Appl. Entomol. Zool. 40 (3): 497–505 (2005) http://odokon.ac.affrc.go.jp/

Chemical and behavioral study on the phytomimetic giant geometer robustum Butler (: Geometridae)

Toshiharu AKINO* Laboratory of Behavior, National Institute of Agrobiological Sciences; Tsukuba 305–8634, (Received 2 February 2005; Accepted 5 April 2005)

Abstract Polyphagous twig-like caterpillars of the giant geometer Biston robustum resemble the host plants not only in the mor- phological appearance, but also the surface chemicals as they change their cuticular chemicals after switching host plants. The object of this study is to investigate whether the caterpillars adjust the cuticular chemicals through direct body contact to the twigs or the ingestion of leaves after changing their host plants. The caterpillars successfully ad- justed their cuticular chemicals to the new host plant when they fed on the plant’s leaves, regardless of the plant species. The caterpillars’ behavior was continuously under scrutiny, but direct body rubbing on the twigs was not ob- served. Thus, the caterpillars adjusted the cuticular chemicals through ingesting the leaves of the new host plant. Con- tinuous behavioral observation revealed leaf clipping and body bending-and-stretching behaviors in the late instar caterpillars. Although the ethological meanings of the body bending-and-stretching behavior is uncertain, the leaf clipping, as well as morphological and chemical phytomimesis, would be adaptive as a countermeasure to predators.

Key words: Giant geometer; Biston robustum; cuticular chemicals; host plant

plant twigs not only in morphological appearance INTRODUCTION but also with respect to surface chemical appear- Camouflage is a deception strategy; it works of- ance (Akino et al., 2004). (e.g. Formica, La- fensively for ambush types of predators, including sius, Camponotus, Crematogaster, Pristmyrmex) the flower mantis, and defensively for mimetic are abundant predators on food plants, but such as twig-like caterpillars. The phyto- workers are not aware of the caterpillars even when mimetic camouflage used by the twig-like caterpil- walking on them. In contrast, ant workers do ex- lars is characterized by a combination of colors, hibit awareness of the caterpillars’ presence on humps and marks on the body that look like buds non-host plants. This indicates that chemical phyto- and scars on twigs, the absence of abdominal legs mimesis would work well for the caterpillars to de- except for a hind pair to grasp the twigs, and a ceive the ant workers. habit of stretching the body at an angle away from I address the question of how the caterpillars ad- branch (Wickler, 1968; Edmunds, 1974; Owen, just their cuticular chemicals to resemble the sur- 1980). Such visual camouflage is quite effective face chemicals of their respective host plants, espe- against avian predators that locate their prey prima- cially after having switched between them. There rily by sight (Heinrich, 1993). However, it is doubt- may be at least two possibilities: (1) the caterpillars ful that visual camouflage works well against inver- acquire the surface chemicals from the host plant tebrate predators that use chemical sensing to lo- through direct body contact with the twigs, or (2) cate their prey. In the case of polyphagous giant the caterpillars acquire the surface chemicals geometer Biston robustum Butler (Lepidoptera: through ingestion of the host plant leaves. The sec- Geometridae), a close relative of B. betularia—the ond possibility is subdivided into the following well-known example of industrial melanism (Ket- three hypothesis: the ingested plant chemicals are tlewell, 1955): the caterpillars resemble their host (a) directly diverted to the cuticular chemicals, (b)

