Entomological Science (2005) 8, 227–228
SHORT COMMUNICATION
Techniques for continuous rearing and assessing host preference of a multivoltine leaf-mining moth, Acrocercops transecta (Lepidoptera: Gracillariidae) Issei OHSHIMA Systematic Entomology, Department of Ecology and Systematics, Graduate School of Agriculture, Hokkaido University, Sapporo, Japan
Abstract Techniques for rearing the leaf-mining moth Acrocercops transecta successively over several generations are described. Based on continuous rearing, some life historical parameters in A. transecta were determined. Because of its short generation time, successive rearing makes the moth suitable for mating experiments and a model organism to elucidate the mechanism of host-associated speciation. Key words: Ericaceae, host race, Juglandaceae, mating, sympatric speciation.
In phytophagous insects, specialists predominate in terms of number of species (Menken & Roessingh 1998). The diversity of specialist herbivores has been ascribed mainly to host-associated speciation through host shift (Bush 1994; Berlocher & Feder 2002). To understand the mechanism of speciation by host shift, it is necessary to know the genetic basis of host preference of ovipositing females and of host adaptation of their larvae (Thompson 1988). The use of a model organism with a short generation time and a multivoltine life cycle is preferable for elucidating the genetic basis. However, almost all studies have used univoltine species for evaluating host fidelity of host-associated species (or host races) and gene flow between them (Berlocher & Feder 2002; Drès & Mallet 2002). A promising model for demonstrating speciation processes through host shift is a phytophagous insect that is multivoltine and is readily reared in the laboratory over several generations. Acrocercops transecta Meyrick, 1931, is a small (forewing approximately 4 mm long) leaf-mining moth distributed in Far East Russia, South Korea, Japan and Taiwan. Its host plants are several species of JuglanCorrespondence: Issei Ohshima, Systematic Entomology, Graduate School of Agriculture, Hokkaido University, Sapporo, 060-8589 Japan. Email:
[email protected] Received 18 February 2005; accepted 3 May 2005.
daceae and Lyonia ovalifolia (Ericaceae) (Kumata et al. 1988), and the association with distantly related host plants (Juglandaceae and Ericaceae) suggests the possibility of a host shift and genetic differentiation between the host-associated populations. This species is multivoltine, with five to seven generations per year. In the present paper, I describe the methods for laboratory rearing of the moth from egg to adult, together with some life historical parameters based on rearing and mating experiments. From late May to early October, leaves with mines containing larvae were collected from Juglans ailanthifolia (Juglandaceae) and L. ovalifolia every third week from Mount Aoba, Sendai City, Miyagi Prefecture, northern Honshu (the central main island of Japan), and maintained in the laboratory at the Miyagi University of Education. The leafstalks were wrapped with wiping paper to which 1% sucrose solution was added. All experiments were carried out at 22 ± 1∞C and under a photoperiod of 14 h light : 10 h dark (LD 14:10) or LD 16:8. Emerging moths were sexed by inspecting external genitalia, and one pair of the moth was transferred to one plastic centrifuge tube (118 mm long, 28 mm diameter) using an aspirator. The tubes were capped with wiping paper. Adults within 24 h of emergence were used for the mating experiment and they were allowed to mate until one of the pair died. One host leaf was placed in each tube. The leafstalk was rolled up with
I. Ohshima
wiping paper to which 1% sucrose solution was added not only to keep the leaf fresh, but also to provide food for the moths. After this experiment, all females were dissected and examined for the status of their bursa copulatrix to check whether mating had occurred or not. Each female that had started laying eggs on the inner surface of the tube was introduced into a plastic container (100 ¥ 100 ¥ 50 mm) aided with an aspirator after the light was turned on. In the container, almost the same size fresh young leaves of J. ailanthifolia and L. ovalifolia were placed, with their stalks rolled up with wiping paper soaked with 1% sucrose solution. The ovipositing females were allowed to select leaves for oviposition. The