Insects comprise approximately 90% of all animal species and play an absolutely essential role in ecosystems. We conduct chemical and ecological studies on a variety of insects. Recent research topics are as follows:
The rice ear bugs are the most serious pests of rice in Japan, causing the economically devastating pecky rice. The invasion of the adult bugs rapidly increases with the heading of rice and decreases abruptly afterwards. However, the mechanism of the invasion of the bugs in paddy fields has remained unclear. The rice leaf bug, Trigonotylus caelestialium, and the sorghum plant bug, Stenotus rubrovittatus, are major rice ear bugs in northern area of Japan. Therefore, we investigate the behavioral responses of above two species to odors of rice in various growth stages and other host plants. We demonstrated that the attractiveness of rice plant odor to the rice ear bugs differs according to rice developmental stage and the part of the plant. In T. caelestialium, bugs are attracted to whole plants in the panicle-formation stage and panicles in the flowering stage (Niiyama et al. 2007, J. Appl. Entomol. 131, DOI: 10.1111/j.1439-0418.2007.01161.x). In addition, the host odor preferences of the bugs change between rice plants and gramineous weeds according to the developmental stage of the rice (Fujii et al. 2010, Agric. Forest Entomol. 12, DOI: 10.1111/j.1461-9563.2009.00457.x). Also in S. rubrovittatus, both females and males are attracted to flowering rice panicles (Hori 2009, J. Appl. Entomol. 133, DOI: 10.1111/j.1439-0418.2009.01398.x). On the other hand, it is known that the pattern of S. rubrovittatus invasion into paddy fields changes when Scirpus juncoides, a cyperaceous paddy weed, is present. Emergence of S. juncoides in paddy fields advances the invasion period of the bugs and increases the bug’s invasion into paddy fields. The odor of flowering S. juncoides spikelets is attractive to S. rubrovittatus (Hori 2009). We revealed that the composition of volatile blends produced by rice plants changes with rice development (Fujii et al. 2010, J. Chem. Ecol. 36, DOI: 10.1007/s10886-010-9839-6). In whole plants in the panicle-formation stage and panicles in the flowering stage, the relative levels of sesquiterpenes—especially β-caryophyllene—are higher than those in the other rice plant structures. Furthermore, we succeeded in identifying the flowering rice panicle volatiles responsible for the invasion of T. caelestialium and S. rubrovittatus into paddy fields. That is, attractants of T. caelestialium and S. rubrovittatus are as follows. T. caelestialium: β-caryophyllene, n-decanal, n-tridecene, methyl salicylate, geranyl acetone, methyl benzoate and β-elemene (Hori and Enya, J. Appl. Entomol. DOI: 10.1111/jen.12019). S. rubrovittatus: β-caryophyllene, n-decanal, n-tridecene, methyl salicylate, geranyl acetone, and β-elemene (Hori and Namatame, J. Appl. Entomol. DOI: 10.1111/jen.12001).
It is well known that some insects, such as those from Orthoptera, Diptera, and Lepidoptera, use tarsi as chemosensory organs in feeding and oviposition. However, the role of the tarsus as a chemosensory organ in feeding and/or oviposition is still unclear in insects from Coleoptera, which is the largest order among living organisms. Gustatory discrimination by tarsi in coleopteran insects has yet to be proven directly in ethological experiments, although the electrophysiological and morphological properties of the tarsus have been investigated in a few coleopteran insects such as Chrysomelidae. Therefore, we investigate tarsal gustatory sense in coleopteran insects by using ethological, electrophysiological, and morphological technique. In Chrysomelidae, all genera observed with a scanning electron microscope possess gustatory sensilla (sensilla chaetica) on their tarsi. In ethological studies, Galerucella grisescens, one of the species of Chrysomelidae, can discriminate sucrose, leaf surface wax of host plant, and antifeedant by using only their tarsi. On the other hand, Henosepilachna vigintioctomaculata (Coccinellidae), which possess no tarsal chemosensilla, cannot discriminate gustatory substances by only their tarsi. In Coleoptera, tarsal gustatory sensilla may develop in only a few groups such as Chrysomelidae. In addition, chrysomelid beetles may use their tarsi as gustatory organs to discriminate their host plants.