* E-mail: [email protected] DOI: 10.1303/aez.2005.497

497 498 T. AKINO selectively diverted to the cuticular chemicals, or light was turned off. There were two replicates for (c) trigger off a change of biosynthetic pathway to the first and second instars, five replicates for the adjust the cuticular chemicals to fit those of the third and fourth instars, and eight replicates of the host plants. The first hypothesis is less likely be- fifth and later instars. Beforehand, the activities of cause of the differences in the surface chemical the fifth and later instar caterpillars were continu- compositions between the leaves and twigs of the ously recorded by an infrared video camera and a host plants (Akino et al., 2004). In the present time-lapse videotape recorder for at least 3 d, and study, I tested two possibilities by performing then behavioral categories were listed (Experiment chemical analyses and continuously observing the I). In the same manner, two to three caterpillars of behavior of the caterpillars to confirm whether or the fourth and later instars were reared on a branch not they rub themselves against the twigs of the placed in an Erlenmyer flask, and behaviors of in- host plant. Through continuous observation, two dividual caterpillars were recorded continuously by characteristic behaviors of the caterpillars, as well an infrared video camera with a time-lapse video as the feeding and resting patterns, were confirmed, tape recorder (Experiment II). of which ethological meanings are briefly dis- In Experiment III, it was tested whether the cussed. caterpillars could adjust the cuticular chemicals to surface chemicals of the new host plants through ingestion of the leaves. A total of six branches (ca. MATERIALS AND METHODS 20 cm long) each of cherry, , and chin- . Approximately 20 adult of B. ro- quapin were punched through a piece of cardboard bustum were collected under mercury lights along (3324 cm) at approximately 10-cm intervals. The a highway on Hachijo-jima Island, Tokyo, Japan, tips of these branches were individually wrapped between 23:00 and 01:00 in early March 1999. The with wet cotton underneath the cardboard to supply moths were allowed to mate and lay eggs in a small water during the experiment. Another set of cage (82035 cm). The eggs were kept at 10°C branches, the surface of which was wrapped with for a few weeks until they hatched. The larvae were Teflon tape, were mounted on another piece of reared in plastic containers (342512 cm) at cardboard in the same manner. The cardboard- 22°C, 14L10D. Twigs with leaves of cherry pieces were placed in a plastic container yedoensis Matsum were provided every day. When (342512 cm), and 20 fourth-instar caterpillars the caterpillars grew to the 4th instar on cherry grown on cherry branches were gently placed on leaves, two groups, of approximately 100 and 20 them. Fresh leaves of cherry, camellia, and chin- caterpillars each, were fed on camellia, Camellia quapin, cut with stems as short as possible, were japonica L., and chinquapin, Castanopsis cuspi- provided to the caterpillars every day. The cut ends data (Thunb) Schottky var. sieboldii (Makino), re- of the leaves were covered with wet cotton and thin spectively. The other caterpillars continued to be aluminum foil, which was additionally wrapped reared on cherry leaves. with Teflon tape to prevent the caterpillars from Behavioral experiments. Fifty caterpillars of contacting them. When the caterpillars grew to the first- and second-instar each, and 20 of third- and sixth instar, the cuticular chemicals were analyzed. fourth-instar each were reared in a plastic contain- This was repeated three times for each combination ers, where cherry twigs with fresh leaves were pro- with different sets of caterpillars. vided. Two to three caterpillars of fifth and later in- Chemical analyses. Single leaves of approxi- stars were reared on a cherry branch (ca. 25 cm mately 20 cm2 in area and twigs of 10-cm length long) placed in 50-ml Erlenmyer flask. Four flasks with a diameter of 3–4 mm were separately rinsed were placed in a plastic container (3222 twice in 5 ml of distilled hexane. The caterpillars 5 cm), so that ten caterpillars of each instar were were kept in the freezer for 1 h, and were then indi- reared in the container. For 2 d, the number of vidually rinsed in 3 ml of hexane without allowing caterpillars feeding on leaves and the locations regurgitation. These hexane rinses were concen- where they rested were monitored, except when trated for gas chromatography (GC) and gas chro- they were fed for 5 min every 30 min. A red light matography-mass spectrometry (GC-MS) analyses, was used for direct observation after the regular and were stored in the freezer at 20°C until Chemical and Behavioral Study on B. robustum 499 needed. profiles of caterpillars and plants was analyzed by GC analyses were performed on a Hewlett quantification theory type III, which was per- Packard 6890 GC equipped with an apolar capil- formed with the “Black-Box Package” in Aoki lary column HP-1 (15 m length, 0.25 mm ID, and (2004). 0.25 mm film thicknesses) and a flame ionization detector. Injection was made through a cool on- RESULTS column injector at 53°C, and the injector tempera- ture was controlled at the oven temperature of plus Behavioral patterns of B. robustum caterpillars 3°C. The column oven was set at 50°C for 5 min, Caterpillars’ behaviors included the following: then ramped up to 300°C at 15°C/min, and kept at (1) Moving: drawing its hind end forward while the final temperature for 10 min. Helium was used holding on with the front legs, then advancing its as the carrier gas at the column head pressure of front section while holding on with the caudal legs, 60 kPa. (2) resting: standing erect and motionless on the GC-MS analyses were archived with an HP 6890 caudal legs, and (3) feeding: feeding on leaves (Ex- GC interfaced to a JEOL JMS SX-102A double- periment I). Additionally, bending-and-stretching focusing mass spectrometer in EI mode of 70 eV, behavior in the late-instar caterpillars was con- and run by an HP model 715/64 computer. GC was firmed several times in a day, at intervals of ap- performed under the same condition as GC analy- proximately 6–8 h. The caterpillars bent their body ses except for the column head pressure of 18 kPa. without releasing the caudal legs, looped the body, Evaluation of resemblance in surface chemi- and then stretched it again (Fig. 1). This behavior cal profiles. A total of 47 components were se- was over in several seconds. In the process, the lected (Akino et al., 2004). The similarity of the caterpillars did not grasp the branch with their profiles was evaluated in terms of the presence of front legs, whereas they did grasp the twig with the the 47 components. The amount of each compound front legs after bending the body and released the in the sample was presented as peak areas by the caudal legs to draw their hind ends forward. After GC-FID detector. When the relative quantity of the the bending-and-stretching behavior, the caterpil- corresponding component exceeded 5% of the total lars held the body erect and motionless on the cau- quantity of these components, it was judged to be dal legs at an angle to the branch. Furthermore, present and was assigned one point. When it was “ballooning” was also confirmed in the first-instar less than 5% of the total amounts of those compo- caterpillars (i.e., the caterpillars produce silk and nents, it was judged to be absent and assigned hang down from the leaves and twigs to wait until “zero” points. The similarity of the surface wax the wind takes them away like a balloon). This bal-

