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We explored the variable adult emergence in summer generations of a multivoltine sawfly [Arge nigrinodosa (Argidae)], larvae of which feed gregariously on the foliage of Rosa spp. (Rosaceae), and its ecological significance. The sawfly showed a trimodal adult emergence under long-day conditions in the laboratory. Following the first and largest cluster of emergence, a small tail of slightly delayed emergence was observed, which most likely was heritable. The third cluster of emergence after nonheritable partial diapause in prepupae seemed to match the synchronous emergence of a portion of adults in September under field conditions, probably as a risk-spreading (i.e., bet-hedging) strategy to cope with food shortage during unpredictable periods of drought in summer.
Bet-hedging, partial summer diapause, polymodal emergence, risk-spreading, stochastic polyphenism, voltinism
Seasonal life cycles are often variable within a population of herbivores (e.g.,
Arge nigrinodosa (Motschulsky) is a common sawfly that feeds gregariously on the foliage of Rosa spp. (Rosaceae) in Japan and the Russian Far East (
To explore the variable life cycle of Arge nigrinodosa, we examined the temporal variability of adult emergence, its heritability, and the prepupal development of the sawfly under constant conditions in the laboratory. The ecological significance of variable adult emergence is discussed with reference to changes in the abundance of larval food resources detected in the field.
Monthly mean air temperatures and monthly precipitation from 1981 to 2010 at the Kobe Meteorological Station.
Arge nigrinodosa eggs and larvae were collected at the campus of Kobe University (34°43.7'N, 135°14.1'E; altitude ca. 200 m), Kobe City, Hyogo Prefecture, western Honshu, Japan, and a field census of wild roses [Rosa multiflora (Rosaceae)] was conducted in the same place. Insects were reared in the laboratory at Kobe University or under outdoor field conditions at Karatodai (34°47.4'N, 135°12.8'E; altitude ca. 350 m), Kobe City. The monthly mean natural day length including one hour of twilight in this area is about 14.1 h in April, 15.0 h in May, 15.5 h in June, 15.3 h in July, 14.6 h in August, 13.4 h in September, and 12.5 h in October.
Figure 1 shows the monthly mean air temperatures and monthly precipitation from 1981 to 2010 at the Kobe Meteorological Station (34°41.8'N, 135°12.7'E) (
Annual changes in the monthly precipitation in July, August, and September from 1980 to 2010 at the Kobe Meteorological Station.
To record the number of days from cocoon spinning to adult emergence, eggs and young larvae collected in May and June 2006 were reared at 20°C with a 16L–8D photoperiod in transparent plastic boxes (14 cm diameter, 7 cm depth) containing fresh leaves of Rosa multiflora, which were renewed every 5 - 10 days. The emerged virgin female adults were placed individually in the same type of transparent plastic box, each containing a young shoot of Rosa multiflora provided for oviposition, and kept at 25°C with a 16L–8D photoperiod. To record the number of days from cocoon spinning to adult emergence, larvae hatched from deposited eggs (all haploid males) were reared at 20°C with a 16L–8D photoperiod.
Adult emergence under field conditions in summer generationsEggs or larvae were collected from the foliage of Rosa multiflora on 24 May and 4 July 2007. They were reared outdoors, under shaded conditions, in transparent plastic boxes (14 cm diameter, 7 cm depth) containing fresh leaves of Rosa multiflora, which were renewed whenever wilted. Cocoons spun by larvae were moved to individual small transparent glass tubes (1 cm diameter, 4 cm length), with the opening covered with aluminum foil. The glass tubes were grouped together in transparent plastic boxes (14 cm diameter, 7 cm depth). The dates of cocoon spinning, adult emergence, and sex of the adults were recorded.
Shoot phenology of wild roses.In early April 2007, 40 stem shoots of the current year’s growth of Rosa multiflora were chosen for successive measurements. The shoot lengths were measured and the number of lateral shoots emerging from the stems was recorded every two weeks from late April to early October.
Prepupal development in cocoonsWe observed the formation of pupal compound eyes through the translucent prepupal head capsules. The outline of the pupal eye (shaped like an eyebrow) increased in length during the transition from eonymph to pronymph (cf.
