Mami Ishizaki

Effect of Plant Species on Survivorship and Duration of Nymphal Stage in Leptocorisa chinensis (Hemiptera: Alydidae)

Abstract Nymphs of Leptocorisa chinensis Dallas were reared on seed heads of various plant species to investigate this bug’s host range. Hatched nymphs were released onto gramineous and nongramineous species. Twenty-four hours after release, the percentage of nymphs that had settled on the plants varied from >90% in all but one gramineous species to 50% or less in the nongramineous species. It is therefore considered that nymphs can discern gramineous plants from nongramineous plants. Nymphs were reared until adult emergence, and the survivors were counted. Nymphs that had been reared on Oryza sativa L. (rice), Digitaria ciliaris (Retz.) Koeler, Poa annua L., or some other gramineous species were able to grow to adulthood, whereas nymphs that had been reared on some other gramineous species such as Eleusine indica (L.) Gaertn., Paspalum dilatatum Poir., Elymus racemifer (Steud.) Tzvelev, Bromus catharticus Vahl, or nongramineous species were not able to grow to adulthood. Results indicate that suitable plant species for the development of nymphs are restricted to a fraction of all gramineous species. Among food gramineous plants, summer plant species that bear seeds in reproductive season of L. chinensis retained high survival rates in young instars compared with spring plant species.

Ovicidal and ovipositional effects of neem (Azadirachta indica A. Juss.) extracts on rice bug, Leptocorisa chinensis (Dallas)

Abstract Ovicidal and ovipositional effects of neem (Azadirachta indica A. Juss.) extracts were tested against rice bug, Leptocorisa chinensis (Dallas), under laboratory and cage conditions, respectively. One- to three-day-old eggs were dipped in the treatments with neem extracts alone and in combination with abamectin (Aba). Treatments with 1.4% neem oil and azadirachtin (Aza) rated at 60, 120 and 180 ppm did not show significant ovicidal effects. Moderate mortality was observed in treatments with Aza at 240 ppm (25–37%) and the combination of neem oil and Aba (36–52%) at day 7, followed by treatments containing Aza in a rate-dependent manner (5–40%). Neem oil moderately reduced oviposition and performed better than Aza at 60 ppm. Adult survival and rice yield were not significantly affected by treatments with Neem oil and Aza used alone. Targeting the adult stage, both treatments did not affect rice yield and quality. Neem extracts and Aba may be used as a supplementary tool in an integrated management programme aimed at controlling rice bug.

Feeding behavior patterns of Leptocorisa chinensis (Hemiptera: Alydidae) nymphs on host and nonhost plants.

ABSTRACT Feeding behavior of the nymphs of Leptocorisa chinensis Dallas (Hemiptera: Alydidae) on various poaceous (grass) species was investigated. The sequence of behavioral phases of the nymph on the food plant was as follows: antennation, rostrum extension, dabbing with the labium, rostrum placement on the plant surface, stylet penetration, and sustained ingestion. Observed behavioral phases of nymphs released on seed heads differed among three categories of plants: food grasses (including rice), nonfood grasses, and nongrasses (nonfood). The nymphs were able to discriminate food grasses from nonfood grasses before they penetrated with their stylets. On nongrasses, nymphs did not exhibit the feeding behavioral phases except for antennation. Nymphs also exhibited behavioral phases such as antennation, rostrum extension, and dabbing with the labium, on paper strips treated with methanol extract of food grasses, but not with extract of nonfood grasses and nongrasses. These results suggest that chemical cues exist in food grass species.

Effects of neem (Azadirachta indica) extracts onLeptocorisa chinensis under choice and no-choice conditions

Two extracts from neem (Azadirachta indica A. Juss. (Meliaceae)) seeds, azadirachtin and oil, and a mixture of neem oil and abamectin, were tested on second-instar nymphs of the rice bugLeptocorisa chinensis (Dallas) (Hemiptera: Alydidae). To clarify the effect of spraying coverage on bioefficacy of test materials, experiments were conducted under choice and no-choice conditions in field cages. In a choice test, treatment with the mixture of neem oil and abamectin was most effective in reducing the survival ofL. chinensis, followed by azadirachtin at 60 ppm, 30 ppm and 3% neem oil, whereas all treatments except neem oil caused 100% mortality within 3 weeks in a no-choice test. When second-instar nymphs had choices of treated and untreated plants within a treatment, no differences in yield and sum of dead and stained grains were found between those two choices, indicating that nymphs neither caused significant reduction in yield nor reduced the quality of untreated plants. Regardless of treatment, the difference in overall yield between treated and untreated plants under choice conditions was not statistically significant (P>0.05). Our results indicate that neem-based formulations, used alone or in combination with abamectin, have the potential to be integrated into the existing programs to control the rice bug.