Fig. 1. Comparison of the movement of the Biston robustum caterpillars in terms of body bending-and-stretching and moving. 500 T. AKINO

Fig. 2. Daily feeding activity of Biston robustum caterpillars. Feeding activity was evaluated according to the number of the caterpillars. looning technique was confirmed only in the first- ing pattern during the night. Before the light was instars caterpillars. turned on, they stopped feeding and stayed away Feeding and resting behaviors of caterpillars from the leafstalk on which they had been feeding. from the first to seventh instars were compared Feeding behavior was rarely observed during the (Fig. 2). Intermittent observation revealed differ- day. The caterpillars, especially the fifth and later ences in daily feeding patterns and resting sites instars, occasionally chewed the leafstalk to get off among the instars. The first and second instars fed the leaf after having consumed it, and then pro- on the leaves both day and night (Fig. 2), staying ceeded to feed on the reminder of the leafstalk left underneath the leaf to remain out of sight. While on the branch. This resulted in having to discard resting, these instars often settled at the edge or the the leaf remnant with the feeding mark on it. underside of the leaves or simply on the branch Such leaf clipping behavior was more frequently (Fig. 3). In contrast, the third and later instars observed when the caterpillars consumed camellia tended to feed on the leaves after the light was and chinquapin leaves (Experiment II). They inter- turned off (Fig. 2). This was more obvious in the mittently fed on one or two leaves during the night- fourth and later instars, less so in the third instars. time, and cut the leafstalk down when they con- Continuous observation of the fourth and later in- sumed almost half of the leaf, which was usually stars revealed that they fed on the leaves intermit- before the light was turned on. They also cleaned tently during the night (i.e., 5 to 20 min of feeding the reminder of the leafstalk on the branch before alternated with 20 min or longer of non-feeding). hiding away from the feeding site. Thus, at least These caterpillars tightly grasped the twig near a one new remnant was found in the cafe every leafstalk with their caudal legs and fed on the morning. leaves. During the non-feeding period, the caterpil- Bending-and-stretching behavior was also con- lars rested near the leaf that they fed on or re- firmed in the caterpillars that fed on camellia and mained nearby. After an interval, they fed on the chinquapin leaves, as well as on the cherry leaves same leaf again, and repeated the feeding and rest- (Experiment II). The caterpillars exhibited this be- Chemical and Behavioral Study on B. robustum 501