Eggs collected on 17 May 2008 were reared under field conditions as described above. Using forceps we dissected eight cocoons from which adults had not emerged within 40 days after cocoon spinning, and we measured the length of pupal eye outlines of prepupae every week until pupation with a digital microscope (VH-8000, Keyence, Osaka, Japan).
Eggs collected in June 2008 were reared at 20°C with a long-day (16L–8D) or short-day (13L–11D) photoperiod. Under the long-day photoperiod, most adults emerged by 20 days after cocoon spinning while a portion of individuals entered what was presumably summer diapause. We dissected cocoons immediately after spinning and those remaining 25 days after spinning to measure the length of pupal eye outlines until pupation. Under the short-day photoperiod, all individuals entered what was presumably winter diapause; we dissected cocoons to measure the length of pupal eye outlines from 39 to 61 days after cocoon spinning.
Results Variability of adult emergence under a long-day condition and its heritabilityUnder a constant temperature of 20°C with a photoperiod of 16L–8D, adult emergence exhibited a trimodal pattern (Fig. 3). The first and largest cluster of emergence occurred 13–24 (males) and 13–28 (females) days after cocoon spinning, while the second emergence occurred 25–36 (males) and 31–40 (females) days after cocoon spinning. The first and second clusters were distinctly separated for females (Fig. 3a), whereas they were rather continuous for males (Fig. 3b). The third cluster of emergence occurred 45–64 (males) and 49–62 (females) days after cocoon spinning. The first, second, and third clusters of emergence included approximately 2/3, 1/6, and 1/6 of adults, respectively, for both sexes (Fig. 3).
Figure 4 shows adult emergence in the sons of mothers in the first, second, and third clusters of emergence. The sons of all three types of mothers exhibited the three clusters of emergence, except that no second cluster of emergence occurred for the sons of mothers in the first cluster (Fig. 4a). Assuming that the range of an emergence period for the first, second, and third clusters of sons is respectively 12–26 days, 27–40 days, and 41–65 days after cocoon spinning (Fig. 5), the proportion of the second cluster of sons to all sons was significantly different among the clusters of mothers (Fisher’s exact probability test, p < 0.001), whereas that of the third cluster was not significantly different (p = 0.277).
No deaths occurred in the cocoon period in all these experiments, whereas we did not record the mortality of eggs and larvae before cocoon spinning.
Adult emergence of females (a) and males (b) at 20°C with a 16L–8D photoperiod.
Adult emergence in the sons from mothers in the first (a), second (b), and third (c) clusters of emergence (see Fig. 3a) at 20°C with a 16L–8D photoperiod. The number of mothers in the first (a), second (b), and third (c) clusters of emergence was 16, 10, and 8 respectively.
Proportion of the sons emerged after three cocoon periods for the mothers of three cocoon periods at 20°C with a 16L–8D photoperiod. Numbers in parentheses indicate the total numbers of sons that emerged.
Of the cohort collected on 24 May 2007 (152 larvae spun cocoons from late May to late June), 99 adults emerged from early June to mid-August, 13 adults emerged around mid-September (Fig. 6a), and the remaining individuals in cocoons died by the end of 2008, except for one male that emerged in early May 2008. Mortality in the cocoon period was 0.257 (39/152). A peak of adult emergence occurred around 26–30 days after cocoon spinning, following by a long tail of delayed emergence (Fig. 6b), and a cluster of adult emergence over 90 days after cocoon spinning was observed in September (cf. Fig. 6a).
Of the cohort collected on 4 July 2007 (93 larvae spun cocoons from mid- to late July), 38 adults emerged from early to mid-August, nine adults emerged around mid-September (Fig. 7a), and the remaining individuals in cocoons died by the end of 2008. Mortality in the cocoon period was relatively high, 0.495 (46/93). A peak of adult emergence was observed around 12–14 days after cocoon spinning with a short tail of delayed emergence (Fig. 7b), and the cluster of adult emergence over 40 days after cocoon spinning was in September (cf. Fig. 7a), which is the same as observed for the cohort collected in May.
