from Miyako Island, Tainan and theRosales were sent by Dr N. Arakaki, Dr Chang K-S and Mr.Amor R. Bertuso. Larvae were reared on an artificial diet(Okamoto & Okada, 1968) at 25 ± 2℃ and 16: 8 L: Dphotoperiod (lights on at 8:00, lights off at 24:00). Theywere given an excess artificial diet in cages.

Four hundred to five hundred larvae were kept in a plasticcase (15 cm× 1 cm × 7 cm) during the first three stadiaand then the number was decreased to 50 per case (33 cm×23 cm×6 cm) to avoid cannibalism. Some larvae wereindividually kept in small plastic cups (6.5 cm diameter, 4cm height) from the fourth instar onward.

Pupae were sexed based on the morphology of theabdominal terminal segments and maintained separately.Moth age was designated as 0 on the day of adult emergence.

All experiments were carried out with individuals of the

first five generations in the laboratory, and the testedfemales were all those that emerged during scotophase.

Observations of calling behavior

Calling behavior of females was observed only duringthe scotophase. Observations were made using a red flushlight at 25℃. Newly-emerged females were individuallyheld in transparent plastic cups (120 mL volume) andprovided with a cottonball moistened with 10% honeysolution as food. The cottonballs were changed every otherday. In the present study, the females extruding the sexpheromone gland from the abdominal terminal were re-garded as calling. The number of calling females wasrecorded at intervals of 30 min. The night of adult eclosionwas designated as the beginning of day 0. From day 0 to thenight female calling started was regarded as her pre-callingperiod (PCP).

To determine if PCP is modified by successive rearing,this was compared between individuals of F1 and F5 for theHigashiyoka and Ginowan populations.

Results

Figure 1 compares PCPs for individuals from eightpopulations. They were all reared under crowded condi-tions until adult emergence. Calling behavior started within4 days of emergence in all populations. In six out of theeight populations, calling started most frequently at day 1.On the other hand, more than 60% of females from Yamatoand Higashiyoka showed calling behavior within a fewhours after emergence. A few individuals did not startcalling until day 4 in the Ginowan population. The patternfor the Saga city population was similar to the patterns forthe southern islands populations from Ginowan, Hirara,Tainan and the Rosales rather than to that for the populationfrom Higashiyoka near Saga city (ANOVA followed byGames-Howell test: F = 9.301, P < 0.05). The PCP for theYamato population was different from the value for the

Table 1 List of sites from which Spodoptera litura was collected.

Collecting site Location

Yamato (Saga prefecture, Japan) 33°12′N 130°17′ESaga city (Saga prefecture, Japan) 33°15′N 130°18′EHigashiyoka (Saga prefecture, Japan) 33°18′N 130°17′EGinowan (Okinawa island, Okinawa prafecture, Japan) 26°14′N 127°45′EKin (Okinawa island, Okinawa prafecture, Japan) 26°27′N 127°56′EHirara (Miyako island, Okinawa prefecture, Japan) 24°47′N 125°17′ETainan (Tainan prefecture, Taiwan region) 23°01′N 120°13′ERosales (Pangasinan prefecture, the Philippines) 15°54′N 120°37′E



Fig. 1 The distributions of the initiation ages of first pheromone gland exposure (pre-calling period = PCP) in females of S. liturapopulations from Japan and southeastern Asian countries. Sample sizes: Yamato = 31, Saga = 48, Higashiyoka = 35, Kin = 24, Ginowan= 52, Hirara = 18, Tainan = 46, Rosales = 25 (ANOVA, F = 9.301, P < 0.05).

Hirara population (ANOVA followed by Games-Howelltest: F = 9.301; P < 0.05).

Figure 2 shows the temporal patterns of calling activityduring the scotophase. In most populations, more callingfemales were observed during the second half of scotophase,especially the last two hours, than during the first half (χ2

test: χ2 = 47.13, df = 7; P < 0.01). In the Rosales populationfrom the Philippines, calling occurred mainly during thefirst half of the scotophase with a peak during the second-hour period (χ2 = 7.28, df = 1; P < 0.05). Calling activitywas almost equally frequent between the two halves ofscotophase in the Saga city (χ2 = 0.17, df = 1; P > 0.1) andHirara populations (χ2 = 1.87, df = 1; P > 0.1), although thepattern appeared to be bimodal in both cases. Among thethree populations from Saga prefecture, a significant dif-ference was found in the temporal pattern (χ2 = 11.92, df =2; P < 0.01). The pattern for the Saga city population wasnot significantly different from that for the Tainan popula-tion (χ2 = 0.052, df = 1; P > 0.05).

The effect of successive rearing on the PCPs was exam-ined using two populations derived from Higashiyoka and

Ginowan. Here, the PCP was compared between F1 and F5in each population (Fig. 3). In either population, no signifi-cant difference was found in mean PCP between the twogenerations (t-test; P > 0.05 each).