a

d

b

c

Fig. 3. Differences in resting sites of Biston robustum caterpillars. (a) The first-instar caterpillars (red arrows) rested at the edge of the cherry leaves. (b) The second instars rested on the underside of the leaves. (c) The third instars rested near their feeding mark. (d) The fourth instars rested on the tops of twigs. havior several times a day, each lasting several sec- saturated alcohols and aldehydes (Table 1). n-Alka- onds. In addition to body bending-and-stretching, nes accounted for ca. 99% in the surface chemicals feeding, resting, and moving behaviors were also of cherry twigs, while they accounted for ca. 70% confirmed in the case of the camellia and chin- and 10% in the camellia and chinquapin twigs, re- quapin, as well as the cherry. However, no other spectively. Alcohols and aldehydes accounted for characteristic behavior, including body rubbing, ca. 20% and 10% in camellia, and ca. 10% each in was confirmed in the caterpillars after switching chinquapin. Unidentified compounds consisted of the host plant species. Nevertheless, the caterpil- ca. 70% of the surface chemicals of chinquapin lars’ cuticular chemicals turned out to be similar to twigs. the surface chemicals of the new host plant species When the caterpillars were reared on camellia (Fig. 4). and chinquapin leaves, their cuticular chemicals closely resembled the surface chemicals of camel- Comparison of the cuticular chemicals lia and chinquapin twigs, respectively, regardless of Experiment III confirmed that the cuticular what plant they had perched on (Fig. 4). The cutic- chemicals of the caterpillars resembled the surface ular chemical profiles of the caterpillars that were chemicals of the plants the leaves of which the fed cherry leaves resembled the surface chemical caterpillars had consumed after having changed the profiles of the cherry twigs. In the process of ad- host plant species (Table 1, Fig. 5). The surface justment, the cuticular chemicals contained addi- chemicals of the plant twigs consisted of n-alkanes, tional components, some of which were found in 502 T. AKINO

Table1. Surface chemical components of the twigs that were the food plants of Biston robustum

Peak No Suspected compounds Cherry Camellia Chinquapin

1 n-Pentadecane 2 n-Octadecane 3 n-Nonadecane 4 n-Docosane 5 n-Tricosane 6 n-Tetracosane 7 n-Pentacosane 8 n-Hexacosane 9 n-Heptacosane 10 n-Octacosane 11 Hexacosanal 12 1-Hexacosanol 13 n-Nonacosane 14 n-Triacontane 15 Octacosanal 16 1-Octacosanol 17 n-Hentriacontane 18 n-Tritriacontane 19 Triacontanal 20 1-Triacontanol 21 Unidentified compound 22 Unidentified compound 23 Dotriacontanol 24 Unidentified compound 25 Unidentified compound 26 Unidentified compound

Corresponding peaks were shown in Fig. 4. Details of identification of the compounds were referred to Akino et al. (2004). the surface chemicals of neither cherry nor the new 18.6%, respectively (Fig. 5a). This was also true host camellia plant, though the replicates were when the caterpillars were given cherry leaves but small (N2). GC-MS analyses following the col- allowed to perch on camellia twigs (grey circle, umn chromatography suggest that the additional Fig. 5b), and vice versa (inverted grey triangles, compounds may be unsaturated long-chained alco- Fig. 5b). Even when the cherry twigs were hols, aldehydes, and sterols, but they have not yet wrapped with the teflon tape, the cuticular chemi- been identified. cals of the caterpillars fed on camellia leaves re- Figure 5 shows similarity of the surface chemi- sembled to the surface chemicals of the camellia cal profiles between the caterpillars and correspond- twigs (inverted black triangle, Fig. 5b). An accu- ing food plant twigs. When Quantification Theory mulated 30.7% was explained up to the two axes Type III was applied to evaluate the similarity of and each axes 1 and 2 revealed 18.7% and 12.0%, the profiles, the profiles of the sixth-instar caterpil- respectively (Fig. 5b). Thus, the caterpillars lars fed on cherry (black circles, Fig. 5a) were changed their cuticular chemicals to resemble the closer to those of the cherry twigs (white circle, twig surface chemicals of the leaves they had fed Fig. 5a). Similarly, the caterpillars fed on camellia on. (black triangles) and chinquapin (black squares) were closer to those of the camellia (white triangle) DISCUSSION and chinquapin (white square) twigs, respectively. Regarding the axis unique values and contribution The giant geometer B. robustum has the ability ratios, an accumulated 52.7% was explained up to to adjust its cuticular chemicals to resemble the two axes and axes 1 and 2 revealed 34.2% and surface chemicals of the respective food plant Chemical and Behavioral Study on B. robustum 503

