Atsuhiko Nagasawa

Host plant use for oviposition by Trigonotylus caelestialium (Hemiptera: Miridae) and Stenotus rubrovittatus (Hemiptera: Miridae)

The main hosts and sites of oviposition for the two bugs, Trigonotylus caelestialium (Kirkaldy) (Hemiptera: Miridae) and Stenotus rubrovittatus (Matsumura) (Hemiptera: Miridae), that cause pecky rice were investigated in 24 poaceous plants. Nymphs of T. caelestialium emerged from both spikelets and leaf sheaths, while nymphs of S. rubrovittatus emerged almost exclusively from spikelets. Suitable plants for oviposition by T. caelestialium are Lolium multiflorum, Digitaria violascens and Hordeum murinum, while Poa annua, Anthoxanthum odoratum, Alopecurus aequalis and D. violascens were preferentially used by S. rubrovittatus. There was a greater difference in the number of nymphs emerging from different plants for S. rubrovittatus than for T. caelestialium. This difference may be because T. caelestialium can oviposit on leaf sheaths if the spikelets are not suitable for oviposition, whereas S. rubrovittatus only oviposits on spikelets. Although both bugs oviposited on spikelets, the internal oviposition sites were different. In D. ciliaris, T. caelestialium laid all eggs between the lemma of the first floret and the second floret, whereas S. rubrovittatus laid eggs almost exclusively inside the second floret. In contrast, in P. annua, T. caelestialium laid all eggs inside the florets, whereas S. rubrovittatus laid eggs both between and inside the florets.

Factors Determining the Host Range of Two Tortoise Beetles, Cassidanebulosa L. and C. piperata Hope (Coleoptera: Chrysomelidae) in Japan

We investigated the relationship between host-plant use by two tortoise beetles, Cassida nebulosa and C. piperata, in a field and their performance under laboratory conditions. Sixteen plant species were grown in the experimen- tal field with C. nebulosa being observed to grow on three chenopodiaceous species (Chenopodium album, C. album var. centrorubrum, and C. ficifolium), while C. piperata grew on the same three chenopodiaceous species plus three amaran- thaceous species (Amaranthus blitum, A. hybridus, and A. retroflexus). Host-plant selection in the field was directly corre- lated to the larval growth under laboratory conditions. Thus, the two tortoise beetles selected plant species that were suit- able as larval food under natural conditions. In the laboratory experiments, the survival period of adults and the number of eggs produced showed that adults have wider food ranges than larvae. Therefore, food quality is less important as a limit- ing factor for adults compared to larvae. In addition, the field observation showed that C. nebulosa laid eggs only on suit- able host plant species, whereas C. piperata laid eggs on both suitable hosts and non-hosts.

Habituation by a tortoise beetle to the deterrents of spinach

Abstract:  Feeding responses of Cassida nebulosa to spinach vary depending on prior dietary experience. Adults reared on spinach, eat much spinach whereas those reared on Chenopodium album var. centrorubrum rarely eat it. However, there are a few reports that address the mechanisms of this phenomenon, which is termed ‘induction of preference’. It has been presumed that this phenomenon arises in response to a reduction of sensitivity to the feeding deterrents contained in spinach as a result of early experience feeding on it. Therefore, we attempted to verify the effects of the feeding deterrent that was isolated from spinach. The feeding‐deterrent response was reduced in adults reared on spinach compared with adults reared on C. album var. centrorubrum. Additionally, consumption of spinach by adults reared on deterrent‐treated leaves of C. album var. centrorubrum was significantly higher than that by adults reared on control leaves of C. album var. centrorubrum. These results indicate that C. nebulosa acquires the ability to tolerate spinach because sensitivity to the feeding deterrent contained in spinach is reduced by early experience with the deterrent.

Suppressive effect of exposure to Chenopodium album var. centrorubrum on the feeding response of the tortoise beetle, Cassida nebulosa, to spinach

Depending upon preceding dietary exposure, the tortoise beetle, Cassida nebulosa L. (Coleoptera: Chrysomelidae), exhibits various feeding responses to spinach, Spinacia oleracea L. (Chenopodiaceae). Adult tortoise beetles reared as adults on the main host Chenopodium album var. centrorubrum Makino (CAC) (Chenopodiaceae) eat little spinach, whereas those reared on spinach accept this plant as food. In this study, we investigated the suppressive effect of exposure to CAC on the feeding response of tortoise beetles to spinach. To investigate the effect of chemical factors in CAC, we provided beetles with Gomphrena globosa L. (Amaranthaceae) as the base food and exposed them to a CAC extract, because bioassays have indicated that exposure to G. globosa did not affect the feeding response of beetles to spinach. Adults reared on G. globosa treated with CAC extract consumed significantly less than adults reared on untreated G. globosa. Thus, we determined that exposure to CAC components is responsible for the suppressive effect on the feeding response to spinach observed in the beetles. In a choice feeding test with membrane filter discs, we found that adults reared on G. globosa treated with CAC extract exhibited a greater feeding deterrent response to the spinach deterrent than adults reared on untreated G. globosa leaves. Therefore, the suppression of spinach consumption due to exposure to CAC is probably caused by the development of a response to the feeding deterrents present in spinach.