Cocoon spinning and adult emergence on the calendar (a) and the period from cocoon spinning to adult emergence (b) in the cohort collected on 24 May 2007.
Cocoon spinning and adult emergence on the calendar (a) and the period from cocoon spinning to adult emergence (b) in the cohort collected on 4 July 2007.
Stem shoot elongation of wild roses started in late April and ceased in July when lateral shoots began to bud from the main shoots (Fig. 8). The accumulated number of the lateral shoots, which would elongate in fall, increased noticeably in July and August and leveled off in early September (Fig. 8).
Prepupal development in cocoonsUnder field conditions, the length of pupal eye outlines on the prepupae that remained over 40 days after cocoon spinning increased gradually until pupation in August (Fig. 9). At 20°C with the long-day (16L–8D) photoperiod, the pupal eye outlines of non-diapause prepupae grew immediately after cocoon spinning, while those of prepupae in summer diapause increased gradually until pupation took place approximately 40 days after cocoon spinning (Fig. 10). At 20°C with the short-day (13L–11D) photoperiod, the length of pupal eye outlines of all prepupae remained short, likely reflecting winter diapauses (Fig. 10).
Elongation of the stem shoots of Rosa multiflora, along with the accumulation of its lateral shoots in 2007.
Changes in lengths of pupal eye outlines on prepupae in summer diapause under field conditions in 2008. Values are means ± SD (n = 8).
Changes in lengths of pupal eye outlines on prepupae not in diapause, in summer diapause at 20°C with a 16L–8D photoperiod, and in winter diapause at 20°C with a 13L–11D photoperiod. Values are means ± SD.
Our laboratory experiments revealed that Arge nigrinodosa exhibits trimodal adult emergence under long-day conditions (Fig. 3). Soon after the first and largest cluster of emergence, a smaller second emergence takes place, which is followed by a third emergence after a discrete interval. Although the stage of delayed development to enable the second cluster of emergence was not identified, the third emergence was most likely caused by prepupal summer diapause (aestivation), in which the elongation of pupal eye outlines is temporarily interrupted at approximately the same size as in winter diapause (hibernation) under the short-day conditions (Fig. 10).
We analyzed the genetic background of polymodal adult emergence by comparing the emergence of sons from mothers of different adult emergences. The second cluster of emergence appears to be under genetic control because it was totally absent in the sons (haploid) of the mothers (diploid) in the first cluster of emergence (Fig. 5). In contrast, no heritability of the third cluster of emergence was supported given that its proportion was the same among the sons of mothers of all three clusters of emergence (Fig. 5).
The first cluster of adult emergence in the laboratory clearly corresponds to the early prevailing peak of adult emergence in the field, while the second one most likely represents a tail of delayed emergence after the peak (Figs 6, 7). The cluster of adult emergence occurred in September for both cohorts collected in May and July, and seems to match the third cluster of emergence in the laboratory. Synchronous adult emergence in September after prepupal summer diapause (Fig. 9) appears to be regulated by the changing day length and/or temperature in late summer (
Given that Arge nigrinodosa lays a large egg mass on a single rose shoot, the larvae often encounter food shortage (
Conversely, the second cluster of adult emergence, which occurs soon after the first and normal emergence, appeared heritable, indicating genetic polymorphism. Furthermore, it probably takes place too soon after the normal emergence to disperse weather risks. Even though reliable data are lacking to gain a thorough understanding of the adaptive value of a slightly delayed adult emergence, note that a gregariously feeding sawfly such as Arge nigrinodosa may incur a high risk of producing diploid males after consecutive sib-mating owing to a system of complementary sex determination (CSD) (
The adult emergence of the rose sawfly Arge nigrinodosa in summer generations appears to consist of three components as follows: the first emergence (i.e., the normal emergence); a second, slightly delayed genetically determined emergence; and a third emergence that takes place randomly after partial summer diapause. Thus, the concept of voltinism, the number of generations in a year, may not be applicable in the life history of sawflies with polymodal adult emergence, as indicated by
We thank Prof. Emeritus T. Naito of Kobe University for his valuable advice and continuous support during this study. Thanks are also due to A. D. Liston and Dr T. Inoue for their helpful comments on the manuscript.