Fig. 4. Comparison of the surface wax profiles between caterpillars and the food plant twigs. Numbered peaks are listed in Table 1. Peaks with asterisks temporarily appeared, and were supposed to be unsaturated alcohols, aldehydes, and sterols based on their mass spectra. species (Akino et al., 2004). It was confirmed that to their respective host plants. It is less likely, how- the caterpillars changed their cuticular chemicals ever, that the caterpillars directly secreted the in- when changing the host plant species, but it was gested leaf surface chemicals in their cuticles, be- uncertain how they could accomplish this. In the cause the surface chemical composition of the current study, no behavioral evidence was obtained leaves and twigs differed slightly from one another to support the hypothesis that the caterpillars ac- in the case of camellia and chinquapin (Akino et quired the surface chemicals of the new host plant al., 2004). In the progress of the chemical alterna- twigs through direct body contact (Experiments I tion, a series of chemicals of unsaturated long- and II). Without direct contact to the host plant chained alcohols, aldehydes, and sterols were tem- twigs, the caterpillars changed their cuticular porarily found in the cuticular waxes (Fig. 4). chemicals to resemble the twig surface chemicals These compounds were not found in the host of the leaves they had fed on (Experiment III). This plants, and completely disappeared when the alter- means that ingesting leaves may be important for nation was over. I therefore believe that ingesting the caterpillars to adjust their cuticular chemicals the leaf chemicals triggers a physiological change 504 T. AKINO

remained at the edge or the underside of the leaf and would intermittently feed on the underside of the leaf throughout the day. In contrast, late instars stayed near the leaf during the nighttime and away from it during the day, feeding intermittently mainly during the night. Lepidopteran caterpillars of many species are known as nocturnal feeders (Edwards, 1964, 1965; Heinrich, 1979; Lance et al., 1986). This habit is adaptive because few birds have nocturnal vision. Heinrich and Collins (1983) suggest the feeding pattern of the cryptic caterpil- lars depends on their methods of disguise: caterpil- lars that are cryptic on leaves or parts of leaves feed day and night since they always stay on the leaves, while caterpillars that are cryptic on twigs or on bark feed during the night. The feeding pat- tern of B. robustum also agrees with that sugges- tion, since early instars appeared to be cryptic on the edge of cherry leaves and the late instars on twigs, respectively. In the current study, behavior was more fre- Fig. 5. Resemblance between the surface wax profiles of caterpillars and the food plant twigs analyzed by Quantifica- quently confirmed in the late instars than the early tion Theory Type III. (a) Open circles, triangles, and squares ones. What is the ethological role of the leaf clip- show the cherry, camellia, and chinquapin twigs, respectively. ping? This behavior is also reported in other lepi- Black circles, triangles, and squares show that the sixth-instar dopteran including Papilionidae (Lederhouse, caterpillars fed on cherry (N3), camellia (N11), and chin- 1990), Sphingidae and Notodonitidae (Heinrich quapin (N 9) twigs, respectively. (b) Grey circles show that and Collins, 1983). These leaf clipping species, in- the sixth-instar caterpillars fed on cherry leaves perching on camellia branches (N10). Grey and black inverted triangles cluding B. robustum, feed only at night, and hide show that the sixth-instar caterpillars fed on camellia leaves away from the feeding site on a branch in the day- perched on cherry branches (N10) and cherry branches time (Dussourd and Eisner, 1987; Dussourd, 1993). wrapped with Teflon tapes (N5), respectively. Such larval behaviors are presumably a counter- measure to avian predators since they are explica- ble if birds learn to search for prey near the sign of in the caterpillars, presumably in the biosynthetic leaf damage. In fact, birds can recognize leaf dam- pathways involving the corresponding enzymes age by sight and search for prey around the feeding and lipophorins, which results in chemical alterna- mark (Heinrich and Collins, 1983). tion for phytomimesis in B. robustum. There is another possibility on the ethological Such a diet-induced is reported only in role of leaf clipping: it is to thwart the host plant’s Nemoria arizonaria, for polymorphism (Green, defenses. Leaf clipping may reduce herbivore-in- 1989). Green suggests that diet is a good candidate duced plant volatiles (HIPVs). HIPVs are elimi- as a general releaser for metabolic reaction to in- nated from various plant leaves damaged by herbi- duce polymorphism. I assume it is also true for vores, which in turn attracts parasitoids and preda- chemical polymorphism, which causes chemical tors (Weinstein, 1990; Dicke et al., 1998). Elimina- phytomimesis in B. robustum. tion of damaged leaves should result in reducing Analyses of the behavioral patterns of B. robus- HIPV production. Stopping any chemical leakage tum suggest that the late instars would be nocturnal may be very important to the mimic trying to es- feeders (Fig. 2). Since early instars fed both day cape from kairomone-guided predators (Stowe, and night, their feeding habit would change as they 1988). Further study, however, is necessary to as- matured. Similarly, they would also change the certain whether leaf clipping is really effective in resting sites as they matured. Early instars usually reducing HIPVs on arbor plant. Chemical and Behavioral Study on B. robustum 505

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