Discussion

Variation in PCP

The present study demonstrated that the PCP of S. liturawas different among several populations when comparedin the laboratory. Naito (1987) mentioned that S. litura isdifferent in reproductive character from other long-dis-tance migratory insects such as Pseualetia separata andAgrotis ipsilon. Female adults are sexually mature at emer-gence in the former but not in the latter species. In thisstudy, PCP was found to vary geographically in S. litura.Yushima et al. (1973) also described that a high rate ofmating was recorded 2 or 3 days after adult emergencerather than on the day of emergence.



Fig. 2 The distributions of the time of calling in females of S. litura populations from Japan and southeastern Asian countries. Asterisksindicate a significant difference between the first and second halves of the scotophase by the χ2 test (*P < 0.05; **P < 0.01). NS indicatesno significant difference.

Fig. 3 Comparison of PCP between the first and fifth laboratory generations in S. litura populations from Higashiyoka and Ginowan.Sample sizes are 35 and 25 in F1 and F5 of the Higashiyoka population; 52 and 15 in F1 and F5 of the Ginowan population. Different lettersafter the mean indicate a significant difference by t-test; P < 0.05).



Females derived from Yamato and Higashiyoka startedto call on day 0, whereas those originated from Saga city,which is located only 10-20 km away from those localities,mainly did so at day 1. These results may indicate geneticheterogeneity in this region of Saga prefecture.

This insect is suggested to be a long-distance migrantcarried by typhoon winds (Murata et al., 1998). From earlysummer to autumn, native populations which overwin-tered inside greenhouses in Saga prefecture were caught bya pheromone trap. However, during the typhoon season inautumn, many adults with their wings injured and withrelatively small amounts of lipids stored were caught by atrap (Murata, unpublished), and they were thought to havemigrated from other districts via typhoon winds (Murata etal., 1998). Interestingly, females that have been flying fora long period of time in the laboratory are able to lay asmany eggs as those that have not flown at all (Murata &Tojo, 2004). In the present study, the adults were collectedat Yamato at the beginning of July before the typhoonseason, whereas at Higashiyoka and Saga city moths werecollected at the end of September within one week after thetyphoon passed through the northern district of Kyushuwhere Saga prefecture is located.

Therefore, it was thought that the individuals of S. lituracollected in Saga city had probably been conveyed fromanother area in the south to Saga prefecture by a specificweather condition such as a typhoon, whereas those of theYamato population were native populations. However,from the present study it seems difficult to judge the originof the individuals collected at Higashiyoka.

Time of calling

Ohbayashi (1973) reported that the calling behavior of S.litura showed two peaks of calling activity during thescotophase: one peak occurred at 3 hours after dusk and theother one at 1 hour before dawn. Miyashita and Fuwa(1972) also observed that this moth exhibited two peaks inmating activity during the night. Unfortunately, the sites ofcollection of the insects were not described in those reports.Oyama (1972) observed the time of mating of wild maleswith tethered virgin females in Kagawa prefecture, Japan,and detected a peak of coupling during the first 2 hours aftersunset, although no data were collected after midnight. Inthe population collected in Shanghai and then reared at 14:10 L: D photoperiod, females started calling at 6 hours afterlights-off and the peak of calling was shown at 8-9 hours(Sun et al., 2002). In the present study, three types ofcalling time were recognized for this species. Diel callingactivity in the eight populations was considerably stableand changed little during the five days after emergenceunder constant temperature and daylength (data not shown).

Therefore, it appears that S. litura has a fixed pattern ofcalling activity depending on the population.

Environmental factors influencing PCP

Pre-calling period of S. litura was not influenced bysuccessive rearing in the laboratory (Fig. 3).

In some species, PCP is affected by environmentalconditions. In Autographa gamma a migrant from Africa toBritain, Mythimna separata migrating seasonally in east-ern China and Japan and M. convecta migrating in Australia,PCP is prolonged by a low temperature and short photope-riod (Han & Gatehouse, 1991; Hill & Gatehouse, 1992;Socorro & Gregg, 1997). In this study, all tested insectswere collected only in one season at each site, and wecannot rule out the possibility that the PCP pattern in S.litura varies with the seasons in response to some seasonalcues. However, the PCP patterns in the laboratory re-mained unchanged between different generations in bothHigashiyoka and Ginowan populations. Therefore, there isno doubt that the PCP is controlled by genetic factors.Further examinations are needed in the future to clarify theenvironmental effects on the PCP in this moth.

Acknowledgments

We sincerely thank Dr S. Tanaka, National Institute ofAgrobiological Science for reading the manuscript. Wethank Dr Chang K-S, National Institute of Chaiyi Technique,Dr N. Arakaki, Okinawa Prefecture Agricultural Experi-ment Station and Mr. Amor R. Bertuso, Philippine Na-tional Tobacco Administraion, for providing S. litura eggs.

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Accepted March 12, 2006

a comparison of the pre-calling period between japanese and southeastern asian populations of...

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