number of eggs on each leaf was counted after 24 h. To obtain eggs for successive rearing, three or four flesh leaves of the host plant, each rolled up at the stalk with wiping paper soaked with 1% sucrose solution, were placed in the plastic container (100 ¥ 100 ¥ 50 mm). Host-plant leaves were replaced every day until a sufficient number of eggs for each experiment was obtained. When more than three eggs was laid on a leaf, I removed eggs to adjust the number of eggs to fewer than four per leaf. Leaves with eggs were separately transferred into a plastic centrifuge tube (118 mm long, 28 mm in diameter) with a plastic cap. Supply of 1% sucrose solution every 3 days kept the leaves more or less fresh until pupation of the moths. When the larvae reached the fifth (final) stadium, plastic caps were replaced with wiping paper, because too much humidity hindered pupation. The rearing of A. transecta revealed the following characteristics in its life history traits. Copulation occurred 4.4 ± 2.2 (SD) days (n = 23) in males and 4.5 ± 2.1 (SD) days (n = 24) in females after emergence. In 67 (91.8%) of 73 trials and 46 (97.9%) of 47 trials mating took place in the J. ailanthifolia-associated and in the L. ovalifolia-associated populations, respectively. Females started laying eggs 2.0 ± 1.3 (SD) days (n = 19) after copulation. Oviposition was concentrated in the period between 2 and 3 h before and after the light was turned off. The females in the J. ailanthifolia-associated population oviposited three to 59 eggs (mean ± SD = 24.6 ± 13.9, n = 30) for 24 h after being introduced into the plastic container, and the females in the L. ovalifolia-associated population oviposited three to 25 eggs (mean ± SD = 10.9 ± 6.1, n = 30) during that time. In all of the 60 trials (30 for the J. ailanthifoliaassociated population and 30 for the L. ovalifoliaassociated population), the females laid eggs on their natal host plant without exception.
228
The eggs hatched 4 days after oviposition, larvae pupated 10.5 ± 1.6 (SD) days (n = 66) after hatching, and adults emerged 9.7 ± 1.0 (SD) days (n = 53) after pupation. These results indicate that A. transecta completes one generation within 1 month. Because A. transecta is associated with deciduous trees, rearing it over winter with the present method is not possible; nevertheless, multivoltinism with five to seven generations per year in this species is sufficient to elucidate the genetic basis of host specialization. Therefore, the present study suggests that the leaf-mining moth A. transecta can be used as a model system for examining the genetic basis for host selection and host adaptation.
ACKNOWLEDGMENTS I thank K. Mizota, T. Fukushi, N. Goto, Y. Hirabuki, R. Deguchi and A. Munakata for allowing me to use their laboratory at the Miyagi University of Education for rearing experiments. I also thank T. Kumata, S. Akimoto, N. Fujiyama and N. Kobayashi for their comments on the manuscript and advice on the project. This study was supported by a JSPS Research fellowship for Young Scientists (DC1) and in part by a 21st Century Center of Excellence (COE) grant from the Japanese Ministry of Education, Culture, Sports, Science and Technology for the “Neo-Science of Natural History” Program at Hokkaido University (Leader: H. Okada).
REFERENCES Berlocher SH, Feder JL (2002) Sympatric speciation in phytophagous insects: moving beyond controversy? Annual Review of Entomology 47, 773–815. Bush GL (1994) Sympatric speciation in animals: new wine in old bottles. Trends in Ecology and Evolution 9, 285–288. Drès M, Mallet J (2002) Host races in plant-feeding insects and their importance in sympatric speciation. Philosophical Transactions of the Royal Society of London B 357, 471–492. Kumata T, Kuroko H, Ermolaev VP (1988) Japanese species of the Acrocercops-group (Lepidoptera: Gracillariidae) Part 1. Insecta Matsumurana New Series 38, 1–111. Menken SBJ, Roessingh P (1998) Evolution of insect–plant associations: Sensory perception and receptor modifications direct food specialization and host shifts in phytophagous insects. In: Howard DJ, Berlocher SH (eds) Endless Forms, pp 145–156. Oxford University Press, New York. Thompson JN (1988) Evolutionary ecology of the relationship between oviposition preference and performance of offspring in phytophagous insects. Entomologia Experimentalis et Applicata 47, 3–14.
Entomological Science (2005) 8, 227–228
