Evaluation of larval growth process and bamboo consumption of the bamboo powder-post beetle Dinoderus minutus using X-ray computed tomography
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The bamboo powder-post beetle Dinoderus minutus is a major pest of felled bamboo in Japan. In this study, X-ray computed tomography (CT) was utilized to non-destructively trace the movement of the larvae of D. minutus inside infested bamboo specimens and to evaluate the bamboo consumption of the larvae. The infested specimens, which had been enclosed with adult D. minutus beetles, were scanned using a microfocus X-ray CT system every 2 to 5 days. Larvae and other stages of the beetles were clearly recognizable in the CT images with a voxel size of 61.9 μm, and they were distinguishable from the bamboo, insect tunnels, and frass. The larvae were traced until pupation, and the length and volume of their tunnels were measured to evaluate their bamboo consumption. The tunnel length and volume bored by one larva was estimated to be 0.98 mm and 1.06 mm3 (0.70 mg of oven-dry mass) per day within the average observation period of 22 days, respectively, and the active larval period was estimated to be 39 days in our experimental conditions.
KeywordsPowder-post beetle Dinoderus minutus Bamboo X-ray computed tomography
Bamboo has been widely used in Japan as a building material for decorative elements in traditional wooden houses, and for furniture or craft products. However, its vulnerability to insect attack limits its use, and thus measures to protect bamboo materials from insects are necessary. This is especially important when bamboo is employed as laths that contribute to the strength of clay walls in wooden houses. Because bamboo laths are covered with clay, insect damage is not easily detected and the damage may become widespread, making the laths brittle and causing the wall to lose strength.
The bamboo powder-post beetle Dinoderus minutus (Fabricius) (Coleoptera: Bostrichidae) is one of the most significant pests of felled bamboo in Japan, whether it is in use or in storage. Mated adults enter bamboo from the cut surfaces, especially the end surfaces of culms, and oviposit inside. The hatched larvae develop by boring into and feeding on the parenchyma in the inner layer of culms and undergo ecdysis four times before pupation. The adults also feed on bamboo after emergence and leave via an exit hole to mate. D. minutus has one to four broods per year in Japan, and adults are seen from March to November. Tunnels bored by larvae run parallel to the grain and are filled with frass, which is a powdery mixture of excrement and bamboo fragments. Tunnels bored by adults, in contrast, run in various directions and are hollow, discharging frass [1, 2, 3, 4].
The most desirable process for insect control for bamboo in storage or in use is to determine first if insect attack exists. If it exists, bamboo being stored before use should be disposed of, and bamboo in use should be either replaced or treated with insecticide if the attack is active. Effective insect-preventative measures should then be taken for new bamboo without attack or old bamboo to avoid attack in the future. The use of chemical preservatives effectively increases bamboo resistance to insects, but it is not always recommendable considering both the expense and impacts on human health and the environment. Felling bamboo in the proper felling season, traditionally said to be from September to November, reduces the risk of insect attack because the starch content in culms is lower in this season . However, prevention based solely on felling season is imperfect, and it is disadvantageous that the felling season is limited to only few months per year. Therefore, appropriate measures of insect control are required. Nevertheless, the biology of D. minutus, especially feeding activity, remains poorly understood. This data is especially relevant to establishing control measures. Furthermore, it is difficult to analyze the feeding activity of the larvae of D. minutus because they never leave the bamboo culm before emergence.
X-ray computed tomography (CT), which allows for the non-destructive analysis of the 3D-density distribution of an object, has been widely applied to assess the internal structures or defects of wood. X-ray CT has also been used to detect termite or beetle damage in wood [6, 7, 8, 9, 10]. For example, X-ray CT was used to visualize the gallery system of two drywood termite species, Cryptotermes secundus  and Incisitermes minor , and the oak platypodid beetle Platypus quercivorus  developed in wood. It was also used to image the bodies of the Asian longhorn beetle Anophophora glabripennis  and the anobiid beetle Priobium cylindricum  in wood. We, therefore, conducted this study to non-destructively observe the boring process of D. minutus larvae inside infested bamboo specimens using a microfocus X-ray CT system. We then used the data to estimate the bamboo consumption by the larvae.
Materials and methods
Experimental insects and bamboo specimens
Adult D. minutus were collected from infested bamboo culms in Oita Prefecture, Japan and were bred using an artificial diet made of buckwheat flour, which was reported to be suitable for laboratory culture . Bamboo pieces (100 mm in length and ca. 35 mm in width), each with a node in the middle of its length, were prepared from non-treated, partly moist culms of madake (Phyllostachys bambusoides Siebold and Zucc.) felled in May 2013 in Kyoto Prefecture, Japan. A total of 25 adult beetles and 4 bamboo pieces were enclosed together in a plastic container with a vent hole and were left undisturbed at room temperature (25–29 °C) for 2 months so that the adults could oviposit in the bamboo pieces. X-ray CT scans confirmed the existence of larvae inside the pieces.