It has been proven that several species of aphids use host and non-host plant odors for host finding (Hori 1999a, Appl. Entomol. Zool. 34). In green peach aphid, Myzus persicae, and onion aphid, Neotoxoptera formosana, repellents from non-host aromatic plants such as rosemary have been found (Hori 1998, J. Chem. Ecol. 24; Hori and Komatsu 1997, Appl. Entomol. Zool. 32; Hori 1999b; 1999c, Appl. Entomol. Zool. 34). However, the attractiveness of headspace components of the host plants to the aphids has been poorly examined. Therefore, we investigate attractants of aphids in the headspace of the host plants. N. formosana are significantly attracted to both Allium fistulosum and A. tuberosum (Hori and Komatsu 1997, Appl. Entomol. Zool. 32). The main headspace components of A. fistulosum extracted with solid-phase microextraction are dipropyl disulfide, 1-propenyl propyl disulfide and dipropyl trisulfide (Hori 2007, J. Appl. Entomol. 131, DOI: 10.1111/j.1439-0418.2006.01130.x). In the headspace of A. tuberosum, diallyl disulfide is detected as the main component. The aphids are attracted to dipropyl trisulfide and diallyl disulfide at a concentration of 0.01%. On the other hand, dipropyl disulfide does not attract the aphids at any concentrations. It is revealed that attractancy of A. fistulosum and A. tuberosum is caused by dipropyl trisulfide and diallyl disulfide, respectively. The findings suggest that N. formosana uses these sulphur compounds, characteristic components of Allium plants, as olfactory cues to find the host plants.
Adzuki bean beetles, Callosobruchus chinensis are the most serious insect threat to stored adzuki beans and, at present, are mainly controlled by fumigation. However, the use of methyl bromide, which has been the most widely used chemicals for the fumigation, is gradually being restricted. In addition, contamination of the stored beans with not only living beetles but also beetle fragments and excreta leads to consumer complains. Therefore, alternative control methods that are capable of directly preventing the beetles from invading stored beans, such as use of repellents, are needed. We investigate the repellency and toxicity of six troponoid compounds against adzuki bean beetles and examine the relationship between their structure and activity. γ-Thujaplicin shows the highest repellency against the beetles among the compounds tested, while the repellency of tropiliden is quite low (Shimizu and Hori 2009, J. Stored Prod. Res. 45, DOI: 10.1016/j.jspr.2008.08.001). The results suggest that the keto and hydroxyl groups are important in the repellent properties of troponoid compounds. Although an isopropyl group is also important for repellency, the effect varies according to its position on the seven-member ring; the farther the isopropyl group is from the keto and hydroxyl groups, the higher the repellency become. As with its repellency effect, the toxicity of tropiliden is quite low. Similar to its repellency, γ-thujaplicin shows the highest toxicity among the thujaplicins. However, the toxicities of α-thujaplicin and β-thujaplicin (hinokitiol) are similar, unlike the repellency.
The cigarette beetle, Lasioderma serricorne, damages a wide range of stored food products, including cured tobacco leaves, cereals, cereal products, soybean flour, cocoa beans, oilseeds, pulses, spices, dried fruits, and some animal products. Currently, insect infestations of stored products are mainly controlled by fumigation. However, the use of methyl bromide fumigant is increasingly being restricted. Phosphine is a fumigant for the cigarette beetle, but resistant beetles are emerging around the world. Therefore, new control techniques are needed. In addition to living beetles, contamination of stored products with beetle fragments and excretions leads to consumer complaints. Alternative control methods need to be capable of directly preventing the beetles from invading stored food products. Insect behavior regulators, such as repellents, attractants, oviposition stimulants, and oviposition deterrents, may be useful as novel control methods because they can control the beetle’s invading or oviposition behavior. Repellents for L. serricorne have been examined in plant derivatives and promising repellents such as β-thujaplicin (hinokitiol) and shiso oil have been found (Hori 2003, Appl. Entomol. Zool. 38; 2004a, Appl. Entomol. Zool. 39, DOI: 10.1303/aez.2004.357; 2004b, Appl. Entomol. Zool. 39, DOI: 10.1303/aez.2004.521; 2004c, Appl. Entomol. Zool. 39, DOI: 10.1303/aez.2004.699; 2005, Appl. Entomol. Zool. 40, DOI: 10.1303/aez.2005.373). However, oviposition stimulants contained in stored food products have been poorly examined. Therefore, we investigate oviposition stimulants for L. serricorne from its hosts. L. serricorne shows high ovipositional activity on roasted coffee beans in which they cannot develop from egg to adult (Hori et al. 2011, Appl. Entomol. Zool. 46, DOI: 10.1007/s13355-011-0062-x). Furthermore, methanol extract of roasted coffee beans shows strong oviposition-stimulatory activity. We are currently isolating the oviposition stimulants from roasted coffee beans.