Results and discussion
Larval growth in CT images
The rest of the larvae exhibited similar movement patterns; they mostly bored along the fiber direction, but they sometimes reversed direction or bored obliquely to the fiber direction, possibly to avoid encountering other individuals or tunnels that could obstruct their movement. However, it was not possible to trace larvae when they were smaller than that shown in the first image of Fig. 4 because they were easily lost sight of in the CT images.
The influence of X-ray irradiation on larval growth was considered in this experiment, but none of the observed larvae died before pupation and the estimated larval period (see the next section) was within the 20–40 days reported previously . This led us to conclude that there was no apparent influence of X-ray irradiation in the experimental conditions employed, yet it needs to be confirmed in a more rigorous study by comparing the growth of irradiated larvae to that of control larvae.
Bamboo consumption of larvae
The consumption in entire larval period is also important for understanding the biology and establishing control measures. The increases in tunnel length, volume, and bamboo mass consumed by one larva per day were multiplied by the estimated larval period of 39 days to estimate that each larva bored 38 mm, produced tunnels that were 41 mm3, and consumed 27 mg of bamboo in its entire larval period. However, these values may be overestimated since the consumption in early larval stage was not investigated in our experiment, and the certainty of these values should be verified in future research.
One major disadvantage of applying X-ray CT to monitor insect activity is that the obtained data are momentary and discontinuous. In our experiment, the scan intervals were mostly 3 days, and uncertainty remained regarding the activity of larvae between the scans. Therefore, a different approach with higher temporal resolution that could be used in a continuous, long-term measurement would be desirable. When wood-attacking insects, such as termites  and lyctus beetles , bite off a wood fragment, elastic waves called acoustic emission (AE) are produced. Because AE is directly related to the feeding activity of insects, AE monitoring may meet these requirements and is expected to be a valuable approach for future analysis.
Bamboo specimens infested with D. minutus larvae were scanned using a microfocus X-ray CT system intermittently. The CT images clearly showed the silhouettes of the larvae and other stages, prepupa, pupa, and adult, and the movement of the larvae was traceable. The length, cross-sectional area, and volume of the tunnels obtained from the CT images were useful for estimating the daily bamboo consumption and active larval period of one larva. These findings draw the conclusion that X-ray CT scanning is a highly effective method for non-destructively and quantitatively assessing the extent of attack of D. minutus larvae in bamboo over time.
The authors thank Mr. Shinji Ninomiya of Oita Industrial Research Institute, Japan for assistance in collecting D. minutus and providing information regarding the methods for breeding this beetle. This work was supported in part by a Grant-in-Aid for Scientific Research (No. 25242032 and No. 26450229) from the Japan Society for the Promotion of Science.
- 1.Yamano K (1969) Kenchiku-konchu-ki (Insect pests of buildings) (in Japanese). Sagami Shobo, TokyoGoogle Scholar
- 2.The Society of House and Household Pests Science, Japan (1995) Kaoku-gaichu-jiten (Encyclopedia of house and household pests) (in Japanese). Inoue Shoin, TokyoGoogle Scholar
- 3.Nair KSS, Mathew G, Varma RV, Gnanaharan R (1983) Preliminary investigations on the biology and control of beetles damaging stored reeds. KFRI Research Report 19, Kerala Forest Research Institute, PeechiGoogle Scholar
- 5.Hirano Y, Shida S, Arima T (2003) Effect of cutting season on biodeterioration in Madake (Phyllostachys bambusoides) (in Japanese). Mokuzai Gakkaishi 49:437–445Google Scholar
- 9.Cruvinel PE, de Naime João M, Borges M, Macedo Á, Zhang A (2003) Detection of beetle damage in forests by X-ray CT image processing. J Braz For Sci 27:747–752Google Scholar
- 10.Kigawa R, Torigoe T, Imazu S, Honda M, Harada M, Komine Y, Kawanobe W (2009) Detection of insects in wooden objects by X-ray CT scanner (in Japanese). Sci for Conserv 48:223–231Google Scholar
- 11.Suzuki K, Kirton LG (1991) An evaluation of some cereal and tuber based artificial diets for laboratory culture of Dinoderus minutus Fabricius (Coleoptera: Bostrichidae) (in Japanese). House Househ Insect Pests 13:59–65Google Scholar
- 12.Fujii Y, Noguchi M, Imamura Y, Tokoro M (1990) Using acoustic emission monitoring to detect termite activity in wood. For Prod J 40:34–36Google Scholar
- 13.Imamura Y, Adachi A, Fujii Y (1998) Acoustic emission (AE) detected from wood attacked by powder-post beetles, Lyctus brunneus STEPHENS (in Japanese). Jpn J Environ Entomol Zool 9:98–100Google Scholar